Islands in the Cosmos
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363 pages

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How is it that we came to be here? The search for answers to that question has preoccupied humans for millennia. Scientists have sought clues in the genes of living things, in the physical environments of Earth from mountaintops to the depths of the ocean, in the chemistry of this world and those nearby, in the tiniest particles of matter, and in the deepest reaches of space. In Islands of the Cosmos, Dale A. Russell traces a path from the dawn of the universe to speculations about our future on this planet. He centers his story on the physical and biological processes in evolution, which interact to favor more successful, and eliminate less successful, forms of life. Marvelously, these processes reveal latent possibilities in life's basic structure, and propel a major evolutionary theme: the increasing proficiency of biological function. It remains to be seen whether the human form can survive the dynamic processes that brought it into existence. Yet the emergence of the ability to acquire knowledge from experience, to optimize behavior, to conceptualize, to distinguish "good" from "bad" behavior all hint at an evolutionary outcome that science is only beginning to understand.

Foreword by Simon Conway Morris

1. Time Travel
2. The Extraterrestrial Pre-Hadean
3. The Hadean Eon
4. The Archean Eon
5. The Proterozoic Eon
6. Phanerozoic Marine Life
7. Origin of Complex Terrestrial Ecosystems
8. Toward the Coal Age
9. Ascendancy of Life on Land
10. Bridging the Eras
11. The Natural History of Natural Selection
12. An Age of Giants
13. One Earth, Two Worlds
14. The Modern Earth
15. Synthesis
Epilogue: The Way of Life




Publié par
Date de parution 14 juillet 2009
Nombre de lectures 0
EAN13 9780253023919
Langue English
Poids de l'ouvrage 1 Mo

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2009 by Dale A. Russell Foreword 2009 by Simon Conway Morris All rights reserved
No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and recording, or by any information storage and retrieval system, without permission in writing from the publisher. The Association of American University Presses Resolution on Permissions constitutes the only exception to this prohibition.
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Russell, Dale A.
Islands in the cosmos : the evolution of life on land / Dale A. Russell ; foreword by Simon Conway Morris.
p. cm. - (Life of the past)
Includes bibliographical references and index.
ISBN 978-0-253-35273-6 (cloth : alk. paper)
1. Evolution (Biology) 2. Life-Origin. 3. Biotic communities. 4. Natural selection. 5. Paleoecology. 6. Paleontology-Mesozoic. I. Title.
QH366.2.R87 2009
1 2 3 4 5 14 13 12 11 10 09
For my wife, Janice, our families, and our children s families -my universe

Foreword by Simon Conway Morris
In his famous closing passage in On the Origin of Species Charles Darwin felt moved to write of how his theory might well explain the diversity and fecundity of life, but for him at least, it still invoked a sense of grandeur. Darwinism remains the air all evolutionary biologists breathe, but so too very many of us see in the history of life an almost epic quality: abysses of time, strange and outlandish denizens, unsolved mysteries, vanished worlds. Unsurprisingly this can lead to a tension. After all, in their day-to-day work paleontologists need to be like any other scientist: prosaic, hard-headed, and objective. But behind this, too often unsung, lies inspiration, imagination, and a sense of wonder. Two apparently separate worlds seem to lie ever further apart: the book of poetry lies unopened beside the electron microscope-but not always, and not necessarily. In this book they are reunited in a sweeping and thrilling overview of the ancient past.
The past is a foreign country; they do things differently there. L. P. Hartley s well-known words are strangely apposite to this book, as they provide a leitmotif for a journey not only toward irrevocably lost worlds, but ones that are discomfortably unfamiliar, brooding, almost alien. Eschewing the worthy but tired dioramas that used to bedeck the walls of natural history museums and that punctuate the books of instruction, ones populated with shuffling trilobites and puzzled-looking brontosaurs, Dale Russell flings open a series of windows into the worlds of the past. It is an open and generous invitation, unafraid of employing vivid imagery and lyrical phrases. Whether it be to visit the earliest Earth, where he describes emergent fields of semiconsolidated lava rubble . . . punctuated here and there by steaming springs exhaling sterility [and] far into the distance, squat volcanoes might barely be distinguished, projecting from a cold leaden sea along the limb of a blue-gray horizon, or what was in geologic time almost yesterday, where together we can stalk through a dinosaur graveyard. But now the usual trope of bleached bones and grinning skulls is turned into something more fertile, but also more macabre. Here each gigantic corpse serves as a biological oasis, teeming with life, but from which also wafts the unmistakable stench of mortal decay.
Again and again Dale Russell combines the familiar with the unexpected, and to good effect. Ancient scenes are dotted with plants and traversed by great rivers, while beyond the horizon lie vast deserts that dwarf the Sahara and immense mountains that overshadow the Himalayas. But there is always a haunting undertone, because while Dale Russell brings to life these dead communities, at the same time he repeatedly emphasizes their archaic nature. And this he does in two intriguingly different ways. First, even in some living biomes, we find more than echoes of those distant worlds, a departing drumroll of former monsters, a flutelike threnody of extinct organic arabesques. Second, and even more intriguing, is the overwhelming sense that these worlds were not only different, but disorientingly so. Here are realms of silence populated by expressionless eyes, where any memories would exist solely as the repositories of the charnel house, of dust and decay, embalmed in the smell of aromatic resins. For us humans they would have been lonely and infertile places, not locations to linger. Now, paradoxically, they are places to relish when viewed through the spectacles of scientific understanding and when molded by our unique gifts of imagination.
Yet however remote these worlds might be, they were a product of evolution, as of course are we. From our privileged perspective, we see them as pregnant with possibilities, a planet that slowly awakens as the first minds begin to stir. Here, surely, is a saga that is even today incompletely told: awareness flickers into existence and intelligences emerge, culminating in the incredible trajectory of human evolution. Dale Russell is surely correct when he proclaims that ours is a truly special time. And as I do, he insists that within the evolutionary tapestry, there are woven inevitabilities, and here perhaps we begin to depart from one part of the present-day neo-Darwinian consensus. This is because a central tenet of neo-Darwinism is its open-endedness and indeterminacy, famously captured by Stephen Jay Gould s insistence that were we to rerun the tape of life, then the outcome would be totally different. But this is flatly contradicted by what we know as evolutionary convergence-the manner in which unrelated organisms repeatedly arrive at the same solution. Perhaps this is best known by the striking similarity between our eye and that of an octopus; each evolved independently into what is very close to an optimal end point. This, along with innumerable other examples, is one line of evidence that evolution is actually reading an instruction manual to which we have by no means yet gained full access. Birds, for example, evolved from the dromaeosaurian dinosaurs, and we see the evidence before our eyes, in the form of Archaeopteryx; but much less well known is that two other lineages were going in much the same direction. Birds are inevitable, and so most likely are mammals-as well as, I would argue, are humans.
These inevitabilities not only provide a compass to the Darwinian adventure, but also in revealing directionalities allow us to reconsider the concept of evolutionary progress, not as an artifact of human wish fulfillment but as integral to this tapestry of life. And Dale Russell brilliantly captures this sense by a dramatic retelling of von Baer s hypothesis whereby the egg, say of a reptile, hurtles through its equivalent evolutionary history, achieving in a few months what it took billions of years for evolution to achieve. Yes, yes, nod the heads of some world-weary embryologists, that is what happens, but in a few deft strokes, Dale Russell has managed to reignite our sense of wonder: embryology, and indeed all life, is extraordinary, and we should never be embarrassed to proclaim its wonder.
Here we have a perspective that gives evolution a cosmic dimension, as the very small-such as the apparently mundane and humble egg-encounters the unimaginably enormous. In one of her visions, the medieval mystic, Julian of Norwich, famously saw the entire creation like a hazelnut held in the palm of a hand. And perhaps it is not entirely otiose to substitute in our mind s eye that symbol of fertility, the egg. In this telescoping of time and evolution, Dale Russell captures the drama of evolution. While he rightly insists on the continuity of evolution, so too he sees compelling evidence for the emergence of order, of increasingly complex ecosystems, of ever larger brains and the spreading of sentience. But Dale Russell is unembarrassed to write, and just as much I to read, At a basic level evolution is not an explanation; it is a mystery. Both of us are firm believers in evolution, but I know that we also share some decidedly unfashionable views. From whence, we ask, comes not only the mysterious beauty of evolution, but its potentialities and indeed the order of the world and the foundations of Reason? Is it not strange how life has navigated on a knife s edge of evolution, between the Scylla of unstructured chaos and the Charybdis of crystalline immobility? Well, no, in fact-not at all. Evolution is, of course, part of the physical world, but it is one that emerged by the finest of threads-as the Anthropic Principle demonstrates-and now houses minds that glimpse the transcendental. Seen from such a perspective, when I read Dale Russell s words, We tread on hallowed ground, I felt an answering call, a deep resonance. Near the beginning of this wonderful book, Dale Russell simply observes that the Laws written within the structure of the visible universe are not arbitrary. Indeed they are not, and as sentient products of evolution, we should not only salute the vanished worlds from which we emerged, but ponder what the future will hold for us all.
Simon Conway Morris
The Hubble space telescope was launched on April 24, 1990, and orbits Earth every 97 minutes at a distance of 600 km above the surface of our planet. Beyond light-disturbing effects of the atmosphere, the telescope sees almost forever. It penetrates distances in units calibrated by the speed of light: a few minutes to the nearest planets, and a few light-years to the nearest stars. The nearest galaxies are 2,500,000 light-years away, and the farthest well over 10,000,000,000 light-years. By assessing the velocities of the most distant sources of light, the age of the event in which the visible universe originated (big bang) can be estimated. With this number, and with radiometric ages from the oldest meteoritic material in the solar system, contemporary technology informs us that Earth is almost precisely one-third as old as the cosmos. Light that originated in a nearby galaxy when the dinosaurs were alive now enters human eyes on starry nights (Voit 2000; McMahon 2006). We can actually see, albeit in highly attenuated form, into the past.
Human behavior is profoundly influenced by the perception of patterns inferred from observation. Once conceived, these patterns may be invalidated or vindicated by additional observations. A dedicated search for pattern is fundamental to the achievements of the scientific method. In theory, all patterns are considered as provisional and are reassessed by means of measurement. Observations of external realities may be quantified, but even basic internal realities are difficult to measure. Each of us is born into the world with a sense of an interior life that is unique, which no other human can fully comprehend. We comprehend with difficulty that the universe we live in is overwhelming in most of its dimensions. Our presence within it floats in a sea of imponderables. If an answer to Why? lies beyond the boundaries of science, must it also lie beyond the limits of inference? We have come to know that just as our bodies change through life, so life on Earth itself has changed with the passing of eons. Three scholars representative of different schools of thought and living at different times have contemplated the phenomenon of life on Earth.
An ancient philosopher inferred the existence of a universal mind that imparts order to all that exists:
mind is infinite and self-powerful and mixed with nothing, but it exists alone by itself. . . . it has all knowledge in regard to everything and the greatest power; over all that has life, both greater and less, mind rules. And whatever things were to be, and whatever things were, as many as are now, and whatever things shall be, all these mind arranged in order. (Anaxagoras, ~450 BC )
A nineteenth-century naturalist, inferring the existence of deep time, deduced that even in the absence of physical change, interactions between living organisms would constantly force functionality in the direction of increasing proficiency:
If under a nearly similar climate, the eocene inhabitants of one quarter of the world were put into competition with the existing inhabitants of the same or some other quarter, the eocene fauna or flora would certainly be beaten and exterminated; as would a secondary fauna by an eocene and a palaeozoic fauna by a secondary fauna. (Darwin 1859: 335)
Here, the terms eocene, secondary , and paleozoic respectively refer to about 50 million years ago, about 150 million years ago, and about 350 million years ago.
A contemporary theologian combines the insights of the ancient philosopher and nineteenth-century naturalist to discern a continuing creativity in a Mind whose domain is universal:
The great projects of the living creation point to a creating Reason and show us a creating Intelligence, and they do so more luminously and radiantly today than ever before. (Ratzinger 1995: 56)
The budding faculty of reason in our children typically encounters the concept of a long-vanished group of animals, the dinosaurs, with the acquisition of speech. A child naturally perceives everything as large and dinosaurs as surpassingly so. The multiple implications of this staggering news are often beyond parental capacities to explain in a satisfactory manner. However, an appetite for knowledge can be nourished further through the wonderful process of learning to read. Fed by actual and mental images in books, it becomes thrilling to imagine great, dumb reptilelike animals moving through open conifer forests, leaving their droppings and bones to litter brushy thickets . . . on a family farm on the slopes of the Wallowa Mountains of eastern Oregon! Clear night skies were illuminated by a brilliant vault of stars traversed by the distant but titanic edge of the Milky Way galaxy. The stark reality of vast time and space collided with an equally staggering perception of internal subadolescent finitude.
The affectionate support and example of an older brother provided the impetus for college entry, with the goal of becoming a paleontologist. Once, a graduate student visited the Jesuit seminary at Saint Andrews on the Hudson, New York. There he contemplated the grave of Pierre Teilhard de Chardin, a priest who was also a paleontologist, whose writings suggest that a way is open toward the unification of the acquired wisdom of humankind.
Much later, as a young professional, the rigors of endless Canadian winters contrasted strikingly with petrified evidence that environments of Alberta dinosaurs resembled those now to be found on Carolina coastal plains in the southeastern United States. Such images, and the possibility that a planetwide collapse of ecosystems accompanied the disappearance of the dinosaurs, underscored the allure of an ancient world in which dinosaurs originated, evolved, and ultimately became extinct. Travels to remote areas of the globe provided unanticipated images, sounds, and smells that seemed to recall those once commonplace in environments inhabited by dinosaurs. If philosophers observe that truth is unitary, how are such impressions to coalesce into a coherent whole?
The following essay was inspired by a search for regularities in the history of life, specifically life on land. Much more was learned from the printed works of others than from personal experience, but the latter breathed life into the former. It is somehow comforting to imagine that a search for pattern in existence moved artists who so long ago beautifully depicted dynamic forms of ice-age animals on cave walls. Much later, with the invention of writing, the search was coded symbolically in ancient inscriptions that silently speak to us still.
I am indebted to outstanding teachers who introduced me to the excitement of exploring the past, including Donald E. Russell, J. Arnold Shotwell, Donald E. Savage, Malcolm C. McKenna, Theodosius Dobzhansky, and Edwin H. Colbert. Memories of their virtuosity in classroom or field are cherished. In their turn, students at North Carolina State University who survived 8 years of exposure to someone learning how to teach are gratefully acknowledged for their survival abilities and inspiring the process of information gathering that led to the compilation of this essay. A highly cross-disciplinary group of scholars too numerous to list cordially and generously shared their knowledge-with a previously unknown colleague-through e-mails and telephone calls. As is abundantly clear, a huge intellectual debt is owed to authors of the works cited in the references.
The support provided by colleagues in academia remains among my fondest memories. Betsy Bennett, in her capacity as director of the North Carolina Museum of Natural Sciences, provided a superb institutional and intellectual environment in which to reflect and write, as well as a trust that remained intact through five long years. Janet Edgerton graciously extended the hospitality and resources of the museum library, energetically dispatching a steady stream of interlibrary loans and feeding an unending supply of paper to overheated library printers. As a retired faculty member, the doors of the Library of North Carolina State University always remained open. The project would have been unthinkable without access to the library s outstanding electronic reserves and the generous assistance of the head of the Natural Resources Branch, Karen Ciccone. A lifelong colleague and leader in research on dinosaurian ecology, James Farlow, with great charity, judged that an exceedingly rough preliminary manuscript might ultimately merit publication. In his capacity as editor of the Life of the Past series of Indiana University Press, his generous support and many positive and valuable suggestions are greatly appreciated. Robert Sloan cordially provided guidance through the publication process, and Karen Hellekson tactfully smoothed text into acceptable English.
Various chapters have benefited from collegial help. Webster Cash and Sidney Van den Bergh generously supplied counsel for astrophysical content, and David Fastovsky penned an encouraging assessment of the entire work from a paleontological perspective. A stimulating exchange of ideas on Carolina vegetation was shared with Jesse Perry, and on fitness and evolution with Lars Witting. Sympathetic grooming was invested by family members, including my son, Frank (classics), and brother, Don, and his wife, Denise Sigogneau Russell (mammalian prehistory), and my wife, Janice, who carefully proofread the manuscript. They are nevertheless not to be held responsible for the many remaining shortcomings in the text. I am indebted to our local paleontological interest group, including Paul Brinkman, Roy Campbell, Julia Clarke, Michael Dunn, Vincent Schneider, Mary Schweitzer, and Elisabeth Wheeler for their most welcome support, and for bringing late-breaking paleontological news to my attention. A formative and long-sustained fellowship in paleontology with an international group of colleagues is gratefully acknowledged. Among them, Don Brinkman, Stephen Cumbaa, Philip Currie, David Eberth, Dick Harington and George Jeletzky (Canada), David Jarzen and John McIntosh (United States), Makoto Manabe (Japan), and Philippe Taquet (France) may prominently be counted. Bonnie Livingstone (Canada), who edited An Odyssey in Time , and Simon Conway Morris (United Kingdom) graciously provided encouragement as the present essay was being written.
Several publications are listed at the conclusion of each chapter to serve as introductions to the chapter s related technical literature. In selecting these suggestions For Further Reading, an attempt has been made to maximize the diversity of authors and topics. For every brief entry, a full citation is provided in the References section that concludes the volume.
Figures and photographs were provided through the courtesy of Michael Benton, the Canadian Museum of Nature, David Dilcher, Melissa Dowland, Michael Dunn, Russell Hawley, Harold Heatwole, the National Aeronautical and Space Administration, David Jarzen, Michael Lee, William Miller, J r me Munzinger, Gregory Paul, Chris Scotese, Stephen Sharnoff, Marius van der Merwe, and Grahame Webb. Paleogeographic maps for the Ordovician, Devonian, Triassic, and Cretaceous are by Chris Scotese, PALEOMAP Project ( ).
Remaining figures and photographs are by the author. People who provided much-appreciated help in locating and processing photographic materials include Aaron Bauer, Herman Berkhoff, Erwin Brodo, John Cooper, Richard Day, Jane Eckenrode, Harold Heatwole, David Herring, Richard Martin, Gordon McPherson, and Rowland Shelley. Special thanks are due to Lisa Yow for painstakingly bending software to image, and processing the photographs that accompany the essay. Stacks of consulted references were ably collated and page proofs scanned by Janet Edgerton and Virginia Creekman.
The wholehearted support and lifelong affection of the historian who is my wife, Janice Alberti Russell, has made this work possible.


Fig 1.1. A group of muskoxen ( Ovibos moschatus ) on Ellesmere Island, province of Nunavut, arctic Canada, inhabits a simple biotic environment that is seasonally subjected to frigid temperatures. The megaherbivores tower above the stunted herbaceous vegetation on which they feed. For further discussion, see p. 18 and compare with fig. 1.2 on p. 20 .
If the dimension of time is difficult to comprehend, introducing the history of life with the ordering of time may seem excessively burdensome. Yet time is intimately embedded in the nature of matter, with vastly differing manifestations on quantum and cosmic scales. The evolution of scientific thought has long been analytic in nature and tends to be increasingly focused on ever more minute scales. However, attention is also being directed toward broad syntheses of physical-biological theory, addressing even the possible influence of complex structures existing today on simpler structures in the remote past through quantum effects (chapter 15; Davies 2007). If one cannot fully comprehend the development of scientific thought, one can at least admire the courage and nobility of spirit that animate it.
The progression of events outlined in this work will be sketched at levels somewhere between the quantum and the cosmic. This is of course in deference to the author s limitations, whose imagination is sufficiently taxed by the more ordinary dimensions of space and time. Consideration will be given to geography, climates, and landscapes of the past. Living things that inhabited those landscapes will be perceived as whole organisms, interacting with their habitats and each other as parts of living communities, much in the way that a modern ecologist regards the denizens of a game reserve. How they changed in size, appearance, and general behavior will be traced through geologic time. For example, dinosaurs belonged to the era of middle life. To understand them as living creatures, they must be viewed within their times, and they must be distinguished from the life that preceded and followed them. A story thereby emerges that, one may dare to suggest, is more fascinating than even dinosaurs themselves. The tale is perhaps a rather simple one, but as will be seen, it is drawn from the considerations of many fine minds whose works are cited in the text.
We remember the past and ponder the future. However, time is encountered as an ever-present stream of consciousness in which are embedded fleeting memories of the past and an indistinct web of future possibilities. Future events are difficult to arrange in sequences; elapsed time is conventionally but paradoxically measured from the elapsing present, and furthermore is reversed. Thus, it is understood at this writing that humans first walked on the Moon about 40 years ago. By extension, it also seems intuitively reasonable to tabulate time as reversed in formal geologic timescales, wherein dinosaurs are described as becoming extinct 65 million years ago. Evolution is possibly best considered according to the sequence in which it occurred: from the past toward the present (cf. Witting 2008). Here an additional conceptual difficulty arises from the vastness of both past and future time. A year, a generation, and a lifetime are durations that are broadly understandable, but longer intervals are far beyond human comprehension. It is not easy to grasp the notion that our species may have been present on Earth for at least 6,500 generations (at 20 years per generation) or 2,600 life spans (at 50 years per life span).
In an example of a calendar of events in human history and prehistory (see the Microsoft Encarta Encyclopedia , 2002), dates are conventionally rearranged in readily understandable units (centuries) in order of decreasing antiquity from the present. There, the domestication of dogs is listed as 14,000 years, or 140 centuries, ago. Perhaps more realistically, dogs could be said to have been domesticated 116,000 years (1,160 centuries) after the appearance of their human domesticators. This of course is the natural direction in which all living things experience time.
In subsequent chapters, lists of events are ordered according to two timescales, each calibrated in millions of years. A natural timescale, representing elapsing time, is presented in italics and increases from top to bottom. It has the virtue of clearly portraying the duration of intervals of geologic time and facilitating comparisons with the pace of events during other intervals of geologic time. A reversed timescale, representing geologic age, is presented in normal type and increases from bottom to top. It facilitates comparisons with standard geologic timescales.
The compilation cited above can be arranged in such a fashion, supplanted with citations for a few additional events:
0 centuries (1,300 centuries, or 130,000 years ago): appearance of skeletal remains of Homo sapiens in the geologic record (or 195,000 years ago; McDougall et al. 2005)
100 centuries (1,200 centuries ago): burial of the dead documented
550 centuries (750 centuries ago): beginning of last glacial interval
870 centuries (430 centuries ago): iron extracted for dye
980 centuries (320 centuries ago): sophisticated cave paintings
1,000 centuries (300 centuries ago): appearance of counting notation
1,020 centuries (280 centuries ago): oldest known dwellings
1,030 centuries (270 centuries ago): oldest pottery
1,160 centuries (140 centuries ago): dogs domesticated
1,187 centuries (113 centuries ago): domestication of figs (Kislev et al. 2006)
1,197 centuries (103 centuries ago): domestication of cereals, invention of agriculture (Kislev et al. 2006)
1,190 centuries (110 centuries ago): end of ice age, sea levels rise, extinction of large ice-age mammals, sheep domesticated
1,235 centuries (65 centuries ago): beginning of Bronze Age, invention of plowing
1,240 centuries (60 centuries ago): horses domesticated
1,247 centuries (53 centuries ago): invention of writing
1,250 centuries (50 centuries ago): germination of the oldest living trees (Lewington and Parker 1999)
1,268 centuries (32 centuries ago): sacking of Troy (radiocarbon dated)
1,275 centuries (25 centuries ago): beginning of recorded history (Herodotus, fifth century BC )
1,280 centuries (20 centuries ago): birth of Christ, the conventional beginning of the modern Western calendar
Several inferences may be drawn from this compilation, some of which are particularly striking. If dinosaurs disappeared 655,100 centuries ago (Hicks et al. 2002), our species did not appear until 653,800 centuries later, a staggering span of time. The invention of agriculture approximately coincides with a global warming and a major extinction of large ice-age mammals. Did the domestication of plants and animals provide a food base from which humans could hunt large mammals to extinction without running the risk of starving to death themselves? Written history (or writing) spans only 4 percent of the total interval of our already brief existence as a species! The earliest narratives are characterized not so much by their technological content (e.g., plans for the construction of pyramids) as by a search for wisdom and an explanation for the origin of the world. The compilation also shows a relatively large number of significant events in the more recent centuries, suggesting that rates of cultural change may have become more rapid with time.
At best, oral traditions preserve an exceedingly hazy memory of large ice-age animals that became extinct about 110 centuries ago, although long after their extinction, large bones buried in the earth were recognized as remains of once-living, giant creatures (Mayor 2000, 2005). Malagasy people entered Madagascar about 24 centuries ago, precipitating the onset of megafaunal extinctions that lasted nearly a millennium. Reports of late-surviving large mammals and giant ground birds are vague, if only a few hundred years old (Burney et al. 2004). Maori legends refer to huge birds, moa, that inhabited New Zealand before their extermination by Maori colonizers seven centuries previously (Holdaway and Jacomb 2000; Wilmshurst et al. 2008). Evidently oral traditions of important, sudden events (e.g., those that took place within a human life span) might endure for several hundred years. That many oral epics must have been spun from interactions between ice-age animals and humans is suggested by a mythic quality imparted to European cave paintings of 320 centuries ago. These sagas are now forgotten. Writing and a culture that is aware of the importance of history are necessary to preserve human memories in detail for longer intervals. Thousands of years in the future, scholars may be very interested in commonplace diaries being written today.
There is no evidence that changes in shape and behavior accompanying the domestication of plants and animals were originally understood as an indication that plant and animal species could be modified artificially. Aristotle (384-322 BC ) observed that mammals reproduced and matured according to a pattern. However, the species to which they belonged did not change significantly between generations. Since then, it has been discovered that mammalian species typically endure for much longer than the few tens of centuries of human history. Aristotle cannot be faulted for concluding that species were essentially changeless; the brevity of the time span over which his observations extended gave him little reason to think otherwise.
Ancient scholars tended to measure long intervals of time in the duration of the reigns of rulers and ruling families, calibrated in years. Examples include dynasties of Egypt and China, genealogies in the Bible, and sequences of Roman emperors. On the basis of biblical genealogies, James Ussher, in his Annals of the World , published between 1650 and 1654, calculated that the first humans appeared 60 centuries ago (4004 BC ). This date approximately coincides with the invention of writing and the recording of preexisting oral traditions (Plaut et al. 1981). Large bones were known long before Aristotle s time, giving rise to heroic myths of former giants (Mayor 2000). In 1669 Nicholas Steno recognized that accumulations of sediment containing fossils represented very long intervals of past time (Cutler 2003).
By two centuries ago it was clearly apparent from sequences of fossils preserved in sedimentary rocks that life must have been present on Earth long before the appearance of humans. The human proclivity to invent names for extinct, newly discovered, or mythical animals, as well as for periods of ancient time, was reinvigorated during the nineteenth century: entering the English lexicon were mastodon in 1811, pterodactyl in 1830, Jurassic in 1831, Devonian in 1837, Pleistocene in 1839, dinosaur in 1841, prehistoric in 1851, and brontosaur in 1892 (see the Oxford English Dictionary , 1989). Expanding lexicons have kept pace with increasingly detailed scrutiny of the record of life and the exploration of deep space through the twentieth century. Yet the staggering extent of geologic time became appreciated at a slower pace. About a century ago it was discovered that radioactive elements gradually fractionate into simpler but stable daughter elements. The ratio of radioactive to daughter nuclei of a given element, when locked within a crystal, indicates how much time elapsed since the crystal was formed. Such evidence revealed that the Earth is thousands of millions of years old. Ratios preserved in crystals formed in rocks at different times in the past provide a framework for prehistory in which events in the planetary past can be situated.
As an example, what is popularly known as the age of reptiles or the dinosaurian era, and what is technically known as the Mesozoic Era, took place between approximately 251.4 and 65.51 million years ago, over an interval of about 186 million years (Smith and Ward 2001; Hicks et al. 2002). ( Million years ago and million years are henceforth abbreviated throughout the text as myr and a thousand years as kyr ; an approximate number is preceded by ~. ) Artists have attempted to reconstruct the appearance of strange creatures that lived during the Mesozoic Era by combining image fragments of modern organisms (eyes, skin patterns, claws) with fossil skeletons recovered from sediments deposited during that long-lost time. Over the past century and a half, restorations have become increasingly realistic (Lanzendorf et al. 2000; Johnson and Raynolds 2002; Rudwick 2002). Similarly, by blending fictional accounts of the experiences of modern explorers with conceptions of dinosaur-dominated ecosystems, writers have acculturated prehistory in increasingly vivid time-adventure classics (Verne 1864; Doyle 1912; Crichton 1990).
The body of available information about the history of the Earth and the cosmos continues to expand. High-technology instruments, such as scanning electron microscopes, radio telescopes, mass spectrometers, and x-ray interferometers, have enormously extended our powers of observation. Explanations of the functions of these marvelous instruments could fill encyclopedias, and a rapid increase in knowledge is nearly certain to continue into the future.
Over the last millennium, scientific methodology has increasingly replaced other routes to knowledge. In our day, evidence of the success of Science is everywhere. Public health and standards of living are continuously improved through applications of scientific methodology. Scientific research has probed the limits of the universe. It informs us that many stars are either too short-lived or too unstable for life to evolve in planetary systems orbiting them. Even within the solar system, most planetary surfaces are almost certainly inhospitable to anything but the simplest forms of microbial life. On the relatively idyllic surface of our own planet, evolution is often perceived as lacking orientation and dominated by erratic changes in physical parameters such as temperature, rainfall, and seasonality that are difficult to predict in detail. The evolution of humanlike organisms is considered to have been far from inevitable on Earth, and even less so in most environments elsewhere in the universe. In practice, the scientific method restricts itself to the material universe, eschewing speculation on nonmaterial questions. Justly impressed by the spectacular and continuing progress of Science, some or many have come to accept a material view of life as realistic and complete. Although Science rests on solid foundations, a perspective that is limited to the material world is considered by some or many to be incomplete.
The present essay resulted from an interest in life as life is successfully lived. The interest has been accompanied by a parallel sampling of various areas of knowledge through a lifetime that has now entered its seventh decade. Necessarily shallow and idiosyncratic, the resulting worldview was greatly shaped by controversies in the domain of the geosciences. The debates have often been passionate, but over time, they have usually given way to a larger, more comprehensive consensus. Many authors have recounted the fascinating flow of concepts that have emerged from scientific disputes. Excellent source references include Adams (1954), Eiseley (1958), Gould (2002) and Rudwick (2005). A few of the more interesting questions debated during the latter half of the twentieth century include:
Are the positions of the continents fixed or do they change (drift) through time?
Were dinosaurs cold- or warm-blooded?
Did dinosaurs become extinct gradually from earth-limited causes, or catastrophically from an extraterrestrial agent?
Will the search for extraterrestrial intelligence (SETI) succeed, or is it destined to be futile?
Did birds descend from primitive reptiles or from dinosaurs?
Is classification better served by gross anatomical comparisons (Linnaean) or analyses of distribution patterns of discrete characters (phylogenetic)?
Is evolution random (contingent) or directional (teleological)?
The positive results of the forgoing controversies pervade the present essay. Its major thesis, however, is best placed within the last category, addressing contingency versus directionality in evolution. Needless to say, evolution is not a topic in which controversy has been wholly replaced by consensus. Dedicated champions of contingency in evolution include Provine (1988) and Gould (2002), and for directional evolution include Morowitz (2002) and Conway Morris (2003). A similarly dedicated advocate of science in place of religion is Dawkins (2006), whereas others (Di Noia et al. 2004) humbly and gratefully place science in support of religion. How, then, is the present essay to be regarded? It was written under the premise that despite prevailing uncertainties in various areas of knowledge, truth is unitary, and within their limitations, humans can recognize truth. Acknowledgments and cited references bear witness to a tremendous intellectual debt owed to numerous colleagues and educational institutions. This essay is therefore offered in a nondiscursive spirit to readers as a means of repaying a debt, albeit inadequately, to colleagues and to society.
Before embarking on an excursion into the depths of time, it would be well to consider some concepts that will be useful in guiding our exploration of the past. Fossil hunters orient themselves in the field by examining pieces of fossil bone for texture and color, and by studying the attributes of strata in which the fragments occur. They thus compile a store of search images that will guide them toward significant discoveries. The history of terrestrial life implies an intellectual voyage into the remote past and an encounter with long-term scales of change that far surpass normal experience. The dozen concepts that follow are provided to assist in the reading of ancient records.
Physical Change
As has been noted, the age of Earth is one-third the age of the universe. Do the materials composing the universe as we know it change through time? On a subatomic level, their history seems to be relatively straightforward (see chapter 2 ). The basic building blocks, be they quanta, subatomic particles, or atoms, show little evidence of change, although particle decay across enormous spans of time has been discussed (Cho 2006b; Reinhold et al. 2006). The basic stuff of the visible universe came into being at the time of its origin.
Biologic Change
The record in sediments dated by radioactive nuclei indicates that before about a thousand million (or a billion) years ago, neither plants nor animals were present on land. Continental surfaces, although crossed by rivers and streams, were barren wastelands. Simple crusts of lichens and bacteria darkened surfaces where moisture was sufficiently abundant. Aristotle would have considered these biological entities to be only marginally alive. A thousand million years later, within a modern tropical rain forest, multitudes of widely differing species vigorously and noisily interact with each other, bound together in intricate webs of mutual dependence. These organisms include some of the most exquisitely bioengineered life forms known, and would have been considered by Aristotle to be highly animated. Accepting that ancient bacterial mats and denizens of modern rain forests represent two extremes in a vast continuum of time, it is reasonable to infer that, unlike the basic components of the physical universe, the living components (e.g., species) of biotic systems do change. Indeed, they have changed enormously through an interval much shorter than the age of the visible universe.
Patterns of Biological Change (Trends)
Organisms are constructed in an organized manner, and their organization reflects realities imposed on them by their surroundings. Internally, protein (molecular) change through time is oriented toward functionality, and therefore in a reasonable manner (Denton et al. 2002; Dokholyan et al. 2002; Dobson 2004; Weinreich 2006). As has been emphasized (Conway Morris 2003), most of the anatomical configurations that could be imagined (and particularly fictional portrayals of extraterrestrial life) are not found in the record of life on Earth because they would not function well in nature. The requirement for functionality implies that patterns of change must also be constrained by functionality.
Many have noted that careful analyses of the simplest building blocks of matter do not reveal all of the potential characteristics that they can express when placed within complex structures (Corning 2002; Morowitz 2002; Davies 2007). The apparent simplicity of atoms and their subatomic components appears discordant with the complex structure of protein molecules, living cells, and central nervous systems. This quality of matter has been described in terms of potentiality in philosophical circles, or in a specifically evolutionary context as emergent (Morowitz 2002) and inherent (Conway Morris 2003). Natural selection, in combination with long intervals of time, discloses otherwise latent but hidden attributes of matter. Some implications of emergence are discussed in chapter 15 .
Terrestrial Life
Continental surfaces are exposed to climatic extremes and altered by continental (plate tectonic) movements. In contrast, oceanic environments are generally more stable, more uniform, and more global. The fossil record of terrestrial life is less complete and less well documented than that of marine life. It has been reviewed by Behrensmeyer et al. (1992) in a thorough and well-documented summation of knowledge available at the time of its publication. Stimulated by work in marine paleoecology, the terrestrial record has since been more closely examined for evidence of long-term patterns of change (Erwin and Wing 2000; see also references in following chapters). Over the most recent 250 myr the direction of biotic dispersal has generally been from the land to the oceans, rather than in the reverse direction. Terrestrial environments thus provide an alternative proving ground for evolution, and the productivity and diversity of terrestrial biotic systems are at least comparable to those of their marine counterparts.
Biologic Systems (Ecosystems and Biomes)
It is commonly observed that living systems are not uniformly distributed across land surfaces, and that certain associations of organisms tend to occur and interact together within certain physical environments. A dynamic association between particular organisms and a particular physical environment is termed an ecosystem , examples of which include a forest, a meadow, and a bog. On a much broader, often subcontinental or intercontinental scale, similar associations of trees, open meadows, and bogs may occur in a repeating pattern across a vast region, such as subarctic taiga, temperate woodlands, and tropical savannas. These associations represent a level of complexity of a larger scale and are termed biomes (Whittaker 1975). Because of the incompleteness of the fossil record, it is not always easy to distinguish an ecosystem from a biome, and following Behrensmeyer et al. (1992), the term ecosystem is generally adopted here in a broader sense to include biomes.
Evolutionary Success
Parents promote their children s health, Olympians train to maximize their athletic performance, and students study to enhance their academic standing. The desired results are understood as contributing to success in life. Organisms interact with their surroundings, the results of which determine which individuals contribute more abundantly to the ensuing generation. The particular case of reproductive success has been recognized as a relatively accessible and quantifiable measure of success in general and is termed Darwinian fitness (see Gell-Mann 1994; Rupert 2003; Le Galliard et al. 2004). However, it has been observed (Corning 2002: 27) that a particular trait may affect differential reproductive success, but it is still the whole organism that must survive and reproduce. Here, evolutionary success is considered in the broader, anatomical-behavioral sense.
Natural Selection
The surroundings in which organisms are immersed induce natural selection by determining the extent to which individual anatomical and behavioral configurations contribute to following generations. Surroundings can be separated into two broad categories, the biologic environment and the physical environment. As will be shown, the effects of natural selection resulting from interactions between organisms and each of these two environmental categories differ over long periods of time. That the physical and biologic environments produce different evolutionary consequences is a central theme in this essay.
Fitness is considered here as being the result of the interactions of organisms with their biologic environment. Fitness promotes survival, either as individuals or as species. It was originally correlated with competitive ability and was seen to increase through geologic time (Darwin 1859: 335; see this volume s preface). Organismal interactions with other organisms within a biological system may be seen as self-stimulating, whether they are the result of competition (Vermeij 1987) or cooperation (Bascompte et al. 2006; Thompson 2006). Mathematically speaking, self-stimulating (autocatalytic) processes tend to be accelerative. Thus, long-term interactions between organisms may be expected to produce a trend toward accelerating improvement in overall fitness with the passage of time.
In both nineteenth-century and modern usage, adaptation refers to changes resulting from interactions with the biophysical environment as a whole (e.g., Darwin 1859). However, the meaning of adaptation is here restricted to the consequences of interactions between organisms and factors in their physical environment, thereby in principle excluding the results of organism-to-organism interactions. For example, physical factors (temperature and rainfall) approximately define the extent of biomes (cf. Whittaker 1975, figure 4.10; Woodward 2003).
In the geologic past, the most extreme changes in physical environments (i.e., those generated by impacts of comets or asteroids, or by continent-wide glaciations) coincided with severe reductions in biodiversity. Such stresses, if sustained, could suppress the evolution of terrestrial life altogether. Conversely, benign physical conditions (i.e., those associated with tropical rain forests, including little or no seasonality, warm and stable temperatures, and abundant rainfall throughout the year) typically support a diverse and highly interactive (self-stimulating) biota.
Living Equivalent Ecosystems
Some biological systems give the impression of being primeval, or sheltering organisms that are regarded as living fossils. Examples include bacterial mounds near the edge of the Indian Ocean in southwestern Australia, hot springs rimmed with multicolored, bacterially stained precipitates in New Zealand, bald cypress swamps in the southeastern United States, and ecosystems on large islands in the Indian and Pacific oceans (Madagascar, New Zealand, and New Caledonia). These ecosystems remain alive today because they occur in extreme (e.g., hypersaline, scalding) or isolated (insular) environments, and they would be menaced by invasive organisms if their environments became more optimal (e.g., less saline, cooler, in contact with continental biota). Living equivalent ecosystems are conceptually useful because they illustrate living low-fitness biological systems that can be compared with more widespread living high-fitness biological systems. They reflect the evolutionarily inhibiting effects of extreme environments. It should be noted that a similar phrase, analogue ecosystem, has been used in the context of environmental history and restoration, in planetology, as well as in nonanalogue ecosystems in reference to aberrantly appearing ice-age botanical associations. Although living equivalent ecosystem is more cumbersome, it is also less ambiguous when applied in a paleogeographical context.
Several living equivalent ecosystems are depicted in the color illustrations throughout the text, approximately arranged in order from the past to the present. Their purpose is didactic. Because the subjects of the photographs are living, they communicate a sense of reality and of accessing the four dimensions of space and time. We examine them for what they are, rather than searching for imperfections that might identify the reconstructions as the work of man. They do not separate us from the core experience with numbers, technical vocabularies, or even the labor of digging for fossils. The color illustrations will instead draw us toward the actuality of the past and invite contemplation of once-living worlds.
There are many areas on our planet in which the harshness of existence has protected surviving examples of life as it once was. How accurately do these living fossil landscapes reflect the past, and in what way might they differ from what was ordinary in the past? It does seem that within the almost Proterozoic of deserts, tundra and ice, one can feel crushingly alone. Companionship is limited to the sound of wind and the pulse of one s heart. There is no quasi-sentient creature with which to communicate. The contrast with what is normal forces a sense of anomaly in the human presence. The present transforms itself into a pseudo-past, calling to mind a long-vanished reality in which the presence of humanity would be alien.
The Balance of Nature
It is commonplace to speak of a balance in nature, as well as a need not to disturb it. Short-term physical disturbances that are restricted in area (fires, droughts, hurricanes, volcanic eruptions) temporarily disrupt this balance, but over a period of one to several centuries, the previous balance is usually restored by a regrowth of a mature (climax) community (Whittaker 1975). The temporary disturbance of the balance followed by its restoration nevertheless indicates that component organisms in the replacing series belong to slightly differing levels of fitness. Importing organisms to island ecosystems upsets a balance in fitness in that the survival of low-fitness island organisms is menaced by continental organisms of higher fitness. Similarly, global warming (or other stressing influences, such as aridification) carries with it the implication that suboptimal temperate biomes may be replaced regionally by subtropical biomes containing organisms of higher fitness levels. Global cooling (or other influences, such as increased seasonality) might have a similar effect on subtropical biomes. The balance of nature is dynamic.
The preceding 12 conceptual tools are not arbitrary definitions. They facilitate the recognition of different effects of natural selection under different environmental conditions and at different intervals in the Earth s history. In the sections that immediately follow, consequences of natural selection in physical and biological environments are reviewed in more detail. These considerations are in turn followed by a sketch of long-term trends in the evolution of marine life, a description of the biological effects of a well-established modern physical gradient, and an imaginary demonstration of how natural selection might have altered biological systems over long intervals of geologic time.
On timescales of thousands of years, many episodes of evolutionary change have generally been interpreted as responses to change in physical environmental conditions. It is herewith posited that over timescales of hundreds of millions of years, accelerating increases in biodiversity, activity levels, and behavioral complexity have also taken place. The accelerative nature of these trends implies they are self-generating (autocatalytic) and therefore biologically driven. Reactions of life to physical stresses are typically not accelerative. Hence, distinctive autocatalytic response patterns usually indicate an organism-to-organism forcing (selective pressure) in a struggle for existence. (In an evolutionary context, forcing is the result of selective pressures generated by interactions of organisms with their environment, whether it be physical or biologic; in this case, the pressures are generated by interactions between organisms and their biologic environment.)
As noted above, organismal change in general is here assumed to be driven by natural selection. In general usage, this change is considered as adaptive or as conferring fitness, without regard to whether it is induced by the physical or biological stresses. A compensatory terminology could be devised so that the results of the two selective processes would be distinguished by word symbols or word combinations (e.g., type A fitness or physical fitness), but this solution seems unnecessarily cumbersome. Therefore, throughout the text, changes induced on organisms through physical forcing have been termed adaptive , and those induced on organisms through biological forcing are taken to confer fitness .
Adaptations include all those attributes which enable an organism to survive in a given physical environment. These include short- or long-term changes in response to such factors as temperature, water availability, salinity, high-energy radiation (including illumination), inorganic toxins, and gas pressure. Because surface conditions on our planet have tended to vary within a range compatible with the existence of water oceans, trends in adaptation would not be expected to show long-term patterns of acceleration.
Conversely, interorganismal competition is self-generated, or autocatalytic, and capable of producing accelerating change, just as do compounding interest or runaway arms races (cf. Vermeij 1987; Witting 2008). Fitness includes all those attributes that enable an organism to survive in competition with other organisms. These include such factors as growth rates, kinetic efficiency, reaction times, foraging and digestive efficiency, reproductive efficiency, behavioral flexibility, and ability to establish mutually symbiotic and commensal relationships. A distinction drawn between the terms adaptation and fitness is required by the thesis presented here.
Adaptation and fitness are each propelled by an amalgam of selective stresses that contribute, respectively, to adaptive and competitive success. Both are abstract concepts, the operation of which can be illustrated by concrete (measurable) examples. Thus, in plants, adaptation through natural selection may promote increased leaf porosity in response to declining levels of carbon dioxide; in insects, it may induce the formation of pupae to deepening seasonality; or in mammals, it may result in thicker hair that better retains body heat in response to cooling climates. In parallel fashion, in plants, competitive fitness, through natural selection, may be augmented by larger, more nutritious seeds that foster more rapid early growth; in insects, by increasingly erratic (evasive) flight; or in mammals, by an enhanced ability to learn. Adaptation and fitness are considered to be basic themes in the history of life. Both are present to a greater or lesser extent in every organism.
The phenomena of adaptation and fitness characterize different intervals of geologic time. Bacteria are extremely adaptive and have colonized scalding hot springs, hyperarid deserts, frigid oceanic troughs, and deep mines. Thousands of millions of years ago they constituted the most complex life forms on Earth. Conversely, multicellular organisms have been abundant for only a few hundred million years, but they now constitute over half of the present living biomass on Earth (Horner-Devine et al. 2004). They occur in greatest abundance in the fraction of terrestrial environments where physical conditions are particularly benign (e.g., tropical rain forests). Their accelerating rise to prominence reflects rapidly increasing levels of competitive fitness . Bacteria and humans are often taken to define the limits of life. Their distributions in space and time suggest these endpoints appear to be linked in interesting ways.
It is important to reiterate that adaptation and fitness are used interchangeably in contemporary evolutionary usage (e.g., Ellegren and Sheldon 2008), and to stress that the presence of a general accelerating trend in the evolution of life has not been widely acknowledged in current evolutionary research. There are nevertheless indications that the evolutionary framework adopted here is not entirely novel. A distinction between the effects of physical and biological selection is present in the climate-diversity hypothesis, wherein harsh environments are seen to host a few organisms that are capable of resisting extremes of physical stress. Both biodiversity and biotic interaction are seen to increase as physical conditions become more benign (Sanders 1968; Johnson 1970). However, in proposing this hypothesis, the particular agents of stress in the defining examples were not immediately recognized (e.g., area effects, Abele and Walters 1979; temperature effects, Cronin and Raymo 1997). More recently, it has also been proposed that over the past ~500 myr, extinctions and originations of marine organisms have been driven by different agents. Thus, the brevity and widely varying intensity of extinctions is taken to suggest physical causes, whereas a gradual and correlated generation of appearances is taken to suggest biological causes (Kirchner and Weil 2000).
In spite of discussions concerning of incompleteness of the fossil record or preservational biases (e.g., Foote, 2000; Badgley 2003; Benton and Emerson 2007; Stanley 2007), the ~500 myr fossil record of marine organisms preceding the present has been rigorously scrutinized for evidence of long-term evolutionary trends. A general increase in biodiversity through geologic time is clear, although the trend is often detrended in a search for evolutionary detail (e.g., Foote 2000; Mayhew et al. 2008). Several rigorously presented and largely concordant generalizations have so far emerged:
Originations of organisms are not bound to extinctions (Kirchner and Weil 2000; Kirchner 2002).
Origination rates exceed extinction rates by an average of 30 percent (Kirchner 2002).
Overall, rates of originations and extinctions decline (Foote 2003), or originations remain constant and extinctions decline through the ~500 myr interval (Stanley 2007).
Marine biodiversity generally increases through the interval (Foote 2000) following an exponential pattern (Benton 1995; Hewzulla et al. 1999; Lane and Benton 2003; Benton and Emerson 2007; Stanley 2007).
Against a background of general biodiversity increase, growth in biodiversity slows, and extinction and origination rates increase under global hothouse climates (Shaviv and Veizer 2003; Mayhew et al. 2008).
Origination rates vary less than rates of extinction (Kirchner 2002).
Early in the interval, change in biodiversity is more strongly linked to extinctions than to originations, whereas origination plays a larger role later in the interval (Foote 2003).
Early in the interval, average duration (longevity) of closely related organisms (genera) is shorter than it is later in the interval (Miller and Foote 2003; Rohde and Muller 2005).
The tropics constitute a center of origination during the latter half of the interval (Jablonski 1993; Jablonski et al. 2006).
Pulses of extinction closely follow a 62 myr cycle (Rohde and Muller 2005), or are more irregular (Stanley 2007).
That there was a turnover of organisms in the marine fossil record was to be expected, for the genetic and physiological construction of even modern organisms falls well short of perfection. Three basic levels of selection have been distinguished: molecular, embryonic, and environmental. Selection for minimal operational efficiency must be focused simultaneously on all three levels for survival to be assured (Lynch 2007; Wilkins 2007; Ellegren and Sheldon 2008). Challenges in maintaining a balance between dynamic and distinct biophysical environments ensures that scope will remain for evolutionary change for a long time to come.
These circumstances, in addition to a strong accelerating signal in the diversity of marine organisms, a shift from change dominated by extinction (physical forcing) to change dominated by origination (biological forcing) through time, and the identification of centers of origin in equatorial (relatively benign) regions, are suggestive of a general increase in competitive fitness, as here considered. A putatively cyclic repetition of pulses of extinction in the above list is fascinating, although its evolutionary implications are unclear (Kirchner and Weil 2005). At least one mathematical model of evolution has been proposed in which local environmental changes produce a global increase in biologic versatility. Two major predictions of the model include an accelerating pace in organismal evolution and a decreasing recovery time from major catastrophes (Turney 2000). An accelerating increase in diversity seems to be well supported (e.g., Stanley 2007). Perhaps an increasing proportion of genera that survive background and mass extinctions (Miller and Foote 2003; Rohde and Muller 2005) reflects an increasing capacity to survive extinction. In summary, what is known about long-term trends in the record of marine life does not rule out the major premises of this essay, and appears to at least mildly support them.
The latitudinal gradient of increasing biologic richness from high latitudes to the equator was one of the earliest ecological patterns to be identified. It may be viewed as analogous in some respects to the here-proposed gradient (trend) in competitive fitness through geologic time. The latitudinal gradient is correlated with a combination of variable physical gradients, such as area, precipitation, temperature, seasonality, and altitude, and is generally correlated with an increase in biodiversity that in some cases (e.g., birds) becomes quasi-exponential toward the tropics. Interspecies interactions, such as competition, predation, or mutualism, also become more intense in the tropics (Gaston 2000; Willig et al. 2003). Because of the catalytic effects of higher temperatures, it has been proposed that evolutionary rates are higher in the tropics. This possibility is currently supported by some data from trees (Wright et al. 2003) but contradicted by other data from birds (Bromham and Cardillo 2003). The understanding of biophysical factors supporting modern latitudinal gradients has been characterized as being in its infancy (Gaston 2000; Willig et al. 2003). Similarities to the proposed fitness gradient through time are evident. The fitness gradient is at least comparably complex and is even less adequately understood.
Consider, then, a journey from the arctic to the equator. The journey will begin during the northern hemisphere summer, when the north-south thermal gradient is least. The jumping-off place is located among the fields of ice and rock rubble of the northern Queen Elizabeth Islands. The route continues south through treeless tundra inhabited by clouds of arthropods (mosquitoes), and enters taiga swamps and endless, species-poor conifer forests. Further to the south lie mixed conifer-broadleaf forests of the midcontinental north woods, followed by cypress swamps of the southern Atlantic states. Passing the buried Chicxulub Crater in Yucat n, Mexico (recalling the end of the dinosaurian era), the journey ends in the vast, biologically rich rain forests of the Amazon Basin. A megafauna survives at similar latitudes in Africa.
The experience would be unforgettable. For some, it will bring to mind a general evolutionary pattern extending hundreds of million years through geologic time.
A thought experiment might usefully illustrate effects of 12 conceptual tools listed above on the evolution life on land. Imagine a lowland coastal floodplain under a moist, warm temperate climate similar to that prevailing today over much of the southeastern United States. Imagine also that the coastal plain is devoid of life except for two adjoining square plots that measure 10 km on an edge. In one plot is a hardwood forest typical of those now growing in the southeastern United States. In the other is a community of organisms typical of life on land 400 myr ago. Underlying physical conditions (elevation, mean annual rainfall, temperature, etc.) are identical in both plots.

Fig 1.2. An elephant ( Loxodonta africana ) wading in an oxbow lake ( dambo ) in Luangwa National Park, Zambia, is illustrative of large-brained animals that inhabit complex biotic environments. The megaherbivore is dwarfed by towering vegetation within a well-watered, tropical environment.
The modern plot contains familiar plants and animals. The more ancient plot is inhabited by a sparse variety of small, spore-bearing green stalks, mites, millipedes, centipedes, small scorpions, and tiny, spiderlike creatures. Organisms living in streams in the ancient plot are somewhat larger and more abundant. These include a dozen varieties of 10-20-cm-long jawless fishes that feed on freshwater microorganisms, which are in turn consumed by gilled scorpion-like creatures up to 1 to 2 m long (see chapter 7 ). Alone, both communities would spread across adjacent lowlands, for they are well adapted to the regional physical environment.
Imagine further that a barrier separating the two plots was lifted and the two communities could intermingle and interact with each other. How might plants and animals of the modern community fare within the ancient community? Pines, frogs, and small salamanders would immediately prosper, for the pines are wind pollinated, and the amphibians typically survive on pond scum and small arthropods. However, dandelions, squirrels, and rattlesnakes would fare poorly at the onset, for they require, respectively, bees for pollination, and seeds or small mammals for food. Although these modern organisms are well adapted to the geography and the climate, their biological needs cannot be met within the older, simpler community. Introduced singly into the more ancient community, they would probably perish.
However, the modern organisms, like their more ancient counterparts, belong to an interacting biological system. Modern hardwoods, pollinated by bees living among them and bearing seeds dispersed by squirrels, would immediately invade the defenseless plot of 400 myr old organisms. Grasses, bushes, and trees would spring up and shade out the more ancient plants. Insects, rattlesnakes, rabbits, and deer would enter the ancient plot along with the trees. Armored garfish, turtles, alligators, and wading birds would invade the streams. Finally, a human family would arrive; they would build a shelter, and plant corn and tobacco. It should be evident on a moment s reflection that the modern community is enormously more fit than the ancient one and would quickly replace the 400 myr old community.
Where might a community of organisms that resembles the 400 myr old community survive today? The plants in this modern community must also be short, and the animals must typically be small, jointed-legged arthropods. Polar and high-elevation tundra come immediately to mind, where biotic communities are simple and organisms are adapted to cold, harsh environments. They would only remain secure there while cold climates persisted. With global warming, for example, tundra ecosystems would probably be invaded by conifers, bushes, and large mammals typical of the arctic taiga representing a somewhat warmer climatic regime to the south.
Although perhaps self-evident, it might be well to point out several conventions concerning the study of the life of the past. Foremost among them is the proposition that scientific research is understood as not proving that any hypothesis (or theory) is true, but rather that some hypotheses are false. The usual sequence of events in the life of a hypothesis is that it is established on a respectable base of information. It is then further examined (tested) and found either to be supported by additional data, or to be in conflict with it. The hypothesis is carefully reevaluated against a background of information, both new and old. If it is found still to be in conflict with a significant body of knowledge, it is considered to be falsified. Otherwise, it continues to exist as a viable hypothesis. However, it always remains vulnerable to falsification by new information. Humans, of course, are human, and therefore are capable of imperfect reasoning; they may err in rejecting or supporting the falsification of a hypothesis (cf. Barr 2003; Allchin 2006). Scientific judgment is not an error-free path to truth. However, over the last two centuries, scientific methodology has had an impressive track record of advancing knowledge of the material universe. Fascinating and probably even mind-bending discoveries may be expected in the years to come.
Introducing the dimension of geologic time dramatically broadens uncertainties, as well as the scope for discussion about the long-term evolution of life. Some of the perspectives adopted in this essay are not often pursued in current thinking on the history of life on land. They are presented as meriting more thorough consideration. It follows that the subjunctive and the conditional could widely replace the declarative mood in many places in the text that follows. The text attempts to be concise and avoid the use of specialized vocabulary-thus, one hopes, making it simpler and easier to assimilate. Those who have written thoughtfully and in more detail on various aspects of the history of life will find it too abbreviated or simple. They may wish to follow citations to the works that grace the list of references. Specialized terms are introduced parenthetically to clarify relationships and facilitate entry into the more technical literature.
As noted at the outset, the perspective herein adopted is the world of middle size, between the microscopic and the telescopic. Its scope reflects the limited insights of a single onlooker who is attempting to conceptualize the researches of his colleagues. The main body of the text ( chapters 2 - 14 ) has been largely compiled from information presented in peer-reviewed scientific journals and books, most of which have been recently published. It must be emphasized that the number of relevant publications is enormous, and personal limitations strongly suggest that many excellent studies have neither been cited nor consulted. I hope that a sufficient number have been cited to convey an idea of the magnificence of the contributions that support our current understanding of prehistory. I hope, too, that the oversimplifications presented here will stimulate others, with the assistance of the primary literature, to carry out more painstaking and meaningful searches for major features in the history of terrestrial life. New approaches will rapidly outdate or deepen the generalizations presented here, particularly those concerning the origin of the universe and the primitive history of our planet.
The present essay focuses on what is currently known about change in life on land, placing dinosaurs and modern terrestrial life into a continuum. In a format that has long been canonical, the sequence of topics begins with brief descriptions of the origins of the cosmos, Earth, and life on Earth. The origin of continents and the course of life on the continents to the present are then outlined and placed within a modern geological timescale (Gradstein et al. 2004). Patterns are proposed in an attempt to describe the evolution of terrestrial life on Earth, and possibly elsewhere in the cosmos. Among other things, these patterns show that dinosaurs and humans, so often joined in our imaginations, represent two distinct phases within the history of life on Earth. Each chapter summarizes a large body of knowledge that has been gathered from the fossil record of our planet, accompanied by a section on inferences specific to the chapter. A summary of long-term patterns of change in the biotic world is presented for consideration in a final chapter.

Fig 1.3. Geologic timescale; dates given in millions of years before the present, after Gradstein et al. (2004).
Adams, F. D. The birth and development of the geological sciences.
Benton, M. J. Diversification and extinction in the history of life.
Darwin, C. On the origin of species.
Gradstein, F., J. Ogg, and A. Smith. A geologic time scale 2004.
Mayhew, P. J., G. B. Jenkins, and T. G. Benton. A long-term association between global temperature and biodiversity.
Mayor, A. The first fossil hunters: Paleontology in Greek and Roman times.
Rudwick, M. J. S. Bursting the limits of time: The reconstruction of geohistory in the Age of Revolution.
Worthy, T. H., and R. N. Holdaway. The lost world of the moa: Prehistoric life of New Zealand.

Fig 2.1. The field encompassed by the photograph is located in the constellation Fornax in the southern hemisphere and is one-tenth the diameter of a full moon. The field contains nearly 10,000 galaxies and penetrates nearly 13 billion light-years of space-time. Tiny red galaxies may be the most distant and oldest galaxies known, whereas the larger spiral and elliptical shapes represent galaxies typically ~1 billion years old. The few bright stars are nearby in the Milky Way galaxy. Photograph courtesy of NASA, the European Space Agency, and Steven Beckwith of the Space Telescope Institute and the Hubble Ultra Deep Field Team .
The origin and evolution of life on Earth took place within a universe that was unimaginably old and inconceivably vast. The structure of life is rooted within this universe. Life as we know it can flourish only under a limited range of conditions that include the availability of liquid water, suitable sources of chemical or radiant energy, and generally stable physical conditions that endure for thousands of millions of years. Such combinations may not be common within the visible universe, which is typically a cold, dark, near vacuum sparsely populated by brilliant nuclear furnaces (stars) emitting lethal high-energy radiation. Environments favorable to life may occur in scattered planetary oases, of which the surface of the Earth is an obvious, if uncommon, example.
High levels of physical ordering and a diversity of subatomic particle energies strongly imply that the visible universe is not average. It may belong to a much larger hypothetical reality variously termed a multiverse, an ultra-large scale universe, a background universe, or a cosmic landscape of universes. For this much larger reality to contain a few highly ordered universes similar to the visible universe, it would of necessity exhibit some staggering attributes. It would contain sites (known as metastable vacua) capable of generating about 10 500 (the number 10 followed by 500 zeros) component universes, and it would be quasi-eternal (Carroll 2006b; Silk 2006). The multiverse would be so huge that light could not pass between local universes, and thus could never be observed from within the visible universe. The multiverse hypothesis is supported by peculiar interactions of space and time at ultra-small levels (quantum effects) wherein the cause-and-effect relationships of classical physics no longer prevail. These and other important and imaginatively stimulating ramifications of quantum theory remain in need of clarifying observational evidence (cf. McFadden 2000; Zellinger 2000; Rees 2001; Morowitz 2002; Ellis 2003, 2005; Seife 2004; Brumfiel 2006; Siegfried 2006; Davies 2007).
The visible (local) universe itself appears to have come into existence in a colossal, virtually instantaneous expansion (inflation) of space and (local) time known as the big bang. Various lines of evidence suggest that the event occurred 13,700 myr ago (e.g., Miralda-Escud 2003; Solomon 2003; Walter et al. 2003; Bennett 2006; Carroll 2006b; Cowan and Sneden 2006; McMahon 2006; Ramirez-Ruiz 2006; Springel et al. 2006).
What follows is a brief chronology of cosmic history after the big bang. Time measured from the initial expansion is in italics; time measured from before the present is in parentheses:
0 (13,700 myr)
~ 10 35 seconds: rapid inflation of universe (Cho 2006a; Springel et al. 2006)
3 minutes: end of production of primary hydrogen and helium (Adams and Laughlin 1999)
0.4 myr (13,699.6 myr): neutral hydrogen and helium condense, microwave background radiation illuminates cosmos with uniform red glow and slowly fades (Miralda-Escud 2003; McMahon 2006)
0.4 myr-60 myr: dark age of the universe (Miralda-Escud 2003)
60 myr (13,640 myr): massive stars begin to form (Springel et al. 2006)
200 myr (13,500 myr): condensation of Milky Way protogalaxy (Pasquini et al. 2004)
300 myr (13,400 myr): stellar radiation ionizes interstellar gas (Springel et al. 2006)
600 myr (13,100 myr): onset of condensation of galaxies (Hogan 2006; Springel et al. 2006)
700 myr (13,000 myr): oldest identified Jupiter-sized planet (Irion 2003; Sigurdsson et al. 2003)
800 myr (12,900 myr): burst of formation of massive stars, synthesis of elements and injection of dust into interstellar space, many galaxies observed (Dunne 2003; Solomon 2003; Walter et al. 2003; Springel et al. 2006)
2,000 myr (11,700 myr): transition from slowing to renewed acceleration of expansion of universe (Bennett 2006)
5,000 myr (8,700 myr): star formation approaches maximum (Hu and Cowie 2006)
9,133 myr (4,567 myr): beginning of condensation of protosolar nebula (Amelin et al. 2002; Jacobsen 2003)
The structure of the visible universe exhibits a litany of seemingly improbable coincidences necessary for the existence of life within it (Gribbin 1994; Denton 1998; Rees 2001; Barr 2003; Carroll 2006b; Davies 2007). By the end of the first 3 minutes, a precise balance between expansion and gravity produced a titanic reservoir of nearly pure hydrogen and helium. Ordinary matter (making up about 4 percent of the universe) and dark matter together provide a gravitational balance so that geologic time would exist (Davies 2007; Hogan 2007). Thousands of millions of years later, the effects of dark energy would gradually begin to reaccelerate the expansion of the universe in a manner that is at present unknown to science (Bennett 2006; Brumfiel 2007; Hogan 2007).
There was a cosmic moment when the future seemed without promise, after the intense red glow from the cosmic background radiation faded and phantom streamers of raw hydrogen and helium slowly swirled in virtually endless wastes of dimension without light (Miralda-Escud 2003). Several tens of millions of years later, the balance between expansion and gravity facilitated the clumping of gases into primitive stars and protogalaxies (Cho 2007). The dark age of the universe ended with starlight.
Photons then streamed through the cosmos from the nuclear burning of nonrenewable (primordial) hydrogen and helium. A process was initiated that would ultimately provide energy to drive biologic (photosynthetic) processes in a remote future. Under high temperatures and pressures within stars, elemental syntheses proceeded smoothly from hydrogen to iron, including two extraordinarily fine-tuned reactions allowing carbon and oxygen, the elemental building blocks of life, to be conserved preferentially. A precise balance of nuclear particles and forces in previously formed atomic nuclei was such that the maximum dimensions of future planets (and organisms inhabiting them) would be defined. In explosions (supernovae) that terminated the brief lives of massive stars, a flood of particles (neutrinos) interacted with stellar materials to maximally disrupt the star, thereby propelling elements synthesized deep within its interior into the vastness of interstellar space. There, atoms of newly produced carbon and oxygen combined with primordial hydrogen to form water ices and hydrocarbons.
Over thousands of millions of years, generations of exploding stars continued to fertilize protostellar clouds with ashes of heavy elements, from which new stars and planets condensed. At present, Earthlike planets may be distributed throughout the Milky Way galaxy (Lewis 1998; Adams and Laughlin 1999; Lineweaver et al. 2004; Lineweaver and Robles 2006; Van Boekel 2007). On planetary surfaces, certain chemical links (hydrogen bonds) connect atoms in a manner ensuring that water will vaporize at unusually high temperatures, ice will float, and complex organic molecules can easily form and fragment (Ball 2008). Twenty light-years away from the solar system, a planet approximately five times the mass of Earth has recently been detected, orbiting a dwarf star one-third the mass of the sun. The planet s surface may be sufficiently warmed by the star to keep water in a liquid state (Udry et al. 2007).
Atomic nuclei undergoing radioactive decay at differing rates suggest that a large star (supernova) exploded in the vicinity of the protosolar cloud. The resulting pressure wave triggered the formation of the solar system (Zinner 2003). The interval between the synthesis of short-lived nuclear isotopes and their injection into the collapsing cloud was on the order of 200,000 to 400,000 years (Alexander et al. 2001). The oldest protoplanetary grains (chondrules) coalesced about 4,567 myr ago (Amelin et al. 2002; Jacobsen 2003). Meteoritic fabrics indicate that before or during the formation of the sun and its planets, clumps of small interplanetary particles provided surfaces upon which a variety of amino acids could condense. As inferred from cosmic and terrestrial abundances, most organic molecules within the solar system are extraterrestrial and nonbiologic in origin (Shock 2003). Among these are tiny hollow protobiologic globules that were protected from high-energy radiation of the infant sun in the interior of massive gatherings of protoplanetary clumps. For thousands of millions of years, these so-called static cartoons of life have continued to fall to the surface of the Earth in meteorites (Nakamura et al. 2002, 2003; Nakamura-Messenger et al. 2006), even long after life had already originated there.
Whether the visible universe is but one of virtually uncountable numbers of other universes in a multiverse or represents the totality of what exists, its physical structure is special, if not unique. We tread on hallowed ground. Because component universes of a multiverse would be isolated in separate space-time bubbles, they could not be observed directly from within the visible universe (Brumfiel 2006; Carroll 2006b), nor would life within the visible universe be affected by the presence or absence of a multiverse.
At this writing, an inflationary (that is, microsecond) expansion of space-time into existence continues to be the preferred model for the origin of the visible universe. Its fundamental structure was apparently fixed at the big bang. The potential for all derivative structures, whether or not they have yet to appear, is considered to have been fixed at the same time. As noted in chapter 1 , there are suggestions that particle masses may change very slowly over huge intervals of time (Adams and Laughlin 1999; Cho 2006b). At any moment in time, however, chemical and physical relationships are apparently uniform throughout the visible universe, thereby rendering it unitary (Cowan 2007). In earliest time, major physical events occurred with extreme rapidity. The pace rapidly decelerated, and cosmic time is often expressed as logarithms so that significant events fall into more evenly spaced sequences that can be placed into graphs easily (Adams and Laughlin 1999).
Today, the effects of events that occurred long before the origin of the solar system are felt everywhere over the surface of the Earth. Terrestrial environments are bathed in a faint glow of cosmic microwave radiation released when the universe was less than a million years old. Life evolves though geologic (cosmic) time. Organisms are formed from elements synthesized in stars and are nourished by photons radiated from the sun. Molecules are not formed within stars but are assembled from elements spread through interstellar space and later drawn together by gravity into planets. Complex carbon compounds also form in space, although there is no indication that even the simplest forms of life can reproduce in the interstellar medium. Bacteria either evolved on planetary surfaces or were ejected by meteorite impacts on their planets of origin and swept up by the gravitational fields of adjacent planets (Davies 1999).
The structure imparted to the universe through the big bang is congruent with life. Through a staggeringly long interval of time, life has been maintained on one planet, which in turn has been maintained within the structure of the universe. All the while, this oasis of life has manifested a general increase in diversity and complexity of form (Conway Morris 2003). It follows that the dynamics of evolution are somehow linked to the physical framework of the universe. As stewards of nature, we should not seek to maintain life in a condition of stasis, but rather to promote change in a manner congruent with the previous history of life. Regularities written into the structure of the visible universe are not arbitrary.
Planetary evolution results from a gravitational collapse rather than a local expansion in the fabric of the universe. Further, the collapse was creative, not destructive. Within the solar system, the initial growth of planetary embryos probably followed a generally similar pattern. However, the rapidly evolving early Earth did not resemble the Earth with which we are familiar. It is perhaps best to consider its initial phases as something entirely new and unfamiliar.
Barr, S. Modern physics and ancient faith.
Davies, P. C. W. Cosmic jackpot: Why our universe is just right for life.
Lineweaver, C. H., Y. Fenner, and B. K. Gibson. The galactic habitable zone and the age distribution of complex life in the Milky Way.
Seife, C. Physics enters the twilight zone.
Siegfried, T. A landscape too far.
Springel, V., C. S. Frenk, and S. D. M. White. The large-space structure of the universe.

Fig 3.1. The Island of Moorea, French Polynesia, southwestern Pacific, is the eroded summit of a submarine volcano, suggesting a common Hadean or early Archean landform. Soft volcanic ash has eroded from around solidified lava in volcanic throats. Similarly formed Hadean-Archean islands would have appeared dark as a result of the virtual absence of oxygen in the atmosphere.
Precambrian time includes the Hadean, Archean, and Proterozoic eras, encompassing nearly all (88 percent) of Earth history. Until recently, evidence for the evolution of Precambrian life was exceedingly meager. However, the search for life within and beyond the solar system has lately stimulated thoughtful investigations of the primitive Earth, in the expectation of better understanding the conditions necessary for the origin of life elsewhere. Rapid progress is now being made in elucidating the history of the Precambrian Earth.
The Hadean Era is here taken to include the interval spanning the appearance of a gravitational field around a minuscule protoearth strong enough to attract adjacent particles of matter and, several hundred million years later, the termination of an intense barrage of interplanetary asteroids, known as the late heavy bombardment. No surface rocks are known from this interval, for they have long since been destroyed as a result of dynamic processes within the interior of the Earth (Kamber et al. 2003; Zahnle et al. 2007). Hadean history is presently inferred from many sources. These include observations of young planetary systems beyond the solar system, elemental abundances (isotopes) in the inner solar system, crystals formed on the ancient Earth and later recycled into younger rocks, and in the genetic code preserved in surviving lineages of ancient bacteria (Schoenberg et al. 2002; Valley et al. 2002; Gaucher et al. 2003; Kamber et al. 2003; Kramers 2003; Menneken et al. 2007; Zahnle et al. 2007).
A brief chronology of the Hadean Eon follows. Time measured from the onset of condensation in the protosolar nebula is in italics; time measured from before the present is in parentheses:
0 (4,567 myr)
10,000 years: gas and dust from solar nebula condense into planetesimals up to 10 km in diameter (Jacobsen 2003; Wood et al. 2006)
100,000 years up to 1 myr: protoearth embryo grows from size of Moon to size of Mars, radioactive minerals and heating from impacts create magma oceans on protoearth (Jacobsen 2003; Wood et al. 2006)
10 myr (4,557 myr): main growth of Earth largely completed, intermittent presence of large bodies of surface water (Jacobsen 2003; Genda and Abe 2005)
~ 62 myr (4,505 myr): collision of Mars-sized planet, protoearth enveloped in impact magma ocean, primitive atmosphere lost, formation of Moon (Jacobsen 2003; Kleine et al. 2005; Iizuka et al. 2006; Koeberl 2006; Wood et al. 2006; Touboul et al. 2007; Zahnle et al. 2007)
~ 117 myr (~4,450 myr): possible initiation of plate tectonic processes, silicon-enriched (protocontinental) crust rapidly circulated through shallower regions of Earth s interior (Harrison et al. 2005; Davies 2006; Iizuka et al. 2006; Wood et al. 2006; Menneken et al. 2007)
167 myr (4,400 myr): presence of water oceans for extended periods of time (Wilde et al. 2001; Kramers 2003)
667 myr (3,900 myr): brief (20-200 myr) but intense interval of impact cratering (Cohen et al. 2000; Kring and Cohen 2002; Ryder 2002; Koeberl 2006)
767 myr (3,800 myr): end of heavy impact cratering, beginning of continuous rock record on Earth (Koeberl 2006)
The particular mix of elements in the planets of the solar system was not unusual, having been set in the big bang for light elements and in stellar nucleosynthesis for heavy elements (Stevenson 2008). Protoplanetary embryos accumulated rapidly from dust and gas in the solar nebula as centrally located gravitational fields became stronger. The protoearth may have grown to the size of Mars within 100,000 years, long before the solar nebula dissipated. Internally, a dense core had by then begun to separate from an ocean of molten rock above. Along with other planets in the inner solar system, our growing planet slowly emerged from a fog of silicon vapor to be exposed to the light of a bright red young Sun. After 10 myr the protoearth became the dominant planetary object in its region of the solar system, having swept up nearly two-thirds of its final mass and accumulated a veneer of water oceans (Genda and Abe 2005). A primary atmosphere, composed mostly of nitrogen, had probably been lost through earlier impacts of giant asteroids (Southam et al. 2007; Zahnle et al. 2007).
About 60 myr after it had begun to accrete, Earth is postulated to have collided with a Mars-sized object provisionally named planet Theia (Yin et al. 2002; Canup and Asphaug 2001; Jacobsen 2003; Touboul et al. 2007; for evidence of an early giant impact on Mars, see Kerr 2008). Debris from the impact formed the Moon, which modeling studies suggest grew to one-half its present mass within a week (Kring and Cohen 2002; Koeberl 2006). The Moon formed at 5 percent of its present distance from Earth, as its larger companion spun at the rate of one rotation per 5 hours. Tides were generated in magma oceans on Earth that were comparable in size to water tides in modern oceans (Abe and Ooe 2001; Koeberl 2006; Zahnle et al. 2007). After the impact, the Earth was again immersed in clouds of incandescent silicon vapor, and it glowed like a small star orbiting the Sun within the confines of the inner solar system. Silicates began to rain out at a rate of ~1 m per day, and magma oceans cooled to solidify within ~1.4 myr. Atmospheric steam then began to condense, raining out at a rate of ~1 m per day and restoring water oceans by ~4,200 myr ago. Within ~100 myr, tidal interactions between the Earth and the Moon had separated them by 58 percent of their present distance, and the Earth s period of rotation slowed to 16 hours (Zahnle et al. 2007).
A carbon dioxide atmosphere remained, mixed with subordinate amounts of nitrogen, carbon monoxide, and methane. Most of its component gases were derived from the interior of the planet or swept up from its orbital path, with minor contributions from the Sun and passing comets. Being much more massive than the modern atmosphere, its heat-retaining (greenhouse) properties were much stronger. Liquid water could thus exist on the Earth s surface in spite of the relatively weak radiation of the young Sun. No free oxygen was present in the atmosphere. Comparisons with primeval asteroids in element abundances indicate that much of the original hydrogen had been swept from the Earth, entraining with it large quantities of atmospheric carbon and nitrogen. However, compared with other light elements, both hydrogen and chlorine (in the form of water and salt, respectively) were preferentially protected from loss by their incorporation into primitive oceans (Drake and Righter 2002; Weichert 2002; Kramers 2003; Engrand and Maurette 2007; Cates and Mojzsis 2007; Zahnle et al. 2007).
It is possible that 10-20 myr after the impact that formed the Moon, oceans were already present that were capable of sheltering life. However, it is equally likely that primitive oceans were covered by heavy pack ice that thinned only in equatorial regions. Frigid climates would have been interrupted by hundreds or thousands of fleeting impact summers. Geological thermometers (oxygen isotopes) in zircon crystals suggest that surface temperatures were compatible with the continuous presence of liquid oceans at undefined temperatures, whether or not they were covered with ice. They would have been about twice as salty and at least twice as voluminous as modern oceans (Bounama et al. 2001; Knauth 2005). At most, only a few areas of silicon-enriched solidified lavas briefly projected above sea level, resembling emergent volcanic peaks of the modern South Pacific but completely devoid of visible life. Only minimal amounts of sediment eroded from these bits of land and were carried to the sea (Kamber et al. 2002, 2003; Whitehouse and Kamber 2002; Koeberl 2006).
Under the assumption that most solar system planets accreted from similar materials, differences between them would have been due largely to the quantity (mass) of material swept up in their formation and their distance from the Sun (Lewis 1998; Lundin et al. 2007). An extraordinarily warm thermal regime was generated within the Earth by high rates of radioactive decay. Accreted materials of different densities separated in the interior of the planet according to their density, leading to the formation of giant plastic masses (tectonic slabs) that slowly circulated within a relatively hot and fluid mantle located between the primitive crust and the growing planetary core. Circulation in the primitive mantle was further enhanced by its high (but soon declining) water content. Levels of hydrothermal activity and volcanism were higher than at present, and the recycling of differentiating crust was more rapid than later in Earth history (Southam et al. 2007; Van Thienen et al. 2007).
Relatively rapid increase in the mass of the planetary core generated a strong magnetic field that shielded water from being swept away by the stream of electrically charged particles emitted by the Sun (solar wind). The latter was particularly strong during the first 1,000 myr after accretion (Lundin et al. 2007). A cessation of differentiation in the Earth s interior would have had at least two major consequences: the collapse of the protective magnetic field, and the cessation of plate tectonic recycling of nutrients to the planetary surface (Dehant et al. 2007). Were plate tectonic movements to stop abruptly today, volcanism would cease, and with it the return of carbon dioxide from the mantle to the atmosphere. Within 1 myr weathering processes would combine all of the carbon (including carbon dioxide) remaining in contact with the Earth s surface into a layer of carbonate rock ~10 m thick. In the absence of atmospheric carbon dioxide, photosynthesis would cease and planetary ecosystems would collapse; without a magnetic shield, the oceans would be lost to the solar wind (Gaidos et al. 2005; Van Thienen et al. 2007).
In the case of Mars, the mantle solidified early in its history, tectonic circulation ceased, most of its surface water was lost to space, and surface temperatures rapidly fell to frigid levels. Earth is about nine times as massive as Mars; its interior remains plastic, and its subcrustal circulation continues. As a result of a gradually accelerating pace of nuclear reactions within its interior, the luminosity of the Sun has increased by ~30 percent over the past 4,500 myr, and will continue to grow (Lundin et al. 2007). To compensate for the otherwise rising surface temperatures produced by increasing solar luminosity, carbon dioxide is withdrawn from the Earth s atmosphere at higher rates through increased erosion and consequent combination with crustal rocks. The atmospheric greenhouse effect is reduced, and surface temperatures fall. Conversely, when surface temperatures cool, erosion slows, and carbon dioxide from volcanic (mantle) sources accumulates in the atmosphere to produce a corresponding global warming. Average global temperatures have thus remained approximately constant (e.g., consistent with the presence of liquid oceans) since middle Hadean time. Assuming thermal stability was due to the regulatory effects of carbon dioxide alone, atmospheric concentrations of the gas must also have declined by ~1,000 times since middle Hadean time in order to offset solar heating. Thus the presence of a long-enduring, self-regulating climatic control system has maintained the Earth s oceans in a predominantly liquid state (Bertaux et al. 2007).
After several hundred million years, the growing tranquility of surface environments of the Earth was shattered. Gravitational effects of a gradual loss of matter in the central part of the disk surrounding the Sun (solar nebula) caused the orbit of giant planet Jupiter to separate slowly from those of the three outer gas planets (Saturn, Uranus, and Neptune). Shortly after 4,000 myr ago, the gravitational effects of the dispersing giant gas planets destabilized the asteroid belt, scattering ~90 percent of its content out of the belt and into the inner solar system. The zone of orbiting comets (Kuiper belt) in the outer reaches of the solar system was also destabilized, but fewer comets reached the vicinity of the Sun. The result was a reorganization of the solar system, and a rain of asteroids and comets that refaced the surfaces of the inner planets.

Fig 3.2. The surface of the Moon remains heavily cratered from the late heavy bombardment that brought the Hadean to a close. Photograph courtesy of NASA .
The record of the late heavy bombardment is best preserved on the Moon, where heavily cratered lunar terrains indicate it reached a climax ~3,900 myr ago (Gomes et al. 2005; Strom et al. 2005; Iizuka et al. 2006; Bottke and Levison 2007; Bottke et al. 2007a; O Brien et al. 2007). The primitive asteroid belt had contained hundreds to thousands of times more mass than it does at present (O Brien et al. 2007), approximating 35 times the mass of Earth (Bottke and Levison 2007). Of this mass, ~200 tonnes/m 2 (Marty and Meibom 2007) were delivered to the surface of the Earth at velocities of ~20 km per second (Ivanov and Artemieva 2001). Enormous volumes of crustal melt were generated, and older crust, together with much of the previous impact record it contained, must have been destroyed (Koeberl 2006).
At its maximum, craters equal to, and frequently much greater than, 20 km in diameter would have been excavated on Earth at rates of one per 100-10,000 years, and possibly as often as one per 20 years. Vast submarine hydrothermal regions (hot springs) were undoubtedly created on the ocean floor. Younger (post-Hadean) sediments deposited between 3,800 and 3,700 myr ago retain elemental input derived from these impacts, and a tenuous signature of the bombardment lingers in the presence of extraterrestrial noble gases in the atmosphere of the Earth today (Marty and Meibom 2007). Only small quantities of water were added to the global ocean. The total mass of accreted carbon was nevertheless 160 times larger than the carbon in all of the organisms now living on Earth (Cohen et al. 2000; Dauphus and Marty 2002; Kring and Cohen 2002; Schoenberg et al. 2002; Iizuka et al. 2006).
The largest terrestrial impacts would have vaporized the top few tens to perhaps hundreds of meters of the ocean, but abyssal waters would have been largely shielded from thermal stresses (Ryder 2002; Koeberl 2006). That life may have originated in the oceans is suggested by trace element similarities between living organisms and seawater (Southam et al. 2007). No fossil record remains in which evidence of life on the Hadean Earth might have been preserved. However, between 4,400 and 4,000 myr ago, stable oceans, an atmosphere mainly of carbon dioxide, and widespread volcanism may have generated ammonia and vast, oily slicks of prebiotic molecules. Complex molecular entities capable of sustaining rapid chemical reactions and self-replication could have been present 4,000 myr ago (Huber and W chtersh user 2006; Dias and Maurette 2007). Protobiotic molecules, or even life itself, may have been annihilated and regenerated several times during the ensuing late heavy bombardment (Davies and Lineweaver 2005). If any archaic life forms survived heat-sterilizing effects of frequent giant impacts, they would have of necessity also been capable of surviving periodic exposure to high-temperature environments, as are some living bacteria (hyperthermophiles; Bada 2004; Battistuzzi et al. 2004). Genetic evidence suggests that bacteria may have differentiated into two major groups (Eubacteria and Archaebacteria) by 4,100 myr ago, and that life had appeared on Earth before the end of the Hadean (Battistuzzi et al. 2004).
It has also been speculated (Davies 1999; Lineweaver 2004) that life may have originated on Mars before it did on Earth. Planet-forming and subsequent bombardments were less intense there, and favorable environmental conditions began, albeit transiently, earlier in its history. Bacteria in a resting state might then have been transferred from Mars to Earth in the interior of meteorites, where they would have been protected from lethal high-energy radiation (Mastrapa et al. 2001). Mars once contained and still retains significant quantities of water (Krasnoplosky and Feldman 2001; Craddock and Howard 2002; Mitrofanov et al. 2003). Heat from large impacts may have produced relatively mild surface conditions, releasing bodies of surface water for at least decade- to millennia-long intervals, and possibly long enough for life to have originated (Segura et al. 2002; Kerr 2003). It has been suggested that oceans existed on Mars as late as ~3,000 myr ago (Perron et al. 2007; but see McEwen et al. 2007), although whether or not life ever existed on Mars remains uncertain. Nonetheless, carbon molecules originating in space surely contributed to the mix of prebiotic molecules on the primitive Earth (Bada 2004).
The history of the Hadean Earth is readily separated into three phases: a ~60 myr interval of accretion ending with a giant impact, a relatively quiescent ~600 myr interval characterized by the presence of water oceans, and a terminal interval of ~100 myr when the inner planets from Mercury to Mars experienced a rain of asteroids that deeply scarred planetary surfaces. The primitive Earth underwent two catastrophic intervals of solar system bombardment that nearly (but not quite) annihilated the geological record of previous surface environments. There is no direct evidence of life on the Hadean Earth. No bacteria could have survived the impact in which the Moon was formed, and for an unknown interval after the late heavy bombardment, any existing land surfaces on Earth must have been as densely cratered as the Moon.
Clues from chemical elements remaining from the middle Hadean strongly suggest that for an interval longer than all of the time that has transpired since the end of Precambrian, the Earth was either a blue or white planet of open or ice-covered oceans. Weather systems were accelerated by more rapid planetary rotation (Bills and Ray 1999) and dominated by ocean-atmosphere interactions. The crust had separated into discrete platelike structures, which vigorously rotated into and out of the mantle below to bring chemicals necessary for life up to the ocean floor. A magnetic field shielded surface water from the solar wind. The carbon cycle protected surface environments from gradually overheating by increasing solar luminosity. Complex carbon molecules from nearby space fell to the surface of the oceans, were recycled into crustal rocks below, and reentered the oceans in petroleum-like aggregations of carbon compounds. Thermal vents were scattered across the abyss, and with them were energy-rich thermal and chemical gradients. What modern exobiologist would not suspect the eminent appearance of life? Rendering the situation even more auspicious are suggestions within the genetic structure of living bacteria that their predecessors were already present in middle Hadean time. And there was another planetary option. Martian impacts still propel meteorites to Earth; should life have originated on the surface of Mars, it may have seeded Earth.
Nevertheless, Hadean exobiologists may not have shared the heady optimism of their current counterparts. They would have observed that the asteroid belt between Mars and Jupiter burgeoned with primitive solar system objects, many of them of considerable size. Ominously, the orbits of Jupiter and Saturn were diverging toward a critical threshold, after which the orbits of asteroids would be wildly disrupted by gravitational effects. If ever there was a time to save Planet Earth, it was then, but the means were not available. The late heavy bombardment ending the Hadean would have seen the upper levels of the oceans pasteurized, probably many times. Huge, hot submarine impact craters would have spawned gigantic storms (hypercanes) calculated to have produced winds approaching the speed of sound (Emanuel 2002). No life would be visible on small, scattered peaks projecting above a periodically tormented world ocean. Their summits would have been deeply eroded, either by water or ice. Narrow coastal plains were probably littered with huge boulders ripped from their foundations by tsunamis and blasted by high-velocity torrents of water mixed with rock, driven before screaming winds. The geological record of the Hadean Earth was continually being erased.
In intervals of atmospheric calm, emergent fields of semiconsolidated lava rubble might be seen, punctuated here and there by steaming springs exhaling sterility. Far into the distance, squat volcanoes might barely be distinguished, projecting from a cold, leaden sea along the limb of a blue-gray horizon. The surface of such land as was exposed above the ocean would have appeared to be dead. Perhaps it was. For millennia upon millennia, Earth may have alternated between ice-covered oceans and a wet, Venuslike atmosphere on the brink of a slow boil. Any oceans on Mars must have been in the process of slowly vanishing. The system of Sol was seemingly without life.
Yet during the Hadean, a part of the solar nebula had collapsed into a geomagnetic and tectonic planetary dynamo with the means of protecting its water oceans for thousands of millions of years to come. Simple bacteria may have already appeared, either around smoking thermal vents on stygian ocean floors (Thyveetil et al. 2008) or within thin films of unfrozen water beneath sheets of continental ice (Price 2007)-or perhaps even in both environments.
Battistuzzi, F. U., A. Feijao, and S. B. Hedges. A genomic timescale of prokaryote evolution.
Bounama, C., S. Franck, and W. Von Bloh. The fate of earth s ocean.
Southam, G., L. J. Rothschild, and F. Westall. The geology and habitability of terrestrial planets: Fundamental requirements for life.

Fig 4.1. Rotorua hot springs mud volcano, North Island, New Zealand, a terrestrial environment representative of those colonized by early Archean bacteria.
Not much land was exposed at the beginning of Archean time. Bacteria, although probably present, had little effect on the appearance of rugged volcanic terrains that periodically emerged from the oceans. By the eon s end, over a billion years later, it looked much the same. No multicellular organisms were present, although the Earth was already over 2 billion years old. Under such circumstances, a similar planet, orbiting another star, might not be viewed as an auspicious cradle for the future evolution of complex life. Yet the planet was infected with life. Bacteria flourished within the upper limits of the Earth s crust and in shallow seas, and had spilled onto lowlands of nascent continents. Change was literally in the wind; bacterial activity was polluting the atmosphere and threatening the primeval balance of greenhouse gases.
A brief chronology of the Archean Eon follows. Time measured from the end of the period of heavy bombardment is in italics; time measured from before the present is in parentheses. For a recent review of Archean biology and environments, see Knoll (2003).
0 myr (3,800 myr): end of heavy impact cratering
21 myr (3,779 myr): oldest marine sediments, and oldest fossil evidence of marine life (Catling and Claire 2005; Dauphas et al. 2007)
350 myr (3,450 myr): diverse bacterial sheets (protostromatolites) in shallow tidal waters deriving energy from sunlight (Allwood et al. 2006; Awramik 2006; Canfield 2006), methane-generating bacteria in hydrothermal environments (Ueno et al. 2006), bacterial biomarkers in volcanic glass (Banerjee et al. 2006)
430-560 myr (3,370-3,240 myr): impact ejecta of large asteroids (Simonson and Glass 2004; Glikson 2005)
600 myr (3,200 myr): formation of protocontinents begins (Castro and Patino-Douce 2001), presence of bacterial mats on tidal flats (Noffke et al. 2006)
900 myr (2,900 myr): presence of supercontinent of Ur (Eyles 2008), oldest known ice age, Pongola glaciations (Young et al. 1998; Kopp et al. 2005)
1,100 myr (2,700 myr): instabilities in heat-retaining properties of atmosphere (Ohmoto et al. 2006; Farquhar et al. 2007), beginning of bacterial colonization of land (Battistuzzi et al. 2004)
1,170-1,260 myr (2,630-2,540 myr): impact ejecta of large asteroids (Simonson and Glass 2004; Glikson 2005)
1,200 myr (2,600 myr): bacterial mats present on exposed soil surfaces (Watanabe et al. 2000)
1,300 myr (2,500 myr): continental mass half that of present continents (Kramers 2002), exhibiting modern landforms (Griffen et al. 2003)
As has been noted ( chapter 3 ), the Earth s mantle slowly circulates. In the process, it transports heat from decaying radioactive elements in the metal-rich core to the surface 2,900 km above. When hot mantle plumes reach the surface, they interact chemically with the oceans and atmosphere, freezing into crustal rocks. The primitive crust resembled seafloor lava in composition. It was later enriched with silicon to resemble granite more closely than modern seafloor lava in composition, and it accumulated in protocontinental aggregations. Lavas are lighter than mantle, and granite is in turn lighter than lavas. Continents thus float on mantle and oceanic crust. They drift and change in configuration in response to the slowly circulating mantle below (Helffrich and Wood 2001; Nisbet and Sleep 2001). The volume of the Earth s oceans had increased to a maximum near the beginning of the Archean, after which the mantle began to cool and its capacity to absorb water increased. Mantle circulation gradually began to transfer water from the oceans back into the Earth s interior, allowing the upper portions of continents to emerge more fully from the world ocean (Bounama et al. 2001; Bertaux et al. 2007; Zahnle et al. 2007).
By early Archean time the Moon had receded to 83 percent of its present distance, and the Earth s day had increased to 20 hours (Zahnle et al. 2007). In contrast to the Moon, the visible topography of which for the most part has remained the same since the end of the late heavy bombardment, the surface of the pre-Archean Earth was continuously recycled. Few rocks remain from the first 250 myr of Archean time, and those that do are now exposed over an area totaling only ~10,000 km 2 (the area of a circle measuring ~113 km in diameter) scattered across three modern continents (Nutman et al. 2001). Protocontinents had begun to form halfway through Archean time (Kamber et al. 2001; Castro and Patino-Douce 2001). By the end of the Archean, silicon-enriched crust had grown to about half of its modern mass (Kramers 2002), and two large continental agglomerations had appeared, Kenorland and Zimvaalbard (Aspler and Chiarenzelli 1998). These largely submarine continents probably resembled the domed highlands of modern Venus. By 2,900 myr ago, the presence of glaciers on both continents indicated that their surfaces were at least partly emergent (Castro and Patino-Douce 2001). The glaciation, the oldest known in Earth history, evidently coincided with a short-term decline in the heat-retaining properties of the atmosphere (Ono 2006).

Fig 4.2. Rotorua hot springs, North Island, New Zealand, hosting bacteria adapted to hyperthermal conditions assumed to have been widespread during the Archean.
During most of Archean time, rocks near the surface of the Earth were warm and buoyant, as is now the case on Venus (Ghail 2001; Marty and Dauphas 2003). Thus, they were not carried into the mantle (subducted) by the subsurface circulation of plastic rock, but rather underplated older surface rocks instead. By the end of the Archean, continental plates became thicker and the oceanic crust cooler and less buoyant. Slabs of crustal rock began to circulate deeply into the mantle. Silicon-enriched crust accumulated more rapidly, and continental topographies came to resemble those of modern, drifting continents. Volcanoes and rugged mountain chains formed along leading edges, overriding denser oceanic crust entering the mantle below, and shallow seas lapped onto trailing coastlines (Castro and Patino-Douce 2001; Griffen et al. 2003; Kato and Nakamura 2003; Marty and Dauphas 2003; Rey et al. 2003; Smithies et al. 2003). Although no craters have been or are ever likely to be identified, glasslike microspherules suggest that asteroids as much as 20-30 km in diameter, or two or three times that of the asteroid that much later exterminated the dinosaurs ( chapter 13 ), continued to strike the Earth. Despite the effects of long-term burial under great heat and pressure, ejecta from ancient impacts will probably reveal significant information on the properties of the ancient crust (Simonson and Glass 2004; Glikson 2005; Hofmann et al. 2006; Glikson and Vickers 2007).
Near the beginning of the Archean (3,500-3,200 myr), exposed surfaces of the nascent continents were subjected to a carbon dioxide-methane greenhouse atmosphere and surface temperatures of ~65-85 C (Knauth 2005; Robert and Chaussidon 2006; Dauphas et al. 2007) that would have been lethal to any modern organism except the most heat-resistant bacteria. Such ocean temperatures would have continued to support hurricanes of extreme violence (Emanuel et al. 1995; Nisbet and Sleep 2001; cf. Emanuel 2003, figure 1). Impact events, however, became smaller; the largest would have brought surface waters only to a boil (Zahnle et al. 2007).
As the continents expanded in area, their erosion probably generated a decline in atmospheric carbon dioxide concentrations and thereby global temperatures. A high-altitude methane haze may have enveloped the planet, further lowering surface temperatures and producing the gla cial age of ~2,900 myr ago (Young et al. 1998; Ono 2006). The glaciations approximately coincided with indications of change in the chemistry (revealed by sulfur isotopes) or transparency of the atmosphere at 3,000-2,750 myr. Although the properties of mid-Archean atmospheres are not well understood (cf. Goldblatt et al. 2006; Kasting 2006; Knauth 2006; Farquhar et al. 2007), they contained virtually no oxygen (Ohmoto et al. 2006; Farquhar et al. 2007).
Temperatures rose again in the late Archean as erosion subsided, and continental surfaces were leveled under a renewed carbon dioxide-methane greenhouse (Battistuzzi et al. 2004; Lowe and Tice 2004; Ohmoto et al. 2006; Robert and Chaussidon 2006; Sleep and Hessler 2006). It would not be for many millions of years and the beginning of a later glacial interval that a decrease in the effectiveness of bacterial oxygen consumers (scrubbers) and an increase in oxygen producers would allow free oxygen finally to accumulate within the atmosphere (chapter 5; Catling and Claire 2005).
It is possible that life originated very early in our planet s history ( chapter 3 ). Proteins in ancient lineages of bacteria suggest that they preferred hot (55-65 C) but not extremely hot environments (Gaucher et al. 2003). The chemical precursors of bacteria, and therefore the ancestors of all subsequent life, may have inhabited tiny vesicles in iron sulfide mounds near submarine hot-water vents. Their fragile, viscous components would have been sheltered from disturbance, and assembly of vital systems could gradually take place (Martin and Russell 2003; Russell 2003). The time required for the prebiotic organization and encapsulation of bacteria is unknown, and conditions leading to the spontaneous origin of bacteria have not yet been duplicated in laboratories (Conway Morris 2003). Yet an undocumented passage occurred at least once between prebiotic molecules and the root of the tree of life. It was followed by the emergence of a thin aqueous cloud of early bacteria, perhaps surrounding hydrothermal vents (Skophammer et al. 2006).
Although living bacteria are simple in external form, they compete against each other by means of a spectrum of toxins and have adapted to a wide variety of physical and chemical conditions (Monastersky 1997; Lenski and Riley 2002; Knoll 2003; Martin and Russell 2003). A living bacterial community has survived for millions of years nearly 3 km below the surface of the Earth, isolated from all photosynthetically derived nourishment. It inhabits hot (over 60 C) water in fractured rock and supports very low metabolic rates, similar to those of bacterial communities in seafloor sediments (Lin et al. 2006). Viable bacterial resting spores have been extracted from salt crystals over 250 myr old (Parkes 2000). Other bacteria have been found to be capable of surviving exposure to the vacuum of nearby space ( chapter 3 ).

Fig 4.3. Bacterial colonies surrounding hot-water lakes near the Old Faithful Geyser, Yellowstone National Park. Scalding waters protect lakeshores from being invaded by forests dominated by multicellular organisms. The local environment is too hot to support complex Phanerozoic-like ecosystems. Photograph courtesy of Michael Dunn and the North Carolina Museum of Natural Sciences .
Evidence of the presence of bacteria in strata of earliest Archean age is often ambiguous. Elemental carbon may have infiltrated more ancient rocks under high temperatures and pressures rather than through the activity of primeval bacteria (Fedo and Whitehouse 2002). Modern bacteria invade cracks on surfaces of ancient rocks fractured by surface extremes of temperature and aridity, where they have been mistaken for fossils of archaic bacteria (Westall and Folk 2003). Bacteria-like microstructures can be produced through inorganic chemical processes (Brasier et al. 2002; Gee 2002; Schopf et al. 2002; Garcia-Ruiz et al. 2003). Ancient bacterial mats may have been precipitated inorganically out of shallow hydrothermal waters (Des Marais 2000; Van Kranendonk 2001, 2003; Westall et al. 2001; Lindsay et al. 2003). Accordingly, purported bacterial fossils must be assessed carefully before they can be accepted as reliable evidence of ancient life. Chemical biomarkers of metabolic activity preserved in the geological record, and molecular indicators of relationship in living bacteria may be more revealing than indistinct fossil forms.
Any strain of bacteria that survived late Hadean time would have been tolerant of high temperatures. The carbon dioxide content of the 3,500 myr atmosphere was about equal to that of the combined nitrogen-oxygen content of the present atmosphere of the Earth. Methane was necessary to further increase greenhouse gas effects, allowing solar luminosity to heat the surface of Earth to biologically hot temperatures. Such temperatures would eliminate multicellular life on Earth today, but they would have been readily tolerated by Archean bacteria (Nisbet et al. 2007; Southam et al. 2007). Archean bacteria already supported a variety of metabolisms (Battistuzzi et al. 2004; Canfield 2006). Bacteria bored tiny tubes into volcanic glass in submarine lava flows nearly 3,500 myr ago (Furnes et al. 2004; Banerjee et al. 2006). These bacteria (archaebacteria) were releasing methane into the atmosphere (Ueno et al. 2006). Organic (isotopically light) sulfur suggests the presence of chemically sophisticated bacteria living in warm saline ponds near the margins of newly exposed land by ~3,450 myr ago (Shen et al. 2001). Well-preserved bacterial colonies mark a thin but extensive microbial reef ecosystem growing 3,430 myr ago in fully illuminated coastal waters (Allwood et al. 2006). At approximately the same time, photosynthetic bacteria growing in shallow water mats beneath an oxygen-free atmosphere were utilizing hydrogen for carbon fixation in photosynthetic processes (Tice and Lowe 2006). The slightly older reef was thus also flourishing under anoxic (oxygen-free) conditions.
About 400 myr later (~3,000 myr ago) oxygen began to enter local, restricted environments in small but significant amounts. The beginning of a dynamic balance between methane and oxygen in the atmosphere ~3,000-2,700 myr ago approximately coincided with the oldest known biomarkers of bacteria that release oxygen (cyanobacteria) and with the early colonization of the land (Summons et al. 1999; Battistuzzi 2004; Canfield 2005; Goldblatt et al. 2006). The oldest known record of a rudimentary but fully terrestrial ecosystem is preserved in ~2,600 myr old semiarid soils permeated with organic carbon produced in cyanobacterial mats. The extensive development of these mats suggests the presence of a local atmospheric screen protecting them from ultraviolet radiation (Watanabe et al. 2000). It also implies the presence of increasing concentrations of organic acids in soils that would have increased rates of erosion on continental surfaces (Kramers 2002).
Particularly interesting are organic molecules (biomarkers) indicating the presence of complex cells (eukaryotes) from strata of the same age (Brocks et al. 1999). Eukaryotes are characterized by the presence of subcellular structures derived from ancestral bacteria (mitochondria) that facilitate cellular respiration and energy production. Eukaryotes have also assimilated cyanobacteria (chloroplasts) that nourish them with energy photosynthetically derived from sunlight. Eukaryote cell walls are flexible, so that they are able to engulf smaller bacterial prey. These unicellular raptors may have driven their miniscule prey toward smaller, rapidly reproducing, and more streamlined forms, suggesting a remote analogy to current relationships between modern lions and gazelles (Kurland et al. 2006). Much later, eukaryotes would diversify into a wide range of single- and multicellular organisms (including animals and green plants) that proliferated in terrestrial environments under oxygenated atmospheres (Baldauf 2003; Hedges et al. 2006).
Early in the Archean, ancestral bacterial stock included methane-releasing (archaebacteria) and sulfur-consuming bacteria. Late Archean time marks the appearance of land-dwelling bacteria (terrabacteria) and carnivorous eukaryotes (e.g., Battistuzzi et al. 2004; Kurland et al. 2006; Pace 2006; Poole and Penny 2007; but see Cavalier-Smith 2006). By the end of the era, a sixfold increase had taken place in the diversity of major bacterial groups (Des Marais 2000; Shen et al. 2001; Van Kranendonk et al. 2003; Battistuzzi et al. 2004). Among the terrabacteria, cyanobacteria were generating oxygen by means of a photosynthetic process so biochemically complex that it has apparently evolved only once (Kasting and Siefert 2002; Baldauf 2003; Allen and Martin 2007). Oxygen- and methane-producing bacteria would soon overtake volcanic emissions in influencing the gas composition of the atmosphere (Hoehler et al. 2001; Nisbet 2002). Although bacterial life was widely distributed, its weight per unit area (biomass) was probably very small. Bacteria did aggregate in thin, simple mats, but productivity was low (Nisbet et al. 2007; Southam et al. 2007). Landscapes remained bleak, darkened by black, gray, and blue-green hues of unoxidized minerals and cyanobacteria (Wiechert 2002).
It would seem that the potential for the existence of atoms, molecules, and life was implicit in the structure of matter at the origin of the visible universe (McMullen 1993; Conway Morris 2003). The very high degree of organization in bacteria relative to that in prebiotic molecules implies a noninstantaneous process of origination. The time necessary for the biophysical configuration of bacteria on the surface of the prebiotic Earth may have been many orders of magnitude longer than the duration of laboratory experiments designed to replicate the origin of life. The origin of life is still not well understood.
Venus resembles Earth more closely in planetary structure, size, composition, and elemental abundances than any other planet in the solar system. Yet the surface of Venus bakes under temperatures averaging 464 C beneath an atmosphere that is without oxygen and that is 92 times heavier than that of Earth (Taylor 2006). It is doubtful whether any bacteria could long endure under such conditions. Near the beginning of the Archean, 3,800 myr ago, temperatures on Earth were also higher than they are now. Its atmosphere was denser and contained virtually no oxygen. Although bacteria were probably already present, the effects of life on physical environments were much less than today. Early in the history of the solar system, the two sister planets resembled each other much more closely than they do now.
How was it that one remained a hellish, cloud-covered furnace and the other was transformed into a fertile substrate for life? The answer appears to be water, to lubricate plate tectonic processes, to cycle subterranean nutrients to planetary surfaces, and to regulate planetary temperatures. Proximity to the Sun and an early presence of water may have triggered a runaway greenhouse on Venus, resulting in the loss of water to space so that only trace amounts remain. Its scarcity may account for great differences in present crustal processes and landforms of Venus and Earth (Nimmo and McKenzie 1998; Nisbet and Sleep 2001; Dietrich and Perron 2006; Kulikov et al. 2007).

Fig 4.4. An image of the surface of Venus was taken by the Venera 13 Lander east of Phoebe Regio under high atmospheric pressures and torrid conditions typical of the planet s surface. The composition of surface rocks appears to be similar to that of terrestrial lava. The Lander ceased to function after about 2 hours. Photograph courtesy of NASA .
Surface environments of the early Archean Earth provided energy sources and water temperatures at least minimally congruent with bacterial survival and evolution. Conditions improved. Hot surface temperatures declined, and continental consolidation progressed. Through the sustained activities of unimaginable numbers of the tiny bacterial quanta of life, atmospheric carbon dioxide levels fell and ocean temperatures moderated. With the advent of photosynthesis, oxygen began to leak into an initially highly inhospitable but ultimately accommodating atmosphere. Bacteria invaded the land, creating gossamer-thin soils and increasing rates of nutrient release by catalyzing erosional processes (Dietrich and Perron 2006). There were reversals, including heavy infalls of large asteroids and compositional fluctuations of an atmosphere on the verge of retaining free oxygen. Extinctions doubtless occurred that remain to be identified in the geologic record.
But by the end of the Archean the balance tipped from initial conditions set by physical regularities toward the biochemical power of life. Life forced environmental conditions in directions more favorable to life; Gaia was in the process of being birthed (Lenton 1998; Nisbet and Sleep 2001). The counterintuitive power of protoplasm began to prevail, and the web of life expanded. In a process of downward causation ( supervenience ), cyanobacteria would alter the primitive physical surface of the Earth in a manner that would promote the diversification of multicellular life (Corning 2002; Allen and Martin 2007). Analogously, and much later, burrowing animals would disturb soils and giant herbivores would open forests, thereby creating new habitats (Erwin 2008). All it would take was time-thousands of millions of years of it.
Allwood, A. C., et al. Stromatolite reef from the early Archaean era of Australia.
Canfield, D. E. The early history of atmospheric oxygen.
Dietrich, W. E., and J. T. Perron. The search for a topographic signature of life.
Farquhar, J. M., et al. Isotopic evidence for Mesoarchaean anoxia and changing atmospheric sulphur chemistry.
Knauth, L. P. Temperature and salinity history of the Precambrian ocean: Implications for the course of microbial evolution.
Nisbet, E. G., and N. H. Sleep. The habitat and nature of early life.

Fig 5.1. The Hoggar Mountains, near the hermitage of Charles de Foucault in southern Algeria, are representative of late Archean and early Proterozoic continental landscapes that hosted only microbial life. The local environment is too dry to support complex Phanerozoic-like ecosystems, such as the rain forests to the south in the Congo basin. Photograph courtesy of Harold Heatwole .
Spanning nearly two billion years, the Proterozoic is the longest division of geologic time. All of later time is less than the average span of its three subdivisions, the Paleo-, Meso- and Neoproterozoic (Amthor et al. 2003; Knoll 2003). The eon may be characterized as an age of microbial evolution. Most organisms, as individuals, remained microscopic during all but the last 3 percent of the eon. It has been necessary to rely on radiometric dates to calibrate its physical history, rather than fossils as is typical of post-Proterozoic divisions of geologic time. Severe glaciations and stepped increases in the oxygen content of the atmosphere occurred near the beginning and end of the eon. As presently understood, various interpretations of the physical and biological history of the Proterozoic are not entirely consistent with each other. The narrative presented here represents an approximation that in the future will be rendered more precise with the rapid accumulation of knowledge.
A brief chronology of the Proterozoic Eon follows. Time measured from the Archean-Proterozoic boundary is in italics; time measured from before the present is in parentheses. For a modern review of Proterozoic biology and environments, see Knoll (2003). Note that atmospheric oxygen concentrations are generally given as the percentage of the modern atmospheric oxygen concentration of 21 percent. Thus, an early Proterozoic level of 0.4 percent of 21 percent implies that the concentration of oxygen in the early Proterozoic atmosphere was ~0.1 percent; similarly, a late Proterozoic level of 15 percent implies an atmospheric oxygen concentration of ~3 percent.
Paleoproterozoic Era, duration 900 myr (2,500-1,600 myr)
0 myr (2,500 myr): Archean-Proterozoic boundary, Kenorland super-continent (Eyles 2008), atmospheric oxygen 0.4 percent of modern level (Canfield 2005), decrease in global temperatures from ~70 C to ~40 C (Knauth 2005; Robert and Chaussidon 2006)
30-37 myr (2,470-2,463 myr): great oxidation event (Canfield 2005; Bekker et al. 2006), ozone ultraviolet shield (Catling et al. 2005), nitrogen biological crisis? (Kasting and Siefert 2002)
100 myr (2,400 myr): Huronian glaciation (Eyles 2008), fluctuating levels of bacterially produced oxygen and methane destabilize climates (Bekker and Kaufman 2007)
300 myr (2,200 myr): bacteria (cyanobacteria) flourished in humid continental climates (Beukes et al. 2002)
500-700 myr (2,000-1,800): atmospheric oxygen levels low (Canfield 2005)
610 myr (1,890 myr): oldest fossils (colonial bacteria) visible to naked eye (Hedges et al. 2004; Catling et al. 2005)
700-1,000 myr (1,800-1,500 myr): Columbia (or Nuna) supercontinent coalesced and fragmented (Silver and Behn 2008)
700-1,700 myr (1,800-800 myr): atmospheric oxygen ~10 percent of present levels (Canfield 2005)
800 myr (1,700 myr): diverse fossils up to 4-5 mm long (Catling et al. 2005)
Mesoproterozoic Era, duration 700 myr (1,600-900 myr)
1,100 myr (1,400 myr): large, complex marine cells (eukaryotes, possibly fungi) (Javaux et al. 2001; Martin and Russell 2003; Butterfield 2005)
1,150 myr (1,350 myr): global temperature ~20 C (Knauth 2005; Robert and Chaussidon 2006)
1,300 myr (1,200 myr): sexually reproducing red algae (Butterfield 2000), microbial (cyanobacterial) mats flourish in limestone terrain on continents (Knauth 2005)
1,400 myr (1,100 myr): flourishing cyanobacterial communities on land (Kenny and Knauth 2001)
1,500 myr (1,000 myr): genetic evidence suggestive of presence of green algae, fungi on land (Heckman et al. 2001a), minimum age for presence of land biota (Kennedy et al. 2006)
1,600-1,740 myr (900-760 myr): Rodinia supercontinent coalesced and fragmented (Silver and Behn 2008)
Neoproterozoic Era, duration 458 myr (1,000-542 myr)
1,700-1,958 myr (800-542 myr): atmospheric oxygen above 10 percent of present levels (Canfield 2005)
1,745-1,920 myr (580-750 myr): Sturtian, Marinoan, Gaskiers glaciations (Eyles 2008)
1,750 myr (750 myr): green algae present (Catling et al. 2005)
1,770-2,000 myr (730-500 myr): organic carbon burial increases sixfold as a result of increasing productivity of land biota as oxygen content of atmosphere increases (Kennedy et al. 2006)
1,850-1950 myr (550-650): Pannotia supercontinent (Meert and Lieberman 2004; Silver and Behn 2008)
1,920 (580 myr): end of Neoproterozoic glaciations (Eyles 2008), oxygen content of atmosphere begins to increase (Narbonne 2005; Fike et al. 2006), major salt deposition in rift valleys (Knauth 2005), lichenlike organisms present in shallow marine waters (Yuan et al. 2005), tiny spinose egg capsules (acritarchs) containing embryos of archaic marine animals (Yin et al. 2007), deep oceans still anoxic (Fike et al. 2006; Canfield et al. 2007)
~ 1,924 myr (~576 myr): diversification of spiny acritarchs, followed by Acraman impact event (Williams and Wallace 2003; Fike et al. 2006)
1,924 myr (575 myr): warming to 20-35 C (Knauth 2005), onset of oxygenation of deep oceans and beginning of diversification of Ediacaran marine biota (Fike et al. 2006; Canfield et al. 2007)
1,945 myr (555 myr): oxygenation of deep oceans completed, atmospheric oxygen levels rise to at least 15 percent of present levels (Knauth 2005), lichenlike organisms in shallow marine waters, mobile bilaterally symmetrical animals present (Fike et al. 2006)
1,952 myr (548 myr): spiny acritarchs reach maximal diversity, appearance of multicellular organisms with hard parts (Fike et al. 2006)
1,957 myr (542 myr): abrupt decline of Ediacaran marine biota (Narbonne 2005; Fike et al. 2006), Precambrian-Phanerozoic boundary (Amthor et al. 2003)
Most of the mass of silicon-rich crustal rocks presently incorporated into the continents was generated during latest Archean and earliest Proterozoic time (3,200 to 2,000 myr ago) (Castro and Patino-Douce 2001). Individual continental terrains (shield areas) have since been repeatedly assembled into supercontinents and then dispersed by changes in mantle circulation, to be reassembled anew in different configurations (Condie 2002). Archean continents had fragmented by the beginning of Proterozoic time (Aspler and Chiarenzelli 1998). An early Proterozoic supercontinent (Columbia) included most of the shield areas of the globe (Rogers and Santosh 2002). A middle Proterozoic supercontinent (Rodinia) was reassembled with South America in proximity to what would become northeastern North America (Torsvik 2003; Collins et al. 2005). The most recent supercontinent (Pangea) formed ~240 myr ago, in temporal proximity to the origin of the dinosaurs, and then it too fractured into the modern continents.
Ocean temperatures were evidently hot (~70 C) during most of Archean time but declined by about 20 C near the Archean-Proterozoic boundary. After briefly rebounding to Archean levels following an early Proterozoic cold interval, they again cooled to temperatures typical of later geologic time by ~1,000 myr ago (Knauth 2005; De La Rocha 2006; Robert and Chaussidon 2006). Initially high greenhouse gas levels prevented a deep and long-lasting glacial age that otherwise would have resulted from the low luminosity of the primitive Sun (Kaufman and Xiao 2003; Catling and Claire 2005). Atmospheric carbon dioxide levels apparently fell during Proterozoic time. Oxygen levels increased, at first abruptly in an early Proterozoic great oxidation event; they then fluctuated, to stabilize and rise more gradually through the remainder of the Proterozoic (Canfield 2005; Bekker et al. 2006; Bekker and Kaufman 2007). The oxidation event preceded an early Proterozoic glacial interval, perhaps coinciding with a brief biological crisis that forced the evolution of nitrogen fixation by cyanobacteria (Navarro-Gonz lez et al. 2001; Kasting and Siefert 2002). The increase in atmospheric oxygen during the latter part of the eon had the effect of reducing methane to a trace gas and generating a return of glacial climates (Pavlov et al. 2003; Canfield 2005; Knauth 2005). During the final ~60 myr of the eon, after multiple Neoproterozoic glaciations, global temperatures may have warmed to ~30 C (Knauth 2005).
The most severe ice ages known occurred near the beginning and end of the Proterozoic. Their causes are not well defined but may have included changes in solar luminosity, greenhouse gas levels, continental positions, and rapid weathering rates of continental flood lavas (Schmidt and Williams 1999; Corsetti et al. 2001; Lindsay and Brasier 2002; Donnadieu et al. 2003; Godd ris et al. 2003; Poulson 2003; Schaefer and Burgess 2003; Knoll et al. 2004). Glacial deposits seemingly occurred in low latitudes, implying global rather than high-latitude glaciation only (Evans 2006; Irving 2006). Deep oceanic waters remained anoxic (Gorjan et al. 2003), so that ice-free shallow marine oases would have only been present locally during intervals of extreme glaciation (Condon et al. 2002; Donnadieu et al. 2003; Poulson 2003). In the case of at least one late Neoproterozoic ice age, the return from full glacial to full greenhouse postglacial climates was abrupt (Nogueira et al. 2003).

Fig 5.2. Continental ice surrounding Depot Peak, near the Mawson Coast of East Antarctica, is representative of frigid late Proterozoic glacial environments. The local environment is too frigid to support complex Phanerozoic-like ecosystems. Photograph courtesy of Harold Heatwole .
It has been suggested that the spacing of glacial ages through geologic time is approximately cyclic. Their occurrence may not be associated so much with extreme cold and low levels of atmospheric carbon dioxide as with high, mountainous terrain near an abundant marine source of water vapor. Glacial climates may be enhanced further by increased cloudiness produced by high levels of cosmic radiation encountered by the solar system as it passes through spiral arms of the galaxy (Shaviv and Veizer 2003; Marcos de la Fuente and Marcos de la Fuente 2004; Geis and Helsel 2005; Eyles 2008).
Photosynthesis provides the only significant source of atmospheric oxygen. The oxidation of the atmosphere, with its chemical consequences for surface environments, is a result of biologic activity. Oxygen-based respiration supplies ~10 times more energy for body maintenance and growth than do other sources of respiration. It allows single-celled organisms to grow to a larger size, supports longer food chains, and led to the proliferation of multicellular organisms. These possibilities are unavailable to organisms that cannot metabolize oxygen. With oxygen levels at ~10 percent of those in the present atmosphere, single cells could reach dimensions of 1 mm. Multicellular organisms possessing circulatory systems could attain dimensions of 10 mm or more, rendering them large enough to be visible without magnification. Tiny but barely visible fossils deposited after the great oxidation event have indeed been recovered in strata nearly 2,000 myr old (Catling et al. 2005).
Microfossil and genetic evidence in living organisms suggest that a radiation of large, single-celled microbes (eukaryotes) containing complex subcellular structures (organelles) was well underway by mid-Proterozoic time, and perhaps much earlier (cf. Bromham et al. 1998; Knoll and Carroll 1999; Budd et al. 2001; Fortey 2001; Siveter et al. 2001; Javaux et al. 2001; Hedges et al. 2004, 2006; Butterfield 2005, 2007; Derry 2006). By late middle Proterozoic time unicellular eucaryotes had given rise to small multicellular organisms (metazoans). Plants (algae) became distinct from fungi by ~1,600 myr, and fungi from animals by ~1,500 myr. Sexual reproduction has been identified within red algae dating from ~1,200 myr ago, in association with differentiated body cells (Butterfield 2000). At about that time, floating single-celled algae (acritarchs) proliferated in shallow nearshore environments, although they rapidly diminished in abundance toward the open ocean (Javaux et al. 2001). The deep oceans may not have been inhabited by organisms large enough to generate readily visible fossils until ~600 myr ago. After the Neoproterozoic glacial interval, an abrupt decline in oceanic salinity, rising global temperatures, and an increase in atmospheric oxygen may have combined to flood the oceans with oxygen for the first time (Knauth 2005).
A few million years after the Neoproterozoic glaciations, acritarchs differing from those belonging to floating marine algae were found to contain embryonic clusters of cells. These capsules were further distinguished by their spines and most closely resembled reproductive cysts of small aquatic animals (Yin et al. 2007). Spiny acritarchs persisted through an interglacial interval and reappeared in the record after a final Neoproterozoic glacial episode. The impact of a large asteroid apparently did not greatly affect the continuity of spiny acritarch evolution (Williams and Wallace 2003; Fike et al. 2006). An abrupt and unprecedented proliferation of large (a few centimeters to 1.85 m long) multicellular marine organisms (Ediacaran biota) commenced ~575 myr ago, during the warming interval that followed the final Neoproterozoic glacial epoch. Their diversification was paralleled by a gradual increase in the productivity of terrestrial biota (producing the so-called Shuram carbon excursion). Atmospheric oxygen levels reached 15 percent of modern levels (Narbonne 2004; Fike et al. 2006; Canfield et al. 2007). Humans would nonetheless have asphyxiated, for oxygen levels were low relative to post-Proterozoic levels.
The marine Ediacaran biota consisted of soft-bodied forms that differentiated into discs, simple segmented forms, and fronds; the latter were often attached to the ocean floor. Most of them apparently filtered nutrients from water or bottom sediments, and they occurred in abundances similar to those of modern bottom-dwelling marine organisms (Narbonne 2004). Their diversification was sustained by the increase in the oxygen content of the water (Knoll and Carroll 1999; Fike et al. 2006), warm global temperatures (Knauth 2005), and runoff of nutrients from the land (Kennedy et al. 2006). After prospering for ~30 myr (see tabulation of events at beginning of chapter), the Ediacaran biota abruptly disappeared, and a sharp increase in the flux of organic carbon simultaneously occurred in the oceans. The extinction has been attributed to numerous factors, including undetermined stresses of a global scale, destruction of the fossil record by newly evolved burrowing detritus feeders, and/or the appearance of relatively efficient predators that soft-bodied Ediacaran biota could not withstand (Bromham et al. 1998; Dzik 1999; Amthor et al. 2003; Narbonne 2005).
Ediacaran marine organisms nevertheless marked an inflection point in the history of life on Earth. Pre-Ediacaran biota had consisted of microorganisms adapted to an irregular but relatively simple spectrum of changes in physical environments that exhibited no obvious overall trend over the previous 1,500 myr. Their photosynthetic base (cyanobacteria) has persisted as stable and undiversified living fossils for over 2,000 myr. Larger, Ediacaran biota, on the contrary, participated in comparatively dynamic biologic environments wherein organismal change stimulated further change. In order to protect themselves from predation, floating cells became highly ornamented. Spiny acritarchs proliferated and then vanished with most Ediacaran soft-bodied organisms at the end of the Proterozoic (Butterfield 2007).
If the marine record is relatively well known, what, then, took place on land during late Proterozoic time? Terrestrial environments would have been protected from the ultraviolet radiation that followed the great oxidation event of almost 2,500 myr ago (Catling et al. 2005). Photosynthetic cyanobacteria were present on land since 2,200 myr ago (Beukes et al. 2002). Terrestrial soils and freshwater bodies were exposed to low levels of free oxygen long before the surface of the open sea was oxidized (Knauth 2005). Oxygen released by bioproductivity from the land may have begun to drive the oxidation of deep oceans 155 myr before the appearance of the marine Ediacaran fauna (see tabulation of events). Terrestrial microbiota of ancient lineage, such as green algae and tiny animals bearing chitinous exoskeletons (e.g., nematodes, tardigrades, mites), are able to resist desiccation associated with terrestrial habitats. Today, spores and dormant microbial cysts are readily distributed by the wind across continental interiors. Genetic evidence suggests that gilled fungi had appeared by ~900 myr, mosses and protoplants by ~700 myr, the remote arthropod ancestors of modern crustaceans and insects by ~650 myr, tardigrades by ~600 myr, and possibly chelicerates (e.g., archaic spiderlike creatures) and myriapods (millipedes) before the end of the Proterozoic (Regier et al. 2004, 2005; Hedges et al. 2006; Kennedy et al. 2006). Fossil remains of early post-Proterozoic arthropods also point to an earlier Proterozoic origin (Waloszek 2003). Thus far, few (e.g., Prave 2002; Knauth 2005) have sought to explore late Proterozoic terrestrial strata for evidence of the evolution of life on land.

Fig 5.3. The color patterns of lichens on slate, in the foothills of the Sierra Nevada Mountains, central California, suggest the possible beauty of pre-Phanerozoic terrestrial life. Photograph courtesy of Stephen Sharnoff .
If ecosystems containing multicellular organisms were present in terrestrial environments after the last Neoproterozoic glacial period ~580 myr ago, it is possible that they had already survived several intervals of extreme global cold. Thermal oases bordering equatorial oceans may have provided areas where oxygen-dependent land life could have survived (Corsetti et al. 2001; Narbonne 2005). Lichens and liverworts may have been well established in terrestrial environments during terminal Proterozoic time. It is tempting to suppose that locally dense photosynthetic carpets might have nourished symbiotic fungi and been seasonally infested with tiny archaic arthropods. Such simple ecosystems could have persevered in sheltered environments at the edges of continental ice sheets (Redecker et al. 2000; Heckman et al. 2001a; Porter et al. 2003; Gibson and Zale 2006).
Microbial organisms (bacteria and single-celled eukaryotes), invisible or nearly invisible to the unassisted eye, defined the presence of life for over two-thirds of the known history of life on Earth. Bacteria were-and are-capable of deriving their metabolic energy from many different sources, among them sunlight. From eukaryotes descended complex single-celled organisms (algae, yeasts, amoebas, paramecia, and euglenas) and ultimately multicellular creatures (Knoll 2003; Martin and Russell 2003). Although these single-celled organisms are capable of surviving great extremes of aridity, temperature, and pressure, it would be inaccurate to suggest that they thrive under such conditions.
As the environmental limits of life are approached, metabolic processes slow. Bacteria living in the near absence of oxygen in sediments beneath the ocean floor possess metabolic rates 100,000 times slower than their counterparts in nearshore environments (Folk and Lynch 1999; Heatwole et al. 2001; Kerr 2001; D Hondt et al. 2002; Mastrapa et al. 2001; Navarro-Gonz lez et al. 2003). Under extreme physical conditions the struggle for existence among simple organisms proceeds very slowly. A red alga ( Bangiomorpha ) has persisted virtually unchanged for 1,200 myr. No more than two or three basic cell types appeared among bacteria during the first 1,400 myr of their existence (Hedges et al. 2004). Oxygen-producing cyanobacteria have remained essentially unchanged for 2,000 myr (Butterfield 2000, 2007).

Fig 5.4. Stromatolites, Shark Bay, Western Australia, consisting of large microbial mounds typical of Proterozoic time, flourish in hypersaline waters. The local environment is too saline to support complex Phanerozoic-like ecosystems. Photograph courtesy of David Jarzen .
Mats of photosynthetic bacteria were present in warm, salty waters along the margins of the seas 2,700 myr ago and are recorded on fossil semiarid soils on land 2,200 myr ago. In contrast to the primeval bacterial mats, by the end of Proterozoic time, bacterial colonies in tidal marine waters formed mounds (stromatolites) a meter or more in height (Nisbet et al. 2007). Oxygen-dependent organisms evolved more rapidly than bacteria (Battistuzzi et al. 2004; Butterfield 2007). By 1,000 myr ago the number of microbial cell types had increased geometrically from less than 10 to approach nearly 100 (McShea 2001; Hedges et al. 2004, 2006). A key transition (symbiosis) occurred ~2,000 myr ago when eukaryotes acquired organelles (mitochondria) that enabled them to process energy rapidly. Another major transition took place a half-billion years later when some eukaryotes assimilated plastids (derived from a photosynthetic cyanobacterium) that enabled them to photosynthesize, and ultimately to give rise to plants.
Sexual reproduction in red algae promoted selection based on the whole organism, suppressing competition between individual cells (Butterfield 2000). Fungi and animals came into existence at about the same time, probably as a result of an algification event that produced simple plants and a more abundant source of food (Hedges et al. 2006). All of these organisms probably withstood long periods of freezing and drought by suspending vital processes. Life on land advanced from drab crusts to tundra that seasonally manifested vividly green hues. Organismal complexity was increasing on timescales of hundreds of millions of years rather than thousands of millions of years, as it had earlier in Earth s history. By the end of the Proterozoic, multicellular marine organisms were beginning to change on timescales of tens of millions of years. The pace of ecological evolution was quickening.
It is reasonable to suppose that the average above-soil height of photosynthetic ecosystems in warm, wet environments increased from 1 cm to a few centimeters through the last ~500 myr of the Proterozoic. Projecting such a trend forward in time by another ~500 myr to the present would predict an above-soil height of ~10 cm for terrestrial ecosystems living today. Instead, one finds that an average height of living tropical rain forests is a staggering ~3,700 cm (Richards 1996), an increase of 370 times. Similarly, a typical large late Proterozoic animal such as a water bear (tardigrade) probably attained a length of 0.33 mm. A modern elephant with body dimensions (shoulder height) of ~3,300 mm would represent a linear increase of ~10,000 times. Rates of change in size of constituent organisms before and after the end of the Proterozoic (Witting 2008, figure 6) suggest an accelerating trend in rates of ecosystem evolution.
To find modern ecosystems similar to those of the late Proterozoic, one must visit sulfurous bogs, hypersaline lakes, hyperarid deserts, and rocky crags surrounded by polar ice. Such environments are inhabited by simple, stressed ecosystems close to the physical limits of life on land. They exemplify living equivalent ecosystems of late Proterozoic biota, for within them, biological time seems to have nearly stood still. Eukaryotes of all sizes figure enormously in terrestrial biomes where environmental conditions are more favorable. They are energy-demanding organisms requiring benign physical environments that promote high levels of biological productivity. Bacteria thrive on organic debris produced by eukaryotes and their multicellular descendants, as well as within their living (and dead) bodies. However, eukaryotes are not displaced from rich environments by bacterial competition. Without eukaryotes, the productivity and biodiversity of productive modern ecosystems might be expected to return to late Proterozoic levels. Thus, low rates of change in microbes (and their poor geologic record) obviate the need here to review their role in terrestrial ecosystems until after the elapse of the additional ~500 myr years of post-Proterozoic time (see chapter 15 ).

Fig 5.5. Body length, cubed, versus time, representing exponentially increasing bacterial/animal body mass through time (after Witting 2008).
The history of Precambrian life thus suggests that interactions between organisms drive rates of evolutionary change in an accelerating manner toward higher levels of fitness. In early evolutionary time, when activity and biodiversity levels were low, evolution proceeded slowly. Later in evolutionary time, as activity and biodiversity levels increased, evolutionary rates accelerated. The physical environment apparently modulates the latter trend, so that acceleration proceeds more slowly under extreme conditions when organisms often survive in suspended animation (hibernation, aestivation). How bacteria could evolve at all in the vacuum of the interstellar medium is difficult to imagine.

Fig 5.6. Lichens and mosses on boulders, north of Qu bec City, Canada, within a fertile late Proterozoic-like environment. Photograph courtesy of Stephen Sharnoff .
It is widely recognized that the physical proportions of organisms change (scale) in a nonlinear (geometric) fashion with respect to size. The legs of an elephant are proportionally more massive than those of a mouse; its movements are smoother and less nimble. It is curious and interesting that the effects of the dimension of time on organismal evolution also seem to be geometric (cf. Vermeij 1987).
There are approximately 1,000 star systems within 30 light-years of the Sun. It is likely that these systems will be scanned for small planets within a few decades. Perhaps as many as 100 nearby systems may contain Earthlike (rocky) planets (Webster Cash, personal communication, 2006). Planets similar to the Proterozoic Earth would be 4,000 myr old, and surfaced with blue oceans and seemingly barren continents. Atmosphere gas composition and floodplains should exhibit indications of the presence of photosynthesis. Someone unfamiliar with the geologic record might not suspect that such planets could be populated by sentient organisms within another half-billion years. However, prospects would certainly look better than those of the present surface environments of Venus and Mars.
Butterfield, N. J. Macroevolution and macroecology through deep time.
Catling, D., et al. Why O 2 is required by complex life on habitable planets and the concept of planetary oxygenation time.
Evans, D. A. D. Proterozoic low orbital obliquity and axial-dipolar geomagnetic field from evaporite palaeolatitudes.
Hedges, S. B., F. U. Battistuzzi, and J. E. Blair. Molecular timescale of evolution in the Proterozoic.
Knoll, A. H. Life on a young planet.
Narbonne, G. M. The Ediacara biota: Neoproterozoic origin of animals and their ecosystems.

Fig 6.1. Patterns in the diversity of multicellular organisms during Phanerozoic time (after Benton and Emerson 2007; courtesy of Michael Benton). For further explanation, see text.
With a duration less than a third (28 percent) of that of the Proterozoic, the Phanerozoic is the shortest of the four eons of Earth history. In contrast to nearly all the preceding history of life, fossils visible to the unassisted eye can be found, often in great abundance, within Phanerozoic strata. The eon is characterized by a great diversification of multicellular organisms that gradually merged into the biosphere we know today. The fossil record of marine life is outlined here because it provides the geologic timescale with subdivisions (periods, epochs, ages) familiar to all students of Earth history. These subdivisions are well calibrated by numerical (radiometric) dates. Warm and cold intervals, changing configurations of continents and oceans, and episodes of mass extinction that influenced the evolution of terrestrial life have all been placed within the marine timescale. The marine record thus provides the temporal framework for the evolution of life on land.
The major divisions of Phanerozoic time (eras) are based on fossil marine shellfish: the beginning of Primary (Paleozoic) time is defined by the oldest abundant appearance of shellfish. The Secondary (Mesozoic) and Tertiary (Cenozoic) eras were recognized by dramatic changes in marine biota resulting from two major extinction events. Phanerozoic eras are now accurately positioned in time by radiometric dates:
Paleozoic: 542-252.5 myr ago (duration 289.5 myr)
Mesozoic: 252.5-65.5 myr ago (duration 187 myr)
Cenozoic: 65.5 myr ago to the present (duration 65.5 myr)
Approximately equivalent divisions based on the terrestrial record ( coal age, age of reptiles, age of mammals ) have been superseded by those of the global marine timescale, and its increasing precision toward the present is reflected in the shortening duration of the major divisions of Phanerozoic time.

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