Life on a Young Planet
Read Life on a Young Planet Online
Authors: Andrew H. Knoll
Life on a Young Planet
Life on a Young Planet | THE FIRST THREE BILLION YEARS OF EVOLUTION ON EARTH |
Andrew H. Knoll
with a new preface by the author
PRINCETON UNIVERSITY PRESS
PRINCETON AND OXFORD
Copyright © 2003 by Princeton University Press
Published by Princeton University Press, 41 William Street, Princeton, New Jersey 08540
In the United Kingdom: Princeton University Press, 6 Oxford Street, Woodstock, Oxfordshire OX20 1TW
First Princeton Science Library printing, 2005
New Princeton Science Library paperback edition, with a new preface by the author, 2015
Paperback ISBN 978-0-691-16553-0
Library of Congress Control Number 2014955273
British Library Cataloging-in-Publication Data is available
This book has been composed in Palatino
Printed on acid-free paper. ∞
Printed in the United States of America
1 3 5 7 9 10 8 6 4 2
For my parents.
In nature and in nurture,
I was lucky.
Contents
Preface to the New Paperback Edition
xi
Chapter 3 Life’s Signature in Ancient Rocks 32
Chapter 4 The Earliest Glimmers of Life 50
Chapter 5 The Emergence of Life 72
Chapter 6 The Oxygen Revolution 89
Chapter 7 The Cyanobacteria, Life’s Microbial Heroes 108
Chapter 8 The Origins of Eukaryotic Cells 122
Chapter 9 Fossils of Early Eukaryotes 139
Chapter 10 Animals Take the Stage 161
Chapter 12 Dynamic Earth, Permissive Ecology 206
Chapter 13 Paleontology ad Astra 225
Acknowledgments
T
HIS VOLUME
distills the thoughts of a quarter century spent trying to understand the early history of life. I first entertained the idea of writing a book more than a decade ago, but fortunately got sidetracked. Children, research, and university responsibilities kept me from reconsidering such a plunge until the fall of 1998, when the alignment of growing kids, the end of my term as department chair, and a sabbatical leave convinced me that the time was right to attempt what for me was a new style of scholarship. Of course, my children weren’t the only ones who had matured in the interim, and so whatever critical fate awaits this volume, I can honestly state that it is far better than it would have been had I completed it at first consideration.
For all the caricatures of scientists as creative loners, science is a richly social endeavor. Our worldviews evolve by reading the works of those who went before, by teaching and learning, through collaboration, conversation, and argument. The ideas and experiences related in the following pages owe much to others, many of them mentioned in one chapter or another. Elso Barghoorn guided my thesis research, providing me with opportunities, support, and a collegiality whose obvious asymmetry he never stressed. My graduate education was shaped as well by Ray Siever, Dick Holland, Steve Golubic, and the late Steve Gould and Bernie Kummel, all of whom seemed to spot in me a potential I would never have seen for myself.
The students and postdoctoral fellows in my lab have been a continuing source of joy and intellectual sustenance, and I thank them all. I have also benefited enormously from wonderful colleagues. My friends in biology and the Earth sciences at Harvard keep me ever on my toes.
In the world beyond Harvard Yard, I am particularly grateful for the friendship and intellectual stimulation of John Grotzinger, Sam Bowring, John Hayes, Malcolm Walter, Roger Summons, Keene Swett,
Yin Leiming, Misha Semikhatov, Misha Fedonkin, Volodya Sergeev, Gerard Germs, Stefan Bengtson, Simon Conway Morris, Brian Harland, Don Canfield, Ariel Anbar, Dave Des Marais, Ken Nealson, Sean Carroll, and my departed friends Zhang Yun, Gonzalo Vidal, and Preston Cloud.
Of course, books are not written in the office or in the field. They get finished in the upstairs study, on weeknights after the homework is done. Book writing, therefore, is very much a family affair. In a profession that commonly rewards obsession, my children have given me the gift of balance in life. And even by confessing that I lack words to articulate my gratitude, I risk trivializing the importance of my wife Marsha.
Much of my research over the years has been funded by the National Science Foundation and NASA, including the NASA Astrobiology Institute. I am grateful for their support. I also thank Dick Bambach, Susannah Porter, Don Canfield, Sean Carroll, Jack Repcheck, Kristen Gager, Lawrence Krauss, and Marsha Knoll for reading my draft manuscript and making many suggestions for improvement. John Bauld, Roger Buick, Stefan Bengtson, Martin Brasier, Birger Rasmussen, Shuhai Xiao, Richard Jenkins, Leonid Popov, Dave Bottjer, Steve Dornbos, Greg Wray, Andreas Teske, Susannah Porter, Bruce Lieberman, and Nick Butterfield provided some of the illustrations that leaven my text. Lastly, I thank Sam Elworthy and Princeton University Press for their unstinting support and confidence. Sam has been my Maxwell Perkins, improving every page of what follows.
Preface to the New Paperback Edition
F
OURTEEN YEARS AGO
, amid millennial predictions of global computer failure and other apocalypses that never transpired, I decided it was time to explain myself. For more than two decades, I had been preoccupied with the attempt to understand an unfamiliar planet—one without plants or animals, and with little or no oxygen in the atmosphere. That planet was the young Earth, and how our globe transited from that early, alien state to the world we know today struck me (and still does) as the greatest story Earth science has to tell. In part the quest was paleontological, requiring the meticulous examination of ancient rocks in search of fossilized microbes. In part it was phylogenetic, using the information recorded in genes to sketch out the tree of life, a universal genealogy that makes predictions for life’s geologic record. And in part, the task was geochemical, using the chemistry of ancient rocks to reconstruct Earth’s environmental past. Could we develop a narrative of life’s deep history that stretches from the origin of life to the spread of animals throughout the oceans more than three billion years later? Could we construct a parallel account of environmental history that traces the rise of atmospheric oxygen and episodic ice ages of global extent? And, most importantly, could we combine our narratives of life and environment to understand how organisms and their surroundings have co-evolved through time? The result was
Life on a Young Planet
, first published in 2003. The decision to reissue this work in a new format affords an opportunity to revisit my earlier effort and consider how the field has progressed over the past decade.
Standing back, the big picture remains much the same: a limited and challenging record of the Archean (> 2.5 billion years ago) Earth that nonetheless documents microbial ecosystems in a world
with little or no O
2
; a long middle age populated by protists as well as prokaryotes, building diversity and cellular complexity beneath an atmosphere with some but not much oxygen; and a dramatic shift in life and environment around 600 million years ago, ushering in our (broadly) familiar world of large animals and abundant oxygen. And the field vignettes still ring true, hopefully providing readers with a sense of how Earth scientists work and not simply a catalog of facts.
That said, many people around the world have made important contributions to this story since I first told my version. For example, while it still makes sense to divide life’s history into three great chapters mediated by oxygen, the details of that environmental history have become far more nuanced, and new debates have emerged. We still think that the Archean atmosphere had, at best, minute traces of oxygen, but several lines of evidence now suggest that more substantial O
2
accumulations began to form at least intermittently within the oceans much earlier. The subtle and still debated record of Earth’s early “whiff of oxygen” predates the Great Oxygenation Event 2.4 billion years ago by as much as several hundred million years. Increasing evidence also suggests that when oxygen began to accumulate in the atmosphere and surface ocean, it rose to relatively high levels before declining again by two billion years ago.
When I wrote
Life on a Young Planet
, the Proterozoic Eon (2.5 to .542 billion years ago) was just coming into focus as environmentally distinct: more oxygen-rich than the Archean but less so than the Phanerozoic Eon (the past 542 million years). Building on an insightful model by Donald Canfield, Ariel Anbar and I had sketched out what Proterozoic environments may have looked like and how this might have influenced the course of evolution, but data were sparse. Fortunately, a decade of research, using chemical proxies that range from the sedimentary distribution of iron to the isotopic composition of molybdenum, has reinforced the idea of a distinct middle age to Earth history. How much oxygen was present in the atmosphere remains a topic of research, but recent estimates suggest that O
2
concentrations remained low through most of the eon, perhaps only a few percent (or even less) of today’s levels.
Some geochemical data indicate major oxygenation of Earth’s deep oceans near the end of the Proterozoic Eon, but other observations suggest that, despite this event, the climb from low Proterozoic oxygen levels to our familiar O
2
-rich world was protracted, continuing well into the Paleozoic Era. The beginning of Earth’s second oxygen revolution broadly coincides with the appearance of large animals in the fossil record, but whether a changing environment facilitated animal evolution or evolving animals drove environmental change remains a topic of debate—both may be true. Recent research on oxygen minimum zones—oxygen-poor water masses in modern oceans—indicates that the levels of O
2
required to support carnivores and other metabolically demanding animals can be relatively modest, so physiologically significant environmental thresholds may well have been crossed near the end of the Proterozoic Eon, even though global oxygen abundances remained well below modern levels. Among other things, this means that the interwoven histories of oxygen and animals didn’t end with the Cambrian radiation but continued well into the Phanerozoic “age of visible animals.”
And what of the fossil record itself? Again, the broad picture presented in 2003 still holds, but new discoveries continue to yield fresh insights. Our understanding of life in early Archean oceans remains limited: more evidence supports a role for microbial mats in building Earth’s oldest stromatolites, and isotopic evidence tells us that 3.5 billion years ago, microorganisms cycled both carbon and sulfur through the oceans. But while Emmanuelle Javaux and her colleagues have now described convincing microfossils from 3.2 billion year old shales, potentially biogenic microstructures in still older rocks remain a challenge to paleontologists. In contrast, the Proterozoic microfossil record, already rich in 2003, continues to grow, with whole new classes of microfossil—for example, phosphatized scales that armored early eukaryote cells—complementing an ever increasing inventory of cyanobacterial remains. Both microfossils and molecular clocks suggest that eukaryotic organisms appeared early in the Proterozoic Eon but radiated much later, an observation that might find explanation in the relatively late evolution of eukaryotic cells that ingest other eukaryotes. Such a
functional innovation would have fueled protistan diversification much in the same way that carnivores are thought to have facilitated Cambrian animal radiation.
New macrofossils continue to be described from Ediacaran rocks, and their foreign shapes still inspire creative interpretations. One sea change: many paleontologists now interpret the quilted fossils so common in Ediacaran strata as structurally simple animals—not much more than upper and lower cell layers surrounding a fluid or jelly-filled interior—that fed by absorbing organic molecules and exchanged oxygen and other gases by diffusion. That idea isn’t so far-fetched.
Trichoplax
, a tiny animal first discovered in a seawater aquarium, lives in just this way, and, if you think about it, so do you. Your body absorbs organic molecules through your intestines, after you’ve swallowed your food and broken it down by digestion.