by Peter Ward
From PW: For Dr. Howard Leonard and Dr. Peter Shalit, life historians, and to the great Robert Berner, Yale University
From JK: To the memories of Dr. Eugene M. Shoemaker, Dr. Heinz A. Lowenstam, and Dr. Clair C. Patterson of Caltech. They and many others left fingerprints all over my brain.
Contents
Introduction
CHAPTER I
Telling Time
CHAPTER II
Becoming an Earthlike Planet: 4.6–4.5 GA
CHAPTER III
Life, Death, and the Newly Discovered Place In Between
CHAPTER IV
Forming Life: 4.2(?)–3.5 GA
CHAPTER V
From Origin to Oxygenation: 3.5–2.0 GA
CHAPTER VI
The Long Road to Animals: 2.0–1.0 GA
CHAPTER VII
The Cryogenian and the Evolution of Animals: 850–635 MA
CHAPTER VIII
The Cambrian Explosion: 600–500 MA
CHAPTER IX
The Ordovician-Devonian Expansion of Animals: 500–360 MA
CHAPTER X
Tiktaalik and the Invasion of the Land: 475–300 MA
CHAPTER XI
The Age of Arthropods: 350–300 MA
CHAPTER XII
The Great Dying—Anoxia and Global Stagnation: 252–250 MA
CHAPTER XIII
The Triassic Explosion: 252–200 MA
CHAPTER XIV
Dinosaur Hegemony in a Low-Oxygen World: 230–180 MA
CHAPTER XV
The Greenhouse Oceans: 200–65 MA
CHAPTER XVI
Death of the Dinosaurs: 65 MA
CHAPTER XVII
The Long-Delayed Third Age of Mammals: 65–50 MA
CHAPTER XVIII
The Age of Birds: 50–2.5 MA
CHAPTER XIX
Humanity and the Tenth Extinction: 2.5 MA to present
CHAPTER XX
The Knowable Futures of Earth Life
Notes
A Note on the Authors
* * *
Introduction
* * *
History in almost any form is the academic subject perhaps most hated by school kids. One of the most thoughtful examinations was by James Loewen, in Lies My Teacher Told Me.1 His conclusion can be summarized in one word: “irrelevance.” Loewen wrote, “The stories that history textbooks tell are predictable; every problem has already been solved or is about to be solved … Authors almost never use the present to illuminate the past—the present is not a source of information for writers of history texts.”
Loewen’s message is quite clear. As American history is now taught in high schools, the past and present are disconnected, such that history has no effect on, or relevance to, our day-to-day lives. Yet that conclusion is so untrue, especially for the history of life, so ancient it is written in rocks, molecules, models, and on the DNA strands found within our every cell. Its relevance is that it gives us place and context. The history of life also just might save us from near-term extinction, if we take note of it and heed its warnings.
In the early 1960s the great American writer James Baldwin wrote: “People are trapped in history, and history is trapped in them.”2 He was speaking to race when he penned those words. But the statement is equally true if the word “people” is replaced with “all of life on Earth, present and past,” for each strand of DNA, in each of our cells, is an ancient record of biological history, written in simple code and passed down from generation to generation. One could say that DNA is nothing but history, one with a physical manifestation that was slowly melded and accumulated over countless eons by the most pitiless of all phenomena—natural selection. DNA is a history that is inside us—and yet one that is our master as well, the blueprint for our bodies, and dictator of what we will pass on to our children, gifts that can be blessings—or deadly time bombs. We are indeed trapped in this particular vehicle of history as much as it is trapped in us.
The history of life gives us the answers to so many of the perplexing questions confronting us all: How did we humans come to inhabit this thin, late-occurring, and very marginal twig on the giant tree of life? What wars did our species have to pass through? What calamities mark the human branch of life’s 4-billion-year-old tree? The past can help us understand our place among the twenty or more million species now living—and the untold billions now extinct. When a species is no longer extant, what is also destroyed is the as yet unrealized future evolution of untold species.
In the pages to come we will look at the long road to our present and the distant trials our long-ago ancestors had to pass through: fire, ice, hammer blows from space, poison gas, the fangs of predators, pitiless competition, lethal radiation, starvation, enormous changes of habitat, and episodes of war and conquest amid a relentless colonization of every habitable corner of this planet—each an episode that left its mark in the total sum of DNA now extant. Each crisis and conquest was a forge that changed genomes by adding or subtracting all manner of genes, each of us the descendant of survivors tempered by catastrophe and quenched by time.
There is a second and perhaps even greater reason to pay attention to the history of life encapsulated in this quote from Norman Cousins: “History is a vast Early Warning System.”3 This wisdom dates back to near the end of the Cold War. Newer generations of humans have little sense of what it was like to grow up in the 1950s and 1960s, when weekly noontime siren tests told us children of that dark time that Armageddon was but one terrifying siren scream away, and that every faint sound of a late-night jet might be the start of the end.
Human warfare has repeatedly, and seemingly incessantly, exacted a hideous toll on humanity—physically, economically, and emotionally. In many ways the history of life has undeniable similarities to human conflict and warfare. The coevolutionary development of offensive weapons by predators (better claws, teeth, gas attack, even poison-tipped barbs to catch and kill food species) caused equally rapid countermeasures in the predators’ prey, including better body armor, speed, hiding ability, and sometimes defensive weapons as well—all of which is technically called the biological arms race. Many of the great events of evolution cannot be repeated; evolution has had a long period to fill the biosphere with highly competitive and efficient organisms, making it unlikely to repeat, for instance, the Cambrian explosion when all of the basic body plans of animals came into being. But what can be repeated are things antipodal to living and diversifying, such as extinction, or extinction writ larger—the dreadful past catastrophes of deep time, the mass extinctions.
With every molecule of carbon dioxide we pump into the atmosphere we are ignoring the early warning sirens that rapid rises in carbon dioxide are the commonality between more than ten mass extinctions of the deep past and what is happening today. Those extinctions were caused not by asteroid impact, but from rapid increases in volcanically produced atmospheric greenhouse gases and the global warming they produced. A terrifying new paradigm of mass extinction has arisen this century: “greenhouse mass extinctions,” a name overtly chosen to describe the cause of the vast majority of species killed off by mass extinctions in the past.4
Evidence of when, where, and how these greenhouse extinctions took place now blares at us from a wide variety of data. To those hearing these siren calls, the danger seems real enough. Yet too many have ignored or missed vast moralities from the past, and what is a possible future. The history of life provides an early warning system that tells us we must reduce human-caused greenhouse gas emissions, but it is human history that tells us that we probably will not heed the warnings and reverse the damage until a succession of climate-induced mass human mortalities gives us no choice.
Scienti
fic information from so-called deep time is the single most ignored aspect of the climate change debate. George Santayana wrote the most oft-repeated aphorism about history, one so commonly used that it is trite: “Those who ignore history are condemned to repeat it.”5 With regard to a clear history of mass extinction brought about by atmospheric carbon dioxide levels rapidly approaching in the near future, however, we should especially take heed of the most important word in Santayana’s prophecy: the word “condemned.”
WHAT IS NEW IN THIS “NEW HISTORY OF LIFE”
No single book could ever do justice to the history of life. Choices had to be made, and those choices were largely dictated by our directive around the word “new.” The last “complete” single-volume history of life was written in the mid-1990s: the exquisite and bestselling Life: A Natural History of the First Four Billion Years of Life on Earth,6 by the British paleontologist and science writer Richard Fortey. His “takes” were marvelous and the book remains a joy to read, or in our case, to reread nearly two decades after its publication. But because science advances so quickly, there is much that was not known then compared to now. There are even two new scientific disciplines that barely existed in the mid-1990s: astrobiology and geobiology. The advances in instrumentation have led to entirely new understanding, while outcrops of strata containing fossils from times or taxa previously unknown have come to light. Even the sociology of how science is done has changed, for now the most important scientific breakthroughs are acknowledged to take place between the boundaries of the previously august and familiar disciplines of geology, astronomy, paleontology, chemistry, genetics, physics, zoology, and botany—each symbolically separated into their own buildings on most university campuses, each with not only a faculty with its own rules and boundaries, but entire fields with their own vocabularies and favored methods of disseminating information coming from research.
We have used three themes in the pages to come as lodestones for the history we have chosen to feature. First, we posit that the history of life has been more affected by catastrophe than the sum of all other forces, including the slow, gradual evolution first recognized by Charles Darwin—based on his training by the dominant teachers of uniformitarianism. The guiding principle of geology for more than two centuries—the Principle of Uniformitarianism—was first developed by James Hutton and Charles Lyell in the late 1700s.7 It was taught to and ultimately became the prime scientific influence on generations of young naturalists, including Charles Darwin.8 The discovery of the dinosaur-killing asteroid that hit our planet 65 million years ago was the start of this paradigm shift toward one that has been sometimes called neocatastrophism,9 a take on catastrophism, the paradigm that preceded uniformitarianism.
As we will show in this book, uniformitarianism, as it applies to ancient worlds as well as the mode and tempo of evolution, is outmoded and largely refuted. The modern world is not the best tool for explaining many times and events in the deep past that indeed were sudden—not gradual. For example, there are no modern examples that can explain the “snowball Earths,” or the “great oxygenation event,” or the sulfur-rich “Canfield oceans” that lasted for more than a billion years and in all that time impeded the first evolution of the animal grade of complexity. Even the dinosaur killing K-T Cretaceous Period–Tertiary Period mass extinction (now termed the K-Pg, or Cretaceous-Paleogene Period, but we hope our colleagues forgive us if we stick to the better sounding and better known K-T) has no parallel today; nor the type of atmosphere and ocean that allowed life to form on Earth; or an atmosphere with carbon dioxide levels so high that there is not a scrap of ice anywhere on the planet. The present is not a key to most of the past; in fact, it is barely a key to the Pleistocene. Making it so has limited us in our vision and understanding.
Second, while we may be carbon-based life, composed of “long-chain” carbon molecules (carbon atoms strung together to form proteins), it is the influence of three different kinds of molecules, simple molecules that exist as simple gases, that have had the greatest influence on the history of life: oxygen, carbon dioxide, and hydrogen sulfide. Sulfur, in fact, may have been the single most important of all elements in dictating the nature and history of life on this planet.
Finally, while the history of life may be populated by species, it has been the evolution of ecosystems that has been the most influential factor in arriving at the modern-day assemblage of life. Coral reefs, tropical forests, deep-sea “vent” faunas, and many more—each can be viewed as a play with differing actors but the same script over eons of time. Yet we know that on occasion in the deep past entirely new ecosystems appear, populated by new kinds of life. The appearance of life that can fly, for instance, or life that can swim or walk—each was a major shift in evolutionary innovation that changed the world, and in each case helped create a new kind of ecosystem.
WHAT WE BRING
An author’s background affects the biases inherent in any written history. Peter Ward has been a paleobiologist since 1973, and has published extensively on modern and ancient cephalopods as well as on mass extinctions of vertebrates and invertebrates. Joseph Kirschvink is a geophysical biologist who began his work on the Precambrian-Cambrian transition, but then expanded to looking at older times (the great oxidation event) as well as being the discoverer of the snowball Earths—major parts of life’s history. Together we have subsequently worked on the Devonian, Permian, Triassic-Jurassic, and Cretaceous-Tertiary (this time interval recently renamed the Paleogene Period) mass extinctions.
We have worked together in the field since the mid-1990s. These trips included study of the Permian mass extinction in South Africa, from 1997 to 2001; the study of Upper Cretaceous ammonites in Baja California, California, and the Vancouver Island region; the study of the Triassic-Jurassic mass extinction in the Queen Charlotte Islands; the study of the K-T mass extinction in Tunisia, Vancouver Island, California, Mexico, and Antarctica; and the study of the Devonian mass extinction in Western Australia.
The voice we bring to this book is meant to be a seamless duet, but there are passages where one or the other of us self-identifies because of the nearness of the topics to some particular interest we have, or because we were integral in the history of some aspect of the science being reported.
NAMES AND TERMS
Earlier we noted that the number of species on Earth is in the millions. Most who study life will acknowledge that the current number of formally defined species (which requires a name for both genus and species) is probably less than 10 percent of the actual number of currently living species.10 But how many have there been in the past? Billions, certainly. That makes the writing of a history of them a daunting process. Paleontology, biology, and geology all have entire vocabularies of highly specific jargon, and it is our job to use the English language in an understandable way to make sense of so much of the multisyllabic jargon—or, in the case of NASA, decipher their endless acronyms. Perhaps even more daunting, by necessity we will have to introduce many of the Latin names for the many creatures great and small that produced and daily continue life’s history on Earth.
Finally, a full acknowledgment of the large number of people helping us in our journey to write this book will come at the end of the text. But Ward would like to specifically shout out to two scientist-writers who have profoundly influenced him: Robert Berner, whose work on oxygen and carbon dioxide is absolutely integral to the work written here, and Nick Lane, a prolific scientist and writer whose books are pinnacles of clarity and insight, whose work profoundly influenced at least one of the coauthors, and whose books remain groundbreaking and current.11
CHAPTER I
* * *
Telling Time
* * *
Until recently, the history of life had an arcane time scale, measured not in years, but in the relative positions of rocks scattered about the Earth’s crust. In this chapter we will look at the geological time scale, the tool used in discovering the relative sequence of life�
�s history on Earth.
The geological time scale is a rickety old contraption, held together by nineteenth-century rules and current European formality. The newer generations of geologists do not love the hoary and very stuffy series of conventions involved with the time scale, and still required by an increasingly aged set of geologists who were trained in the old tradition. To this day, any change has to be approved by committees;1 all time units have to be associated with a “type section”—a real stack of sedimentary rocks chosen to best represent a given time interval. The type section is supposed to be readily accessible and must be undisturbed by tectonism, heating, and “structure” complexity (such as faults, folds, and other tricky mashing of the originally horizontal sedimentary beds). The section should not be upside down (which happened more often than one would think), should have lots of fossils (both macro and micro), and should also have beds, fossils, or minerals that can be dated with “absolute” ages (a date in actual years) through some combination of radiometric age dating, magnetostratigraphy, or some form of isotope age dating (such as carbon or strontium isotope stratigraphy).
The time scale is complicated and often useless in the sense that when someone says a rock is Jurassic in age, they are in reality saying the rock in question is of the same age as the designated type section for the Jurassic, which was in the Jura Mountains in Europe. But it is what we Earth and life historians have to work with to discover the age of rocks by their fossils, as well as to communicate their actual age to others. Although more modern tools than dating events and species based on their relative position in piles of sedimentary rock are sometimes available2—including the determination of a fossil’s actual age through the use of isotopic dating, such as the well-known use of carbon 14 or other kinds of “radiometric” dating using the known rates of decay of various elements contained in the rock—in fact very few fossils are found in beds or are made up of materials allowing this kind of absolute age dating. Usually it is fossil content only that is available, yet from this the rock must be dated.
