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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere INTRODUCTION For practitioners of the vibrant new field of astrobiology, the study of life in the Cosmos, who peer ever outward into the universe, the Holy Grail is to discover at least one other Earth-like planet (and, of course, at least one other kind of life—alien life). Yet what exactly is an “Earth-like” planet? As anyone who has watched or read any science fiction knows, such a planet has water and a breathable-oxygenated atmosphere. But we are coming to realize that our view of an Earth-like planet usually means one with an atmosphere similar to that found on Earth at the present-day. Yet our current atmosphere is but a slice of a forever-changing entity and is greatly different from the atmosphere at most times in Earth’s history. It is currently suited to us mammals—hence the high diversity of mammals alive today. But again, this has not always been the case. Two not-so-ancient versions, in astrobiological timescales, of our “Earth-like atmosphere” very nearly wiped out our furry ancestors some 250 million years ago and then tried again some 200 million years ago. If a small premammal named Thrinaxodon, whose delicate skulls have been collected in lowest Triassic strata, had not survived, what would life on Earth be like now? Perhaps we would have a diverse and unbelievably beautiful world of birds, in the air, on the ground, diving deeply into the sea, and perhaps they would be the dominant animals on Earth.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere But we mammals did survive. The characteristics we now equate with “mammalness” are the consequence of these near-disasters during times of low-oxygen atmosphere, and also from times when oxygen was far higher than now. For most of our history we were the size of rats, sluggishly gasping in a low-oxygen world as we avoided the active dinosaurian overlords. The thesis of the pages to come is deceptively simple: The history of atmospheric (and hence oceanic) oxygen levels through time has been the most important factor in determining the nature of animal life on Earth—its morphology and basic body plans, physiology, evolutionary history, and diversity. This hypothesis means that the level of oxygen influenced every large-scale evolutionary adaptation or innovation that is the history of animal life on Earth, that oxygen levels dictated evolutionary originations, extinctions, and the architecture of animal body plans. Support for this hypothesis will make up the chapters that follow. THE HISTORY OF LIFE We begin a very brief version of the standard history of animal life on Earth with Charles Darwin. Darwin hoped that the history written in the fossil record would sooner or later support his contention about his then (1859) new theory of evolution: that change came about in small increments. In his On the Origin of Species he wrote: From the beginning of life on earth there was a connection so close and intimate that, if the entire record could be obtained, a perfect chain of life from the lowest organism to the highest would be obtained. But this was hope, not history. A new understanding came a half-century later from Charles Wolcott, discoverer of the Burgess Shale, a Canadian rock deposit that gives our best look at the nature of what is known as the Cambrian Explosion, a short time more than 500 million years ago when most animal body plans rather suddenly appeared in the fossil record: In early times the Cephalopoda ruled, later on the Crustacea came to the fore, then probably fishes took the lead but were speedily out powered by the Saurians, the Land and Sea Reptiles then prevailed until Mammalia appeared upon the scene when it doubtless became a struggle for supremacy until Man was created.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere Wolcott wrote this passage a century ago and since then much has been learned about the sequence of events making up the history of animal life. Recent books by Richard Dawkins (The Ancestors Tale) and Richard Fortey (Life) emphasize the history of life during the time of animals, from about 600 million years ago to the present-day. New discoveries about Earth’s atmosphere, however, render even these most recent histories outdated. The major events that all authors agree on as being consequential include (1) the origin of animals during the Cambrian Explosion, resulting in the first appearance of common animal fossils in the geological rock record of sedimentary strata; (2) the lower Paleozoic diversification in the seas, when a further expansion in numbers of marine species following the Cambrian Explosion resulted in a widespread shallow-oceanic shelf-fauna dominated by calcareous shelly organisms and the first coral reefs; (3) the mid- to late Paleozoic colonization of land, with successive and overlapping colonization by land plants, arthropods, vertebrates, and mollusks; (4) the late Permian mass extinction, an event that eliminated the majority of marine and land life and in so doing allowed a new mix of survivors to become dominant on land and in the sea; (5) the transitional world of the Triassic, a time on both land and sea when there were many new body plans appearing, including the first dinosaurs and true mammals; (6) the Jurassic and Cretaceous Age of Dinosaurs, including a characteristic marine fauna of shelled cephalopods and large flat clams in the sea; (7) the late Cretaceous diversification of modern-day land and sea life, followed by the enormous, asteroid-induced mass extinction that ended the Mesozoic Era; and (8) the Tertiary diversification of mammals. Each of these intervals or events in the history of life constitutes a chapter here. Before beginning this examination, let’s look at the concept of history and how it is studied and interpreted in the physical sciences—especially the history of life. EXAMINING HISTORY There are four important questions in the study of any history: What? When? How? Why?
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere The first is the most basic: what happened? The what is the data of the story—in our case, the actual occurrences in the fossil or genetic record. The second, the when, is also straightforward, for it is the chronology that binds together the events of history into a linear, temporal succession. The third, the how, is often deeper and more difficult to learn than the previous two. It is the actual mechanism or motive that drove the historical record. Then, there is the lasting question—why. Sometimes it is very straightforward. More often that not, however, the underlying why of a history is the most difficult question to answer and is the subject of endless debate and revision for good reason. Along with being the most difficult question to answer, it is often the most interesting. Stephen Jay Gould was clearly concerned as well with why? in studying the history of life, when in his book, Wonderful Life, he wrote: How should scientists operate when they must try to explain the results of history, those inordinately complex events that can occur but once in detailed glory? Many large domains of nature—cosmology, geology and evolution among them—must be studied with the tools of history. The appropriate methods focus on narrative, not experiment as usually conceived. The stereotype of the “scientific method” has no place for irreducible history. But is the history of life—in this case, animal life—really irreducible? The scientific method, even laboratory experiment, is indeed not only relevant but also crucial to explain this particular history, the history of life. In various laboratories around the globe scientists are increasingly conducting experiments, many dealing with growing animals under varying oxygen content, to explain life’s history. So how do these questions apply to the history of animals and their great groupings of individual body plans that we call phyla? In terms of evidence, this history is a series of beginnings and endings: A particular group first appeared at a given time, flourished (or not), and finally died out (or continued into the present-day) in a linear, chronological order. The what of this history is the identity of the organisms themselves and comes from two sources of data: from a zoology of the living that is based on two centuries of taxonomic study and more recently on decoding and comparing genetic sequences from groups of extant
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere animals and from a paleontological study of the past over about the same two-century interval. Both of these endeavors have consumed the lives of men and women who have traveled to the ends of Earth to discover, classify, and catalog the extant and extinct biodiversity. The when of this history has proven as difficult to discover as the what but for different reasons. Early geologists had no conception of the true age of Earth and had absolutely no way of discovering that great age. The one or two efforts to estimate the age of the planet, such as Lord Kelvin’s innovative (and ultimately hugely wrong) effort to use the heat flow from Earth to estimate its age, yielded woefully low estimates. Discovering the age of individual fossils was even more impractical, and thus the chronology of life’s history obtained from the fossil record was only a relative one. The succession of strata, one layer piled upon another, was slowly and painfully discerned by a century of stratigraphic study, eventually producing the geological timescale of eras, periods, and ages in use today. The first and last appearances of the major vertebrate groups that make up the what of life’s history could, with the construction of the geological timescale, be put into relative succession. But the actual age of each event, such as the first appearance of mammals, or the last of dinosaurs, remained unknown until the inception of rock dating using radioactive age determination. Yet even without knowing the actual period in which mammals arose or dinosaurs died, paleontologists had, by the middle part of the 1950s, arrived at an accurate picture of animal evolution. While in the 50 or so years since then science has filled in innumerable details and developed a better understanding of the length of this history, in the larger view little has changed. The third question of life’s history—the how—has also remained little changed for a long time—since the nineteenth century, in fact. Charles Darwin, with his great theory on the evolution of organisms, gave an explanation as to why there has been a history of life. The diversification of the great vertebrate groupings came about through evolutionary processes. We obviously now know a great deal more about how evolution works than did the life historians in Darwin’s day, but the overall explanation of how this works remains the same: evolution is responsible for providing the mechanism in this history of life.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere We thus come to the final and most troubling question of history: the why? Why did the history of life unfold as it did? From the time of Darwin all the way until today there remains a group of nonscientists who find little trouble in answering this question: it has been God’s will. But for those who choose to follow the methodology and philosophy of science, the why question concerning the history of life has been most troubling. Enormously interesting and complicated questions can be asked: Why does a group of organisms diversify? Why do we observe particular body plans and morphologies and what functions do they serve? Why are there so few (or so many) of any particular taxonomic unit, such as the orders or families of reptiles, or of mammals, for instance? Indeed, why are there mammals and reptiles at all? Why not some other biological reality? Why not truly flying fish, or water-breathing mammals, or even fire-breathing dragons, for that matter? Why are there no more dinosaurs, for instance? Now we can begin to see that the world of animals that now exists is just one of any number surely possible, at least theoretically. But for a particular battle in human history the map of nations would be radically different, and but for a particular disaster so too could the book of animals now on the planet be a radically different volume. The animals of Earth are the way they are because of their history and because of the history of physical and biological conditions on Earth. They are as they are so as to be able to live on a planet with a given temperature range, a given overall pH, a given level of water, and given levels of other chemicals active in the biosphere. They are also as they are because of the history of events affecting the planet. New discoveries in our understanding of fundamental changes in the composition of Earth’s atmosphere, especially its oxygen levels, through time require us to reexamine the accepted whys of animal evolution. AN OUTLINE OF CHAPTERS This book is a reinterpretation of selected and important events and evolutionary breakthroughs during the past 550 million years, the time of animals, showing in chronological fashion why this author believes that it was the varied kinds of adaptation to varying oxygen levels that was the major stimulus to evolutionary change among the animal
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere phyla. The evidence comes from both the history we have observed and what we know of animals living today. Following two introductory chapters, one discussing why animals need oxygen and the other recounting how various researchers have deduced the history of oxygen levels through time, the focus is on the new appearances and disappearances of various taxa and illustrating how specific adaptations among the various animal groups support the larger thesis of the importance of oxygen in forging evolutionary changes and results. Many of the evolutionary results are themselves newly hypothesized. By the end of this book, readers will come to appreciate the critical role of changes in atmospheric oxygen levels on the history of animal life on Earth and why there were dinosaurs and why there are birds.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere A late Triassic mammal-like reptile under predatory attack by two early dinosaurs. Here, the inferior lung of the mammal group is pitted against the superior lung system of the dinosaurs.
Representative terms from entire chapter: