How Vertebrates Left the Water

Preface

xi PREFACE Life appeared in the oceans in a past so distant that it is difficult to imagine. The exact age of life on Earth is debated because the structures once considered to represent the oldest fossils (remains of ancient organisms, or traces which they left) have been reinterpreted as mineral crystallization in microscopic fractures by some paleontologists (this reinterpretation is itself debated). The first life forms were very simple and resembled extant bacteria, some of which formed stromatolites , the oldest of which are about 3 Ga (billion years) old (Fig. p.1). Stromatolites are still being formed today in some coastal regions. For at least 1.5 Ga, life remained in its native aquatic environment. Thus, for the greatest part of the history of the biosphere, life remained in water, diversified, and radiated into several ecological niches. The oceans and seas teemed with life well before the first animal ventured out of the water. In the last few hundreds of millions of years (Ma), life has come onto dry land. This transition was very gradual; it was initiated by simple life forms, such as bacteria. Later, more complex organisms ventured onto land: lichens, simple green plants (the first of which were mosses, horsetails, and lycopods), arthropods (arachnids, insects, crustaceans, etc.), mollusks (slugs and snails), annelids (earthworms, leeches), and Figure p.1. Cnidarians. The first metazoans (animals with several cells) were all marine. Cnidarians are among the oldest and simplest metazoans. Reproduced from Haeckel (1904). Preface / xiii vertebrates. Despite their late arrival in this new environment, vertebrates will be emphasized in this book because they include humans and nearly all our domestic animals (dogs, cats, birds, cattle, sheep, pigs, horses, etc.). Thus, most readers are probably more interested in vertebrates than in any other group. The conquest of dry land is a fascinating evolutionary problem because all systems and organs of our distant ancestors had been adapted to their aquatic habitat through hundreds of millions of years of evolution. This episode in the history of life on Earth is probably one of the most difficult to understand, and precisely because of this, it is no doubt one of the most interesting. The problems that our ancestors had to solve were so severe that some creationists have used them to try to cast doubt on the scientific study of biological evolution and to try to strengthen the case of their creationist “explanation” (this word is not entirely appropriate in this context) of biodiversity. We will see that scientists have formulated several theories that explain this fascinating history, and that one of the main challenges of modern paleontology consists of testing these theories through more or less indirect methods. This book summarizes what we know about this history, without hiding the gaps that remain in our knowledge. It also presents the methods used by paleontologists, these “detectives” of life history, to reconstruct our distant past. To avoid the excessive simplifications that too often reduce this type of book to “just-so stories,” a few technical terms, for which there is no vernacular equivalent, must be introduced. The reader should refer to the Glossary, which includes all these technical terms. Despite the modular organization of this book, I advise reading Chapter One, “How Can We Reconstruct Evolutionary History ?” first. A brief section on extant vertebrates illustrates the surprising amount of data that can be extracted from contemporary species, but for obvious reasons, the emphasis of this synthesis is on fossils and the evolution of the first land vertebrates. Finally, in the conclusion, the reader will discover that, contrary to the “Indiana Jones” stereotype, paleontologists do not necessarily spend a great proportion of their xiv / Preface time excavating fossils in the field, and that a major part of the most fundamental discoveries results from the study of fossils first described by older generations of scientists, or from sophisticated analyses of databases that centralize data that have long been available but used to be scattered. This book is mostly for life and earth science students who want to learn the basics of modern paleontology, systematics, and evolutionary biology, or those interested in the history of the conquest of land by vertebrates. It requires little prior knowledge in this field. Some points are covered in sufficient detail to give the reader a sense of how science works, but this book does not attempt to cover all relevant facts, because this would result in a...