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 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.
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).
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 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 much larger work. Those who want to know more will find the key publications in the bibliography; they can also consult the exhaustive reviews of Devonian limbed vertebrates by Clack (2002, 2006). Another recent and very technical synthesis (Hall, 2007) covers the diversity, function, and evolution of fins and limbs and presents points of view not all of which are compatible with those found in this book (see Laurin, 2007). The history of our ideas about the origin and first evolutionary radiation of limbed vertebrates was recently summarized by Coates et al. (2008). Finally, a detailed review of the hypotheses about the origin of extant amphibians was recently published (Anderson, 2008), along with a commentary presenting a different perspective (Marjanović and Laurin, 2009).
This is a translation of a book initially published in French (Laurin, 2008b). The text and bibliography were updated (several papers published in 2008 and 2009 were added), and a few references to especially important older studies were also added.
To the reader who may wonder how paleontological research can be useful, I answer simply that it enables us to know our distant history. Like archeology, paleontology is a historical science. Such research does not normally lead to patents, but it enables us to satisfy our curiosity and it has played an important role in the development of science fiction, especially since the discovery of Mesozoic dinosaurs. From Jules Verne’s Journey to the Center of the Earth through Michael Crichton’s Jurassic Park, paleontology has played a central role in popular culture. The reader will discover that reality can be as fascinating as fiction.
I thank the colleagues who helped me write this book. Joseph Segarra has given me much advice and many comments on the French edition of this book. Various colleagues (Vivian de Buffrénil and Louise Zylberberg) and students (Aurore Canoville, David Marjanović, and Laëtitia Montes) of the team “Squelette des vertébrés” have proofread chapters of the French edition of this book. Christopher A. Brochu, Stephen Godfrey, Michael S. Y. Lee, David Marjanović, Sean P. Modesto, and Robert R. Reisz read chapters of this English translation. Douglas Henderson allowed me to reproduce his very nice reconstructions of early stegocephalians in their habitat. My former thesis advisor, Robert R. Reisz, has played a central but indirect role in drawing my attention to Paleozoic stegocephalians and in communicating his enthusiasm for the study of this episode in vertebrate evolution. My greatest debt lays with my parents, who have always actively supported my studies, and even the fairly bold project (which I had first imagined in the 1970s) of becoming a paleontologist.