Embryos in Deep Time: The Rock Record of Biological Development

Prologue

xi The diversity of life is usually presented in evolutionary trees: A branching pattern culminates in figures of animals and plants. This is good, as trees convey the common history that organisms , including ourselves, share. But there is a limitation in this kind of representation. The organisms portrayed are static entities , usually adults with the recognizable features of their species . In reality, organisms change throughout their lives. If we wanted to portray the biology of biodiversity, we could show for each organism a high-speed film of the different phases of its life history—for a multicellular organism, from fertilization to death. The kinds and number of changes that occur over the life course are so numerous and complex that different scientific disciplines are devoted to each phase of the process. The first steps of cell division and formation of an embryo are associated with embryology. Much research in this area concerns, for example, gastrulation, the phase in which the germ layers are formed and the body plan of the mature organism is established. Developmental biologists tend to study the point of occurrence or oriPrologUe xii / Prologue gin of tissues and organs. Comparative anatomists and zoologists are usually concerned with changes occurring after birth, which some biologists like to call “growth,” as opposed to development. For all these disciplines, molecular biology has offered methods and concepts to address a whole new set of questions concerning the mechanistic bases of life history evolution, also reviving old ones posed by morphologists. Individual development is a rich subject of investigation in biology. But what about the fact that most species represented on the tree of life are extinct? Some may think that developmental biology needs paleontology like a fish needs a bicycle. They might also claim that the fossil record appears to be totally silent about many aspects of developmental evolution and genetics . I do not think this is the case, and I have written this book to explain why. Of course, paleontology remains largely silent about major topics such as gastrulation. Defining the limits of what paleontology can achieve is thus important. In this context, I am reminded of the contributions of a famous fish anatomist to the discussions in the 1980s about the role of fossils versus molecules in reconstructing the tree of life. Colin Patterson (1933–98), a major figure in the study of fossils who was based for decades in the paleontology department of the Natural History Museum in London, strongly advocated the primacy of information on living species over fossils when investigating evolutionary relationships . He was severely criticized by most fellow paleontologists . Several years later his ideas became much appreciated, and in 1996 he was awarded the Romer-Simpson Medal, the highest recognition of the Society of Vertebrate Palaeontology. The role of paleontology in reconstructing the tree of life is largely recognized by most biologists, in spite of the limitations of working with fragmentary data, which contrasts greatly with the Prologue / xiii large database of information (e.g., genomic) now known for an increasing number of living species. The integration of information from paleontology and embryology has a long tradition. During the Victorian era, for example, Thomas Huxley made major contributions in both fields; he was the first to suggest that birds are related to dinosaurs , for example, and he discovered major aspects of the early life history of cnidarians, the group to which corals and jellyfish belong. In fact, many researchers are continuing to make major contributions in these diverse fields. For example, Phil Donoghue , in Bristol, studies conodonts, basal vertebrate animals that have been extinct for about 200 million years, and also has a research program concerning micro RNAs and their role in morphologic diversification. Brought together, his paleontological , developmental, and molecular studies are leading to a better understanding of the history of life. In this book I examine what we may learn about development directly from the fossil record. Fossils are not just the static objects of defunct animals. With a discerning eye and the appropriate information and conceptual background we can learn much about the reproduction and development of the extinct animal . It would seem that the snapshot of some stage of development that a fossil provides is surrounded by so many unknowns that the interpretation of the often incomplete anatomy remains speculative at best. But there is a method in the study of incomplete fossils, and the inferences drawn by most...