Brief History of Animals

The central focus of this book is to identify the genetic mechanisms underlying the evolution of animal design, particularly with regard to the patterning of animal body plans and body parts. To approach this mystery, new discoveries and ideas from developmental genetics must be integrated into the larger framework of the evolutionary history of animal life. This history is reconstructed from many fields of study—in particular, paleontology, systematics, and comparative biology. In this chapter, we present a brief overview of animal evolution from these three perspectives. This discussion provides a historical foundation for the consideration of the mechanistic questions that are addressed in subsequent chapters.

First, we discuss the origin of animals and the radiation of the major animal phyla based on evidence gleaned from the fossil record. Most living phyla have ancient origins, and the fundamental differences between them evolved long ago. Two milestones in early animal history that are of special interest are the evolution of bilaterally symmetrical animals and the explosive radiation of these forms in the Cambrian period more than 500 million years ago.

Second, we examine the phylogenetic relationships among animals. Understanding the direction of evolutionary change in morphological, developmental, or genetic traits and the ability to make inferences about animal ancestors requires knowledge of the structure of the animal evolutionary tree. While traditionally based upon morphological comparisons, new phylogenies based on DNA and protein sequences have revealed unexpected relationships among anatomically disparate animals, refuting long-held notions about which phyla are more closely related.

Third, we consider the comparative anatomy of selected phyla with the aim of identifying some of the major trends in the evolutionary diversification of individual phyla. In particular, we focus on the modular organization of the body plans and body parts of larger animals—the vertebrates, arthropods, and annelids. Much of the large-scale morphological diversity within these phyla (for example, between different classes) involves differences in the

. . . an understanding of regulation must lie at the center of any rapprochement between molecular and evolutionary biology; for a synthesis of the two biologies will surely take place, if it occurs at all, on the common field of development.

—Stephen Jay Gould Ontogeny and Phylogeny (1977)

number and pattern of modular elements (segments, appendages, and so on). The recognition of the modular organization of these animals is an important conceptual link to understanding the genetic logic controlling their development and the mechanisms underlying the evolution of diversity.

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