COMPARATIVE ANATOMY

the comparative study of the body structures of different species of animals in order to understand the adaptive changes they have undergone in the course of evolution from common ancestors. The field is largely confined to the study of the vertebrate animals. Modern comparative anatomy dates from the work of Pierre Belon, who in 1555 showed that the skeletons of humans and birds are constructed of similar elements arranged in the same way. From this humble beginning, knowledge of comparative anatomy advanced rapidly in the 18th century with the work of the Count de Buffon and Louis-Jean-Marie Daubenton, who compared the anatomies of a wide range of animals. In the early 19th century, Baron Georges Cuvier placed the field on a more scientific basis by asserting that animals' structural and functional characteristics result from their interaction with their environment. Cuvier also rejected the 18th-century notion that the members of the animal kingdom are arranged in a single linear series from the simplest up to humans. Instead Cuvier arranged all animals into four large groups (vertebrates, mollusks, articulates, and radiates) according to body plan. Another great figure in the field was the mid-19th-century British anatomist Sir Richard Owen, whose vast knowledge of vertebrate structure did not prevent him from opposing Charles Darwin's theory of evolution. Darwin made extensive use of comparative anatomy in advancing his theory, and it in turn revolutionized the field by explaining the structural differences between species as arising out of their evolutionary descent by natural selection from a common ancestor. Since Darwin's time, the study of comparative anatomy has centred largely on body structures that are homologousi.e., ones in different species that have the same evolutionary origin regardless of their present-day function. Such structures may look quite different and perform different tasks, but they can still be traced back to a common structure in an animal that was ancestral to both. For example, the forelimbs of humans, birds, crocodiles, bats, dolphins, and rodents have been modified by evolution to perform different functions, but they are all evolutionarily traceable to the fins of crossopterygian fishes, in which that basic arrangement of bones was first established. Analogous structures, by contrast, may resemble each other because they perform the same function, but they have different evolutionary origins and often a different structure, the wings of insects and of birds being a prime example of this.