Comparing Anatomy and Embryology

What do homologous structures and similarities in embryonic development suggest about the process of evolutionary change?

By Darwin's time, scientists had noted that all vertebrate limbs had the same basic bone structure, as shown in Figure 16–14. Yet, some were used for crawling, some for climbing, some for running, and others for flying. Why should the same basic structures be used over and over again for such different purposes?

Homologous Structures Darwin proposed that animals with similar structures evolved from a common ancestor with a basic version of that structure. Structures that are shared by related species and that have been inherited from a common ancestor are called homologous structures. Evolutionary theory explains the existence of homologous structures adapted to different purposes as the result of descent with modification from a common ancestor. Biologists test whether structures are homologous by studying anatomical details, the way structures develop in embryos, and the pattern in which they appeared over evolutionary history.

FIGURE 16–14 Homologous Limb Bones Homologous bones, as indicated by color-coding, support the differently shaped front limbs of these modern vertebrates. These limbs evolved, with modifications, from the front limbs of a common ancestor whose bones resembled those of an ancient fish. If these animals had no recent common ancestor, they would be unlikely to share so many common structures.

dd

Similarities and differences among homologous structures help determine how recently species shared a common ancestor. For example, the front limbs of reptiles and birds are more similar to each other than either is to the front limb of an amphibian or mammal. This similarity—among many others—indicates that the common ancestor of reptiles and birds lived more recently than the common ancestor of reptiles, birds, and mammals. So birds are more closely related to crocodiles than they are to bats! The common ancestor of all these four-limbed animals was an ancient lobe-finned fish that lived over 380 million years ago.

Homologous structures aren't just restricted to animals. Biologists have identified homologies in many other organisms. Certain groups of plants, for example, share homologous stems, roots, and flowers.

Table of Contents