8.20: Comparative anatomy and embryology reveal common evolutionary origins.

If you observe any vertebrate embryo while it’s developing, you will see that it passes through a stage in which it has little gill pouches on the sides of the neck. It will also pass through a stage in which it has a long bony tail. This is true whether it is a human embryo or that of a turtle or a chicken or a shark. The gill pouches disappear before birth in all but the fishes. Similarly, we don’t find humans with tails. Why do these features exist during an embryo’s development? Such common embryological stages indicate that the organisms share a common ancestor, from which all have been modified (FIGURE 8-38). Study of these developmental stages and the adult body forms of organisms provides our third line of evidence for the occurrence of evolution.

Among adult animals, several features of anatomy reveal the ghost of evolution in action. We find, for example, that many related organisms show unusual similarities that can be explained only through evolutionary relatedness. The forelimbs of mammals are used for a variety of very different functions in bats, porpoises, horses, and humans (FIGURE 8-39). If each had been designed specifically for the uses necessary to that species—flying, swimming, running, grasping—we would expect dramatically different designs. And yet, in each of these species we see the same bones—modified extensively—betraying the fact that they share a common ancestor. These features are called homologous structures.

At the extreme, homologous structures sometimes come to have little or no function at all. Such evolutionary leftovers, called vestigial structures, exist because they had value in an ancestor species. Some vestigial structures in mammals include the molars that continue to grow in vampire bats, even though these bats consume a completely liquid diet (FIGURE 8-40); eye sockets (with no eyes) in some populations of cave-dwelling fishes; and in whales, pelvic bones that are attached to nothing (but in nearly all other mammals serve as an important attachment point for leg bones). Even humans may have a vestigial organ—the appendix. It is greatly enlarged in our relatives the great apes, acting as host to cellulose-digesting bacteria that aid in breaking down the plants in the apes’ diet, but in humans it seems to serve no purpose (although recent studies suggest it provides some immune system support).

Not all organisms with similar-looking adaptations actually share recent ancestors. We see flying mammals (bats) and flying insects (locusts) (FIGURE 8-41). Likewise, dolphins and penguins live in similar habitats and have flippers that help them swim. In both examples, however, the analogous structures developed from different original structures. Natural selection, in a process called convergent evolution, may act on different starting materials (such as a flipper or a forelimb) and modify them until they serve similar purposes—much as we saw in the marsupial and placental mammals in Figure 8-37.