Homologous structures

Evidence from Living Organisms
By examining fossils and by determining their relative and absolute ages, scientists have collected evidence that supports the theory that species changed over time. Further evidence is derived from living organisms. In order to determine if species change scientists compare common ancestry, structure, biochemistry, and development of organisms alive today. As you read this section, study this evidence and critically evaluate whether it indicates that species may have arisen by descent and modification from ancestral species. Evidence of Common Ancestry
If species change over time, then scientists should be able to cite examples showing that a group of living species may have come from a common ancestor. Let us examine one of many cases for which this seems to be true. Gracing the islands of Hawaii is a family of birds commonly called the Hawaiian honeycreepers. All Hawaiian honeycreepers have similarities in skeletal and muscle structure that indicate they are closely related. However, each of the Hawaiian honeycreeper species has a bill specialized for eating certain foods. Scientists suggest that all 23 honeycreeper species apparently arose from a single species that migrated to Hawaii.


If a bat, a human, an alligator, and a penguin all evolved from a common ancestor, then they should share common anatomical traits. In fact, they do. Compare the forelimbs of the human, the bat, the penguin, and the alligator. Find the humerus, radius, ulna, and carpals in each forelimb. Though the limbs look strikingly different on the outside and though they vary in function, they are very similar in skeletal structure. More significantly, they are derived from the same structures in the embryo. Structures that are embryologically similar, but have different functions, are called homologous structures. Though these animals look different, a comparison of homologous structures indicates that they are quite similar. This suggests that these animals evolved from a common ancestor.