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.

Homologous Structures

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.

Vestigial Organs

Some organisms have structures or organs that seem to serve no useful function. For example, humans have a tailbone at the end of the spine that is of no apparent use. Some snakes have tiny pelvic bones and limb bones, and some cave-dwelling salamanders have eyes even though members of the species are completely blind. Such seemingly functionless parts are called vestigial organs or structures. Vestigial organs are often homologous to organs that are useful in other species. The vestigial tailbone in humans is homologous to the functional tail of other primates. Thus vestigial structures can be viewed as evidence for evolution: organisms having vestigial structures probably share a common ancestry with organisms in with organisms in which the homologous structure is functional.

Biochemistry

Biochemistry also reveals similarities between organisms of different species. For example, the metabolism of vastly different organisms is based on the same complex biochemical compounds. The protein cytochrome c, essential for aerobic respiration, is one such universal compound. The universality of cytochrome c is evidence that all aerobic organisms probably descended from a common ancestor that used this compound for respiration. Certain blood proteins found in almost all organisms give additional evidence that these organisms descended form a common ancestor. Such biochemical compounds, including cytochrome c and blood proteins, are so complex it is unlikely that almost identical compounds would have evolved independently in widely different organisms. Further studies of cytochrome c in different species reveal variations in the amino acid sequence of this molecule. For example, the cytochrome c of monkeys and cows is more similar than the cytochrome c of monkeys and fish. Such similarities and differences suggest that monkeys and cows ate more closely related than are monkeys and fish. Scientists have similarly compared the biochemistry of universal blood proteins. Their studies reveal evidence of degrees of relatedness between different species. This evidence implies that some species share a more recent common ancestor than other species do. From such evidence scientists have inferred the evolutionary relationships between different species of organisms.

The similarities described above are not the only ones scientists have noticed among organisms of different species. The image to the left shows that embryos of certain species develop almost identically, especially in the early stages. Such physical similarities indicate that there are genetic similarities between the organisms. These similarities can be considered evidence that the organisms shown probably descended from a common ancestor.

The similarities between living species-- in ancestry, in homologous and vestigial structures, in embryological development, and in biochemical compounds-- all could be explained as extremely remarkable coincidences. However, a far more probable explanation of these similarities is that species have arisen by descent and modification from more ancient forms. Additionally, the fossil record contributes compelling evidence that species have changed over time. The fossil evidence and evidence from living organisms strongly suggest that species evolve.