Evidence of Evolution

Direct evidence of evolution is obtained when animal or plant breeders raise new varieties by cross-breeding. Further evidence is provided by laboratory experiments with bacteria and other primitive forms of life. Since these organisms produce new generations within minutes or hours, scientists can observe evolutionary changes that would take thousands of years in more complex forms. Another type of direct evidence comes from paleontology, the study of fossils, in which scientists trace the history of life on Earth.

There is also a great deal of indirect evidence of evolution. For example, before birth, all animals with backbones—including humans—pass through a stage of development in which they have gill clefts. This fact suggests that these animals are descended from organisms that once lived in water. Another example is the fact that all organisms are similar in certain essential ways. By comparing the traits of different species, we can find certain similarities that indicate that they evolved from a common ancestor. All living cells, whether in a bacterial colony or an elephant, in a tree or a human, undergo similar processes of metabolism, growth, and reproduction.

Some species have organs that are not useful to them. These vestigial organs have been passed down to them from an ancestor who once needed these organs. The human appendix is an example of a vestigial organ. Many species with dark caves as their habitat have vestigial eyes.

Evidence of evolution can also be found in certain genes, such as those for the proteins cytochrome cand hemoglobin, which occur in many species. The subunits, or nucleotides, that make up such genes vary from species to species. However, these nucleotides have been found to be most similar among species whose classification indicates a close relationship. For example, the gene for hemoglobin in humans is much more similar to the corresponding gene in monkeys than to that in kangaroos.

Artificial selection parallels natural selection in many ways, and provides further evidence of evolution. Crop breeders breed plants with traits that are useful, and this results in offspring with new traits. In a few generations, the traits of the species would be different from their ancestors. Wild animals were selectively bred and domesticated this way by our human ancestors.