Comparison of genetic material, homologous structures, and fossil records can be used to support common ancestry amongst organisms.
DNA and amino acid sequences can be used support the claim that two organisms share a common ancestor. Simply put, the more DNA sequences and/or amino acid sequences that organisms have in common, the more closely related they are.
Homologous structures are bones or organs that are found in different species that are share anatomical and functional similarities. Therefore, organisms that share homologous structures are though to also share a common ancestor. Examples of homologous structures are the arm of a human, the limb of a horse or cat, the wing of a bat, and the fin of a whale.
Fossil records are also used to support the claim that two species share a common ancestor. Fossil records provide evidence of the progression of the formation of organisms over time. Such records may demonstrate divergent evolution. Divergent evolution results when two or more species evolve from a single ancestor. Environmental isolation of subpopulations of a species may result in divergent evolution. The subpopulations will evolve due to natural selection of each environment’s unique characteristics. An example of divergent evolution would be the different finches found the Galapagos islands.