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How do homologies provide evidence that species share a common ancestor?

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A diagram showing the hierarchy of animal classification in taxonomy, with colored boxes arranged from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, and species. — taxonomy Animals and other organisms are classified within a succession of nested groups that ranges from the general to the particular.

In biology, homology refers to similarities in structure, physiology, or development between different species that result from their descent from a common evolutionary ancestor. These shared traits provide evidence for common ancestry because they represent modifications of an inherited structure rather than designs created independently for a specific purpose.123

For example, the forelimbs of humans, bats, and whales are constructed from the same basic pattern of bones. While these limbs have been modified through evolution to perform diverse functions—such as writing, flying, or swimming—their underlying structural correspondence indicates they were inherited from a common ancestor.432

This evidence is further supported at the molecular level. Scientists compare homologous DNA or protein sequences across species; the degree of divergence in these sequences reflects the evolutionary distance between organisms. By analyzing these inherited similarities, researchers can reconstruct the phylogeny, or evolutionary history, of organisms, helping to map the "tree of life" and determine how species are related through common descent.453