Homologous structures

Homologous structures are features found in different organisms that are built on the same basic plan because the organisms inherited them from a shared ancestor. Their jobs may be very different. The forelimbs of humans, cats, whales, bats and birds are the classic example: each has one upper arm bone, two forearm bones, wrist bones and digits, even though the limbs are used for grasping, walking, swimming or flying.

Homology is one of the main lines of evidence for evolution, because descent with modification explains why such different animals share a hidden skeleton. It is also the raw material for building phylogenetic trees, since shared inherited features reveal which groups are related. Homologous structures contrast with analogous structures, which look or work alike but evolved independently through convergent evolution, like the wings of insects and birds. A feature can be both: bird and bat wings are analogous as wings but homologous as forelimbs.

Vestigial structures, such as the small hind leg bones of whales, are a special case in which an inherited part has lost most of its original function. The same idea applies to genes that descend from one ancestral sequence.