Why are the forelimbs of humans, whales, dogs, and bats homologous?

The forelimbs of humans, whales, dogs, and bats are considered homologous because they share a common evolutionary origin.12

In biology, homology refers to structural similarities between different species that are inherited from a common ancestor. Although these animals use their forelimbs for vastly different functions—such as writing, swimming, running, or flying—the underlying skeletal structure remains remarkably consistent. The bones in these limbs are arranged according to the same basic pattern, representing adaptive modifications of the forelimb structure present in their early mammalian ancestors.123

This structural correspondence is a key piece of evidence for evolution. Because these limbs were modified from an inherited structure rather than designed from scratch for specific purposes, they often exhibit imperfections that reflect their ancestral history. This is distinct from analogy, where structures perform similar functions but do not share a common evolutionary origin.312