What are homologous structures?

Homologous structures are anatomical features in different species that share a common ancestry, even though their current functions may differ. These structures arise from evolutionary processes, illustrating how species have adapted over time. For example, the forelimbs of humans, whales, and bats have similar bone structures (humerus, radius, ulna) despite being adapted for different functions: manipulation, swimming, and flying, respectively. This similarity reflects their shared evolutionary origin.

In contrast, structures with different origins but similar functions are termed analogous structures, and options describing completely different forms or those serving the same purpose do not capture the essence of homology, which is rooted in shared ancestry. Thus, the focus on the structural similarity arising from evolution rather than current functionality is what defines homologous structures.