What type of structures are similar due to common ancestry?

Structures that are similar because of common ancestry are called homologous structures. They may perform different functions, but they retain a comparable underlying anatomy inherited from a shared ancestor.

The forelimbs of vertebrates provide a classic example. A human arm, a bat wing, a whale flipper and a horse foreleg contain corresponding bones, including the humerus, radius and ulna. Evolution modified the ancestral limb for grasping, flying, swimming or walking.

Homology therefore reflects divergent evolution: related lineages inherited a basic structure and adapted it to different environments. Similarity alone is not enough; scientists also examine position, development and detailed anatomy when determining whether features are homologous.

Homologous structures are commonly confused with analogous structures. Analogous structures have similar functions but evolved independently, such as the wings of birds and insects. Vestigial structures are different again: they are reduced remnants whose original function has been diminished, such as the human appendix or whale pelvic bones.