Homologous structures

Homologous structures are anatomical structures in organisms that share the same basic form. For example, the bones in the appendages of humans, dogs, birds, and whales all share the same overall construction resulting from their origin in the appendages of a common ancestor. Over time, evolution led to changes in the shapes and sizes of these bones in different species, but they have maintained the same overall layout.

Figure 1: Homologous Arm Structure of 4 Animals

The similar construction of these appendages indicates that these organisms share a common ancestor. Analogous structures, in contrast, have the same function but do not necessarily derive from the same ancestry, as some analogous structures evolved convergently.