1. Homologous Structures
Unit 8

3. This evolutionary tree shows the relationships between different tetrapod lineages, all of which evolved from a single common ancestor. This 350 million year old animal, the first tetrapod, had limbs with one long bone (the humerus) attached to two other long bones (the radius and ulna). Its descendents, including whales, lizards, humans, and birds, as well as many others, inherited the tetrapod limb from this ancestor.
Similar structures inherited from a common ancestor are called homologous structures, or homologies. 3