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Inheriting homologies

Whales, lizards, humans, and birds all have the same basic limb layout. But how did such different animals wind up with the same sort of limb? The answer is that they inherited it from a common ancestor, just as cousins might inherit the same trait from their grandfather.

In a family tree, traits such as hair color and poor eyesight are passed from generation to generation.

This family tree shows several generations starting with a pair of grandparents. In the same way, an evolutionary "family" tree shows relationships over much longer periods of time.

In an evolutionary tree, traits are passed from ancestor to descendent.

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) with a branching series of smaller bones (carpals, metacarpals and phalanges) on the end. Its descendents, including whales, lizards, humans, and birds, as well as many others, inherited the tetrapod limb from this ancestor.

Structures inherited from a common ancestor are called homologous structures, or homologies.

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