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.
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.
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.