Biologists believe that for 100 million years the only vertebrates on
Earth were water-dwelling creatures, with no
arms or legs. At some
point these "fish" began to develop hips and legs and eventually were
able to walk out of the water, giving the earth its first land lovers.
Once the land-dwelling creatures evolved, there were some mammals that
moved back into the water. Biologists estimate that this happened about
50 million years ago, and that this mammal was the ancestor of the
modern whale. Despite the apparent uselessness, evolution left traces
of hind legs behind, and these vestigial limbs can still be seen in the
modern whale. There are many cases where
whales have been found with
rudimentary hind limbs in the wild, and have been found in baleen
whales, humpback whales, and in many specimens of sperm whales. Most of
these examples are of whales that had only leg bones, but there were
some that included feet with complete digits. It was reported recently
that whales and hippos were distantly related.