Peter Egerton had it right the first time: The five digits are a very old
trait, dating back to the early amphibians. Most vertebrates have developed
other numbers of digits by mutation, e.g., the occasional human mutation for
six digits. But a few vertebrate groups have maintained this ancient trait
to the present day.

It may be odd to think of humans as "primitive" when we pride ourselves on
being the pinnacle of creation. The fact is that many of our traits are
relatively unchanged and unspecialized, and this, coupled with high
intelligence, turns out to be a very effective tool kit for survival. Our
feet and hands are not all that different from the original amphibian feet,
as a look at any salamander will show. Our forefeet have not become wings,
as in birds and bats and pterodactyls. Five-fingered hands can climb trees,
pick up and throw objects, or even convey language. Neither cats nor horses
can do most of these things; they are far more specialized than we are.
Think of how specialized these animals are in diet and habitat: anteaters,
tree sloths, rhinoceroses, pandas, beavers, panthers, crocodiles, ostriches.
Yet all made of the same stuff, and all with an ancestor that had five
digits on each limb. The brain is our most specialized organ.

Andrew K. Rindsberg
Geological Survey of Alabama