Five fingers

Were the Earth to be started over again with all its physical features identical, its extremly unlikely that anything closely resembling a human being would ever again emerge. There is a powerful random character to the evolutionary process. A cosmic ray striking a different gene, producing different mutation can have small consequences early but profound consequences late. Happenstance may play a powerful role in biology, as it does in history. The farther back the critical events occur, the more powerfully can they influence the present.
For example, consider our hands. We have five fingers, including one opposable thumb. They serve us quite well. But I think we would be served equally well with six fingers including a thumb, or four fingers including a thumb, or maybe five fingers and two thumbs. There is nothing intrinsically  best about our particular configuration of fingers, which we ordinarily think of as so natural and inevitable. We have five fingers because we have descended from a Devonian fish that had five phalanges or bones in its fins. Had we descended from a fish with for or six phalanges, we would have four or six fingers on each hand and would think them perfectly natural. We use base ten arithmetic only because we have ten fingers on our hands. Had the arrengement been otherwise, we would use base eight or base twelve arithmetic and relegate base ten to the New Math.

Carl Sagan, Cosmos.