An interesting question --
The persistence of pentadactyly is to me one of the obvious proofs of species evolution. It is what might be called the "default" condition of all tetrapods. It is true of Amphibians and Reptiles. The mammalian order of Primates is one of the more conservative ones, and its retention of the pattern is clear. The same is true of the Insectivora (shrews and moles), Chirpoptera (Bats), Monotremes (Platypuses and Echidnas), Marsupials, Sirenia (Manatees and Dugongs), Edentata (Armadillos, Pangolins, Aardvarks, Anteaters), and even seals and their relatives (Pinnipeds), with the bones of five fingers in their flippers. Departures from pentadactyly are later adaptations: Cetaceans, for example, often have hyperphalangy-- more than the usual number of joints per digit. Most of the more derived mammalian orders have fewer than five digits, often three or four, though usually with some vestigial evidence of the missing digits. Most Dinosaurs had five digits, though later forms lost one or two-- theropods and their descendants, the birds, being a case in point. Like the Cetaceans, the marine forms like Plesiasaurs and Ichthyosaurs often had more than five digits with many joints in their flippers, a convergence . Even coelacanths have five incipient digits in their limblike fins. So the reason we have ten fingers is because that was the number the first tetrapods had. Apart from mutations, genes are quite conservative -- also an argument against "Intelligent Design", along with many other things (such as, why design from scratch a human female who gives birth via a narrow channel within the pelvic girdle, when it would be easier and safer to have babies born through the stomach, as in a Caesarian?).
