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Our five fingers is presumably an efficient amount, though, given that the earliest land amphibians had 8 fingers or so, but never lost more than 5, so that must be the most usable amount for us.
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Early amphibians had a variety of digits, but it was those with five that became the ancestors of successive tetrapods. It is likely to have been nothing to do with the number of digits, rather the other adaptations that the pentadactyls had.