Why do we have five digits on each limb?
Dear Straight Dope:
Why do we have five digits on each limb? If we had only four, our natural numbering system would presumably be octal, a much more sensible arrangement. In that vein, why do we have the same number of toes as "fingers"? (I use the quotes to forestall the "thumbs are not technically fingers" controversy.) Lesser lights (biologists, chemists, physicians) have let me down. You are my only remaining hope for enlightenment.
Why? Because the ancestor of all mammals had five digits on each limb. The five-digit condition evolved among the first land tetrapods (four-footed creatures), so the evolution went--as far as we can tell--very rapidly from fins to five toes, possibly with seven and/or six toed intermediates. (There are some aquatic or semi-aquatic fossils with more than five digits, but they may be evolutionary dead ends--side branches of the proverbial tree--rather than ancestors of ours.) Pentadactyly was passed on to the Therapsids, and this feature was passed on to their surviving descendants, the mammals, many of which retained five digits, including the ancestral primates.
If you're seeking a bioengineering reason for five digits, there isn't necessarily an adaptive explanation. Five doesn't have to be optimal in order for it to persist. Mutations causing extra or missing digits are common (occasionally people are born with six on each limb, for instance), and as long as this mutation doesn't interfere with survival, it may persist and be passed on by chance, and it can also die off by chance. There simply haven't been any such mutations on the line leading to primates, though many other vertebrates do have reduced numbers (the extreme being animals such as horses which have one functional digit, though the vestiges of the other digits are still there). Evidently four works in many cases, but that doesn't prove that four is better than five.
Many people forget that "natural selection" means the eventual selective elimination of variants that are at a disadvantage relative to other existing variants. Five doesn't have to be "best" from a design standpoint; it just has to have no significant difference in reproductive value from any other heritable mutant variations. (There are many minor features of human anatomy that are suboptimal from a design standpoint, but still persist, and even major flaws such as our backwards retina and our crossed larynx and esophagus.) Would a four or six-fingered person be expected to have significantly more children simply because their hand design was somehow slightly better? Under neutral selective conditions, the only way a four or six-digit mutant will spread is by genetic drift, which is an incredibly long shot. If six was no worse than five, and the ancestral primate had by chance been a six-fingered mutant, then you'd be asking me about why we have six digits instead.