This question occurred to me whilst washing my hands: why do we have five fingers? Why not four, or six? Is five so much more efficent than those numbers? (And what’s the deal with the “four fingers and a thumb” argument? Is it just smartassery?)
We don’t have five fingers (or more precisely, four fingers and a thumb) unless you have only one hand.
My understanding why we have five digits on each hand is luck. AFAIK it is just a configuration that worked well enough.
Make it “You don’t have five fingers…” in the previous post.
I guess the Darwinian answer would be “Because that what suits our environment and allows us to best survive in it.” Truth is, somewhere way, way back, it probably made a difference and other variations just got naturally-selected out. The first thing that occurs to me is that five digits allows for a nice balance of dexterity with (every engineer’s favorite) excess capacity.
And for what it’s worth, some people do have six fingers on each hand.
We have four fingers and a thumb per limb because 'Eomaia" our oldest known placental mammal had a similar arrangement. Since we are here to observe ourselves, it follows that this arrangement was the best one available that combined the dexterity, strength and flexibility necessary to shinny up shrubs and trees, both to find food and to avoid proto-cats which liked to nibble at proto-mice, so the design did not dead-end.
Or, if you prefer the ‘intelligent design’ theory, we have that configutation of fingers and thumbs so that we can operate keyboards and trackballs effeciently.
To distinguish us from the Simpsons characters 
Hox4 genes control (among other things) digit expression in a developing embryo. There are four subgenes within the Hox4 complex - 4.4, 4.5, 4.6, 4.7 and 4.8 - which directly control the formation of digits. These genes are not expressed sequentially, but rather in a nested form. Thus, digit I might have the expression (4.4+4.5+4.6+4.7+4.8), digit II (4.4+4.5+4.6+4.7), and so on, with digit V having only 4.4 expressed (these digit numbers do not necessarily correspond to our anatomical numbering, wherein the “thumb” is digit I, and our pinky finger is digit V).
There are, then, only five “addresses” whereby a digit can be produced. In instances of polydactyly, the extra finger is a duplicate of one of the existing digits, not a new invention. Note also that the Hox genes control the identity of the digits formed, but not their exact morphology (the same hox genes are used to specifiy the identities of both fingers and toes, for example).
Because Hox genes are essentially “master development genes”, messing with the genes themselves has rather wide-reaching effects. As such, mere duplication of one of the Hox genes (thereby providing an extra “address” for the expression of an extra true digit) is likely to have more detrimental effects elsewhere during development.
Oh…another thing. That we have five fingers does not necessarily have anything to do with any adaptive consequences. It is more likely the result of a combined genetic & developmental constraint, which in turn limits the degree of variation that can occur. That is, while a sixth (true) digit may be evolutionarily advantageous (as, for example, in the case of the panda), it can’t happen because of this constraint. Thus, again using the panda example, other means had to “found” in order to create its faux sixth digit.
It’d be confusing if we had less/more fingers than toes. 
I’m a psych major, but here goes. The thumb is different from the fingers. Fingers have three joints; the thumb has two. All toes have two joints, that why the big toe is the big toe rather than the big digit or something like that. Its’ a bit smartassish, but its’ true.
Technically, the thumb has three joints as well; the third one, which makes it opposable, is at the base of the wrist. The main difference between the thumb and other fingers is that the thumb has only two phalanges (digit bones), while the other fingers have three each (the first metacarpal is used as a third phalange in the thumb). Similarly, the “big toe” (hallux) only has two phalanges, while the other toes have three each.
Really?
:recounts:
Whoops! I missed that little one in the middle. Thanks!
Good show Darwin’s Finch!
Hell you just opened my mind in ways indescribable by deftly pointing out the practical consequences posed by “combined genetic & developmental constraint” as an evolutionary mechanism. To boot I think I understood what you were talking about, even when you went ‘deep gene sequencing’ on us.
Thanks!
Sparc
Mr. Alfonseca, meet my friend Indigo. He has something he’s been wanting to tell you.
Actually, the great thing about Alfonseca is that he always gives 120%.
Darwin’s Finch:
“That we have five fingers does not necessarily have anything to do with any adaptive consequences. It is more likely the result of a combined genetic & developmental constraint, which in turn limits the degree of variation that can occur.”
That we have no MORE than five fingers may be due to genetic constraints, but that we have had five for well over 125 million years certainly speaks to adaptation. Could all placentals have randomly developed five digits by coincidence and maintained that configuration through an evolutionary process which developed H. Sapien from Eutheria?
Well, all placentals HAVEN’T maintained the 5-digit configuration - horses, for instance, have only one functional digit (plus some vestigal remnants of earlier digits, known as the splint bones). It just happens that the lineage that gave rise to us DID maintain that primitive 5-digit characteristic, probably because it was useful in the contrext of a tree-climbing lifestyle, but possibly just by coincidence.
No, it doesn’t speak for adaptation, it’s just that in humans, and other mammals (and in fact, all tetrapods beyond the most primitive of all) five digits is the ancestral condition. As such, it is simply retained unless there are adaptive pressures to change it. One may regard this simply as a kind of evolutionary “inertia.” It requires no adaptive explanation.
Even some primates have lost digits. Spider monkeys lack thumbs, possibly because they would be in the way when they are moving through the trees by brachiation (swinging by their arms).