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Humans (and other humanoid mammals) have five fingers in each hand.

Curious to know, why five? Wouldn't more fingers be more useful?

Is there any evidence that it used to be different and natural selection caused those with five fingers to survive?

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    $\begingroup$ Related post $\endgroup$
    – WYSIWYG
    Nov 13 '14 at 13:43
  • $\begingroup$ Thanks @WYSIWYG, missed it due to not knowing "digits" is used to describe fingers and toes. $\endgroup$
    – Shadow Wizard Is Vaccinated V3
    Nov 13 '14 at 14:15
  • $\begingroup$ Cost / Benefit. More fingers need more neurons devoted to controlling and coordinating them which means less neurons for other stuff. And I suspect the amount of coordination need probably increases exponentially which each additional finger. I'd also argue that the fact you can't move your pinky completely separately from your ring finger is a sign we're already on the limit of what we can support. The benefit of a sixth (or seventh, or eighth) finger on the other hand is probably pretty minimal (diminishing returns). $\endgroup$
    – Matt Burland
    Nov 13 '14 at 21:19
  • $\begingroup$ @Matt interesting, if you can elaborate with scientific quotes this might make a fair answer as well. :) $\endgroup$
    – Shadow Wizard Is Vaccinated V3
    Nov 14 '14 at 21:39
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I found an interesting article in Scientific American (Coates, 2005), and I quote part of it:

The condition of having no more than five fingers or toes [...] probably evolved before the evolutionary divergence of amphibians (frogs, toads, salamanders and caecilians) and amniotes (birds, mammals, and reptiles in the loosest sense of the term). This event dates to approximately 340 million years ago in the Lower Carboniferous Period. Prior to this split, there is evidence of tetrapods from about 360 million years ago having limbs bearing arrays of six, seven and eight digits. Reduction from these polydactylous patterns to the more familiar arrangements of five or fewer digits accompanied the evolution of sophisticated wrist and ankle joints--both in terms of the number of bones present and the complex articulations among the constituent parts.

So Coates states that around 360 million years ago there were tetrapods with more digits (6, 7, 8), but that this was reduced to 5 about 340 million years ago. Losing things in evolution is generally relatively simple, yet re-gaining them is hard. So while there is still a tendency for evolution to create species with less digits, there are no examples of land dwelling species with more than 5 digits.

The author reasons that more digits do no good on land as more digits impair locomotion. Most polydactylous species were indeed found to be aquatic.

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    $\begingroup$ How do more digits impair locomotion? $\endgroup$
    – Etheryte
    Nov 13 '14 at 18:54
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    $\begingroup$ @Nit: You need to coordinate all those digits as you walk making the process more complicated. $\endgroup$
    – Matt Burland
    Nov 13 '14 at 21:40
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    $\begingroup$ So, why we do not have three fingers? $\endgroup$
    – emanuele
    Mar 21 '16 at 19:41
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    $\begingroup$ Well, doesn't more digits create more traction? That's kind of the whole point of having fingers and toes, that an animal can create more surface are contact with any given object in a controllable manner. However, more digits are also more complex and would require more metabolism to keep alive as well. $\endgroup$
    – RayOfHope
    Jul 2 '17 at 8:00

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