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The most distinctive characteristic of a human is it's face - it is unique among each individual (with the exception of identical twins). It is uncertain to me if whether we best identify other humans by their face due to special brain parts developed for this (there are people with disease that cannot recognize faces) or for actual very distinctive physical face forms; maybe it is a combination of this.

Anyway, the question is about evolution. Why would distinct faces (or ability to recognize distinct faces) be something useful that would be retained by evolution? How is this better at survival?

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I've read that penguins are also pretty good at identifying family members(visually and/or vocally) amongst hundreds of other similar looking birds. – Alex Stone Jun 3 '13 at 1:23
Speculation: It seems that identification would be important in society. Having a distinctive appearance means one gets credit for good behavior and being able to distinguish would allow giving credit only where due. Helping gene-sharers (similar appearance) and breeding with dissimilar others could have selective advantages; both require recognizing similarities and differences. Distinctive aspects of appearance may also accentuate, honestly expose, or disguise fitness markers, with obvious selective advantages, and discerning actual fitness markers would be advantageous. – Paul A. Clayton Jun 3 '13 at 14:39

Given the prevalance of myopia in humans I would have thought that facial recognition was secondary to other indicators such as gait recognition which is also unique to individuals though can be copied to an extent. Voice and shape are also used to identify people.

Being able to recognise individuals would allow the formation of groups, allow for structures of trust, reciprocal altruism to develop as well as punishing transgressors. Repeat interactions with other people require some kind of identification, but I dunno if facial recognition is the only/primary one.

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