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A common question posed is, "how did the eye evolve?", because the eye is so complex. However, this has been answered rather clearly and there are several examples around the world of animals in which the eye has evolved independently.

The human brain, however, only seems to have evolved once in the history of life on Earth - a brain which is capable of advanced speech, predicting the future given information on the past, producing art, imagining advanced abstract concepts such as algebra, creating advanced tools, etc.

How, then, could the human brain have evolved? What stimulus was there for progressive Homo individuals to have brains that were slightly more advanced than the previous one's? I've heard Richard Dawkins summarize it as "big brains are sexy", but that's a comment made in hindsight (and not even shared by all members of the human species today!) At the time, why would a human female choose a male with a slightly larger brain, so slightly larger that the increased intelligence probably wouldn't be noticeable? Or why would such an individual have an evolutionary advantage?

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Note to mods: I don't have the permission, but I think this should be tagged with brain. –  Jez May 3 '12 at 21:44
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You should't confuse "big" with functional. Brain size is a side-effect. –  Shep May 4 '12 at 4:40
    
Strongly suggested reading: The Accidental Mind by David J Linden –  nico May 4 '12 at 6:39
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3 Answers

New work suggests a prominent role for genes in the SRGAP2 family, as explained here. Data suggest a mechanism where incomplete duplication - most likely at SRGAP2C - created a novel gene function some 2–3 mya, which is a time corresponding to the transition from Australopithecus to Homo and the beginning of neocortex expansion. Further, in the mouse neocortex, Srgap2 promotes spine maturation and limits spine density, supporting sustained radial migration and leading to the emergence of human-specific features.

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Very interesting, and if one gene duplication did indeed result in our brains suddenly getting more sophisticated, it would mean that Dawkins is dead wrong - there really would be one mother who didn't have "high intelligence capability" and a child that did suddenly have "high intelligence capability". I can see how that's easier to explain evolutionarily than brains very slowly increasing in size. –  Jez May 4 '12 at 19:54
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@Jez, I don't think this means Dawkins was "dead wrong". Just because one gene mutation sets a species on a new evolutionary trajectory, it doesn't mean anything changes overnight. Note that while this mutation produced human-like features in rodents, the rodents didn't get any smarter. The mutation may have allowed human brains to develop, but it certainly didn't cause the jump you're talking about. –  Shep May 6 '12 at 17:58
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You made a few statements in the beginning of your question, I'm just going to focus on the last paragraph (which seems like your question).

When people try to understand evolution, a common fallacy is to assume you're looking at a linear progression, in this case that each surviving child is slightly more intelligent than his parents. This is confusing in part because it's at odds with day to day observation: dumb people have smart kids, tall people have short kids, etc.

It may help to visualize a it as a cloud rather than a line, with the really incompetent people being constantly clipped off the low end and the really competent being, as Richard Dawkins puts it, "sexy". Slowly, genes which tend to land people in the "really dumb" side are weeded out, genes that sometimes produced above-average intelligence are propagated. You can't think about evolution without considering the entire population.

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OK, so I ask you the same question I did Preece; if this is such an obvious selection mechanism, why is it so extremely rare in evolution (only ever happened in humans)? The next-most intelligent animals, chimps, can barely learn simple sign language, and they're on the same twig of the tree of life as us! –  Jez May 4 '12 at 19:43
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no one said it's an obvious selection mechanism. Having a large brain is expensive, for one thing, our brains consume about 20% of our energy, and run exclusively on glucose. In addition, human birth is far more excruciating than most mammals because babies have such huge heads. Our bodies are, in many ways, in a very awkward evolutionary state because the benefits of a big brain have only recently come to fruition. –  Shep May 4 '12 at 20:39
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why would such an individual have an evolutionary advantage?

First thing to keep in mind, is that natural selection will seize upon even very slight advantages and evolve them until the point where evolving them is no longer viable. So, to you a slight increase in intelligence might be unnoticeable, but in a naturally selecting system it would be very noticeable. Don't mix up sexual selection (where the preference of one sex shapes the selection of genes) with natural selection (where fitness to the environment shapes the selection of genes). Sexual selection may or may not have helped shape brain evolution, I can't produce evidence that it did.

But the increased fitness that slight gains in intelligence convey are easier to create evolutionary anecdotes about. It's simplistic to speak of just "intelligence" as evolving. Many different nervous structures evolved through out the animal kingdom, each with a function that directly increased fitness. I get the feeling that you're specifically talking about the evolution of the neocortex, because you reference human intelligence.

So, what I guess you're really asking is what evolutionary advantage does the neocortex convey? The answer isn't that hard, once you understand a little about the neocortex. It is an organ that simply models the external and internal world as perceived by your sensory organs. By external world, I mean modeling things like vision, hearing, touch, etc... By internal world, I mean modeling the actions you can perform, the plans you make, and so on. Increasing the size of the neocortex is essentially increasing the possible complexity of these models. Not only can you find more subtle and complex patterns out there, you can develop increasingly subtle plans and ruses too.

Watching documentary footage of tribal hunters provides a really stark look at just how useful the neocortex is. It frames our intelligence in terms of a situation similar to one we evolved in. It's not hard to imagine why the growth has been explosive. Outsmarting other animals is a great niche that in our evolutionary history was virtually unexploited.

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But if it is so easy to see the advantage of our large neocortex, why is it so extremely rare (has never evolved outside of humans)? Why wouldn't it also be beneficial enough in some other species to evolve to the same extent? –  Jez May 4 '12 at 19:39
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You're imagining that the human brain is unique in some way. It really isn't, we have the exact same structures as almost every mammal, from rodents to apes to dogs and cats. The main difference is that out neocortex is larger. Asking why the neocortex has only evolved in humans is like asking why white fur has only evolved in polar bears. Every mammal has hair, it's not "rare", it's just slightly different in different species. Also, consider that it's possible that intellectual gains are exponential to linear increases in cortical size. –  Preece May 4 '12 at 22:12
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