ELI5: How did humans become so smart and why are we the only ones?

When the human ancestors separated from chimpanzee ancestors, they ended up on the African savannah. The problem with this is that many of the ape adaptations do not work on the savannah, except for one: intelligence. It turns out that our ancestors had just enough intelligence to survive. But since the environment was so different from the jungle environment of their ancestors, this gave them a lot of opportunity to evolve.

The evolution was both physical and mental, and which worked symbiotically to allow humans to evolve to exploit the savannah that other organism just couldn't. Bipedalism allowed us to expend energy in migration much more efficiently; it also let us see above tall grasses so we could see an approaching predator from a long distance. Our ancestors quickly became scavengers, which lead to making tools allowed us to process a carcass fast enough that we were not just scared away from a kill by some larger group of scavengers. We then adapted the ability to throw spears. This required a improvement in dexterity to accomplish this. We then lost our hair and gained sweat glands so that we could engage in a strategy of hunting called persistence hunting. Each step of the way, our brains had to get better to employ these changes in strategy.

So the key in our evolutionary history is to realize that many of our recently adaptations don't work unless there is an accompanying mental improvement as well. So increased mental abilities had real concrete, measurable payoff. Which is exactly how natural selection works.

We are simply the first organism for which mental improvement has such an enormous pay-off. Think about it -- chimpanzees basically eat all the fruit, ants, and colobus monkeys they can practically eat in the jungle and move from territory to territory to gain access to more. But at some point there are diminishing returns. If the chimpanzees were 50% smarter, could they exploit this intelligence for any purpose? Unlikely -- they are already doing very well at their jungle exploitation strategy. Since our ancestors entered savannah, we started by scavenging, then spear hunting, then traps, then large scale migration, fishing, etc., etc. Since there were so many ways for us to improve our survivability in our new and very large environment, it gave the opportunity for many improvements to our brain.

/r/explainlikeimfive Thread