Continuing the discussion from "There is no such thing as a 'transitional fossil'…", @johnZ said:
Ironically, if we assumed human evolution from some hairy ancestor, we would see that in order for the benefits of a bigger brain to be advantageous, it would have to be accompanied by a weaker body, and by hairlessness, a vulnerability to weather. Otherwise, there would be no need to use the bigger brain to survive, and thus no advantage to the bigger brain. The hairy human with powerful hands and feet and long tail could simply survive in the open like all the hairy apes, instead of wearing clothes, building houses, growing gardens, and herding sheep.
This post is a couple months old, but it seemed backwards to me. Bigger brains don’t evolve because a weaker body demands it; a weaker form would be less likely to find enough calories to support a high-energy brain, or even survive to reproduce at all. Bigger brains evolve for two main reasons in the animal kingdom: to be better at maintaining social groups, and to be better at hunting or scavenging.
It wouldn’t make any sense for the ancient population that would eventually become humanity to evolve weaker bodies for no reason, or just to give brains more reason to develop. Instead, we should look at what our bodies specialized for: every single ‘weakness’ compared to apes was a trade-off for something it was important to do better. The anatomy of the human body, especially compared to apes, tells us that humans have been highly specialized for long-distance ground travel, especially in hot weather. Apes can stand or walk on two legs, but not efficiently. Human can walk and run long distances that no ape would bear. Pretty much the entire animal kingdom can out-sprint us, but only a handful can out-marathon us, and if you schedule the marathon for a hot summer’s day, fit humans can run down almost any quadruped on the planet.
Rather than large brains only being useful for protecting weakened bodies (a hypothesis which begs the question of where all the extra calories to fuel energy-intensive brains could have come from), it is far more likely large brains developed in tandem with bodies that grew progressively more capable of long-distance scavenging and cooperative endurance-running based hunting strategies, since non-olfactory tracking of prey, cooperation with other group members, and strategizing to overheat and exhaust other animals would all have benefited from the increased brainpower that would be possible due to consuming more meat.