Brains & Brawn
Hominid aerobic capacity for endurance running improved 2 MYA. Around the same time, brain size showed a notable increase: a trend that accelerated during the Middle Pleistocene, 800 TYA. Also ~800 TYA, Homo began having longer adolescence, as contrasted to the more rapid development of earlier hominids.
The single biggest change of hominin evolution was in metabolic allocation: brains demanded more calories while muscles became more parsimonious. Ravenous brains developed by their becoming more molecularly complex and more intricately integrated.
Contrary to common conjecture, nothing suggests that hominid brain evolution improved acumen. Hypotheses about hominids progressing in problem-solving or sociality via brain changes are unsubstantiated. A trove of evidence dispels the notion that brain size or organization is related to intelligence. For instance, technology is not evidence of astuteness. With their industrial technologies modern men engineered their own extinction within a few generations. How smart is that?!
Oddly, larger brain size led to smaller guts. This life-history trade-off – enlarging the brain leading to a smaller digestive system – is common among fish, birds, and mammals. Why is not understood.
Typical consequences of larger brains are fewer offspring and greater altriciality. Primates have fewer offspring than other mammals, and humans fewer still.
As their brains got bigger, human muscles atrophied. For their size, monkeys and chimps are far stronger than even the burliest men.
There is more to the story than trading brawn for brain. In losing strength, hominin muscles adapted for a different power: endurance. Tellingly, walking is excellent mental exercise, as it demands sensory awareness.
Endurance allowed foraging over greater distances. It also engendered the diaspora that led to the human infestation of the entire planet.