•Scientists estimated blood flow to the brain by looking at holes for arteries
•They compared the holes in fossilised skulls from several early humans
•Blood flow to the brain in modern humans is six times that in our ancestors
•It suggests human brains needed more fuel as complex thinking evolved
That did not happen during a period when resources were free and the game was to just eat and have sex as often as possible. If that had been the case, energy would never have been diverted from raw reproduction to something as esoteric as higher cognitive processes. In such conditions, useless expenditures like grasping calculus or understanding future consequences will be selected against.
What I suspect is that the primary impetus for this evolution was the ultimate K-stimulus, war. There is nothing which will advance the evolution of intelligence like humans meeting and pitting wits against each other in a competition that will kill the losers and only leave the smartest and most capable to reproduce.
A simple environmental stress is a static evolutionary goalpost, toward which evolution would appear as an asymptotic curve. If you must evolve the ability to defeat a pathogen, or survive some extreme temperature, eventually you will evolve that ability and further evolution will cease. Competing against the evolved intelligence of the group however, is a moving evolutionary goalpost, which is lifted and carried further with each generation that is smarter than the last. The guy who wins one generation’s competition easily may not stand any chance among the comparable geniuses who comprise his great, great, grandchildren’s generation, themselves idiots by the standards of later generations.
Imagine if the upper 1% of the winners of one generation are the mean of the next generation. And that ends up always being true, no matter how smart your species evolves. So long as a sufficient number of the idiots are always killed back, further evolution of intelligence is assured, and any major leap is assured to confer great advantage.
Obviously as you look out over society, it has been a long time since we killed off our imbeciles. Fortunately for our species, the imbeciles have a plan for that, even if they don’t know it yet. I expect in the next decade or two, we will all be hip deep in dead imbecile bodies.
The challenge for us will be not becoming collateral Darwinian damage in that imbecile-fueled bloodbath.