It is generally accepted these days that, based on the fossil record, the precursors to humans developed a modified knee that allowed them to stand and move about on two feet, and, shortly thereafter, these hominids’ brains started getting bigger at a dramatic rate.
There is no proven causal relationship between the events that I know of, but I believe it is reasonable to imagine that once the hands were freed up, cleverness and nimbleness would be rewarded. (And the ability to throw things well.)
Suddenly this animal’s brain had the tools to perform much more complex tasks, and nimble fingers became a force multiplier for more intelligent owners. Smart became very sexy.
The brain expansion kept right on going, even when it became dangerous to push that big head out of a mother’s birth canal. As a compromise human babies are born completely helpless, brains still developing, just so the mother would survive. It was a heck of a compromise to make, and required more complex social groupings for the species to survive. And here we are.
Unimpeachable sources will tell you “Idle hands do the Devil’s work.” But it was the idle hands of our ancestors that literally defined who we became as a species. Those locking knees created idle hands, and it was idle hands that gave rise to our brains.
There are those who believe the human organism was created through a process of intelligent design. But perhaps it would be more accurate to consider we were made instead by malevolent design. Those locking knees, and all the Devil’s work that has come since, certainly have me wondering.