Form tends to follow function is the compelling trend of natural selection. So why is the human cranium perched as it is on a three curved spine? The argument that bipedalism requires a balanced cranial stance with 180 degree freedom of motion is not compelling, since other bipedal animals have not converged to this form. However, a gimbaled, binocular device for accurate targeting during rock throwing would confer a distinct survival advantage since it would permit action at a distance.
Man, the generalist, is generally attributed with a very good ability to run, swim, brachiate, or climb, but not the best, as would be expected if those modes of transport were the primary functional design natural selection was working on. An evolutionary period of millions of years should have permitted convergence to a more efficient method of locomotion. Even his eye design is flawed. Where he is the undisputed king is in throwing accurately.
It is probable that both Darwin and Wallace were correct: tool use and environmental changes both were the agents driving human bipedalism. But, the primary functionality that allowed proto-human survival, both interspecies and intraspecies, and was the forcing function for evolutionary modifications leading to bipedalism was throwing.
The existence of manuports that could have served as grenades or fastballs could be evidence that this was the first tool used in human warfare or hunting. And, would also have served defensive purposes. Of course, hunting sticks, clubs, spears, atlatls, and other projectiles that emerged with time would all have required a highly developed method of aiming and throwing that had already been preselected.
The ensuing spiral of group warfare and environmental pressures from improved hunting efficiency coupled with environmental changes could have been the spark that led to several human diasporas. And, more importantly the increased hunting efficiency and protection afforded in our escalating arms race would have allowed further neoteny and accelerated selection for a larger brain.
