The parallel emergence of these two evolutionary events illustrates nicely how natural selection works. Those mothers with hereditary large pelvises were able to bear large-brained babies who because of their superior intelligence were able to compete successfully in adulthood with the smaller-brained offspring of mothers with smaller pelvises. He who had a stone axe was more likely to win a vigorous difference of opinion in Pleistocene times. More important, he was a more successful hunter. But the invention and continued manufacture of stone axes required larger brain volumes.
