Stanley Garn, writing in “Culture and the direction of human evolution” (1963: 222):
But to speak of higher things, when we turn to the size of the brain there is no doubt of an increase from the African and Asiatic megadonts through the Javanese and Chinese fossils and on through to the forms we accept as cospecific with us today. Some of this increase in brain size is necessarily allometric, the simple growth-associated relationship between bigger frames and bigger brains. Some of this brain increase, perhaps the largest portion, is a true increase in the volume of the brain; and here we must rack our brains to explain our brains. It is no longer enough to attribute even the first increase of hominid brain size to the mere rudiments of technology, and certainly not the second increase that followed Pithecanthropus. Surely man did not double and nearly redouble his cerebral volume merely to pick up sticks.
Garn SM (1963) Culture and the direction of human evolution. Human Biology 35:221-236.