From 3 to 2 million years ago, there was another long period of cool, dry climate in which Africa's forests shrank once more... The changing climate also put pressure on the australopithecines to develop new sources of food. Their diet, to judge by the nature of the microscopic wear on their teeth, was mostly vegetarian until about 2.5 million years ago.
At this time the australopithecines, already adapted to living in open woodland, had evolved two quite different solutions to the problems of survival, according to the evidence of their fossil remains.
One of the two species, known as the robust australopithecines, had developed larger cheek teeth, suitable for eating coarse leaves. The other had emerged with a much more original solution than chewing away at vegetation. It seemed to have decided to try its hand at carnivory. Meat-eating allowed for a smaller gut and furnished the extra nutrition that made possible a larger brain.
This second species is known as Homo habilis. The title of Homo is one it does not clearly deserve since, far from being fully human, it retained its apelike body form and still used the trees as a refuge. But it possessed a striking new adaptation.
The australopithecines had lived for 2.5 million years with brains scarcely bigger than a chimp's, but with habilis the brain at last started to expand. Chimpanzee's brains have a volume of 400 cubic centimeters, compared to the 1,400ccs of the average human brain.
The australopithecine brain size ranged from 400cc to 500cc. The brain volume of the known habilis skulls ranges between 600cc and 800ccs.
For a species to put resources into growing extra neurons is not as obvious an investment as it may seem. Brawn and teeth count a lot in the struggle for survival. Brain cells are greedy consumers of glucose and oxygen. "When costs are taken into account, the rarity of the human evolutionary phenomenon is at last understandable" writes Robert Foley.
It's easier to explain how habilis sustained its larger brain than why it got it.
Brains require a high quality diet to sustain them, such as meat but not vegetation can provide. Meat eating requires less tooth power than does chomping through mounds of vegetation and habilis indeed had smaller teeth.
And habilis appears on the scene at the same time, 2.5 million years ago, as do the first stone tools. If, as it seems likely, habilis was the maker and user of these implements, that would explain its smaller teeth and how it managed to nourish a larger brain; it didn't need large teeth because it was using tools to hunt or scavenge meat, and the richer diet supplied the energy for its greater cognitive capacity.