Basic Musings on Big Brains
"Why have apes not acquired the intellectual powers of man?"
Thus asks Darwin in his book on the Origin of Species (6th ed., p. 201), and does not hesitate to give the answer: We don't know.
Historically (geologically speaking that is), large brains are more common among predators than among prey. Perhaps, in stalking prey one has to have a model of what the prey might do. When being prey, it doesn't pay to ponder the matter. The task is to get away. Quickly. Another clue: in the Cenozoic, the larger brains seem to be with social animals. Many mammals are highly social, presumably as a result of the strong mother-offspring bond resulting from extended care-giving. Animals that live together in groups have a need to signal to each other, and to receive the signals and use them to modify behavior. Dogs maintain a social hierarchy employing quite complex behavior, which includes predicting how an adversary or partner will react to provocation or appeasement. Certain monkeys warn each other about the presence of predators (as do many other social animals) but they also specify whether it is a snake or large cat.
Basically, we still do not know why humans acquired a large brain (or why dolphins did so, or elephants). What mechanism kept the brain growing once it had started to do so, three million years ago?
Several things kept changing about humans. Upright gait and getting better at throwing things. Tool-making. Beginnings of language. Perhaps larger groups, a more complicated social structure. Trading and warring with other groups. Somehow, on the margin, having a slightly larger brain was of greater advantage than running a bit faster or throwing rocks farther. In the end, we must assume that the growth of the brain itself provided the conditions that led to further growth. Humans entered a positive feedback loop just like the giraffe with its neck, the elephant with its trunk, the dolphin with its fluke. Once you have it, it pays to improve on it. It is like building on expertise, in business. We must not forget that it was a very slow process. From one millennium to the next, the change would have been immeasurably small. Summed over half a million years, it is very remarkable. For the last 30,000 years, as far as can be ascertained, nothing much has happened regarding brain size (or anything else). Our ancestors from that time, if fetched in a time machine, would fit right in and quickly acquire the skills we have. (In return, they might volunteer to show us how to hunt mammoths.)