Becoming Human documentary. According to the documentary, hominid brain size remained relatively stable for 4 million years, and then suddenly there was a brain size explosion coinciding with the emergence of the genus Homo. Why should brain size suddenly have increased after 4 million years of near-stagnation? The answer, according to the narrator, might be climate change. Evidence from samples taken from the sea bed off the coast of Ethiopia show that the climate in the Rift Valley was volatile in the period leading up to the emergence of Homo. Rapid alternation between moist verdant periods, dry plains, desert, and back again might have selected for a creature which could deal with novelty. This would have necessitated a larger brain.
It's not the first time I've heard a theory about environmental factors affecting human evolution. The Mount Toba catastrophe theory is used to explain why there is an apparent bottleneck in human evolution. Evidence from mitochondrial DNA shows that every human on earth today is descended from an extremely small ancient population. There's more genetic diversity in a community of chimpanzees than there is in the entire human race. There must have been some point in history where the human population was reduced to perhaps a few hundred individuals. One explanation is that the eruption of the Mt. Toba supervolcano caused a severe drop in global temperatures which led to the near-extinction of humans.
It's possible that these hypotheses are true. However, we should be skeptical of hypotheses according to which environmental factors are the chief engine of evolution. Evolutionary biologists have accepted for awhile now that the principle driver of evolution is not the impact of the environment but the impact of species on one another. This was recently proved for the first time in a laboratory experiment. It's not the environment that selects our genes so much as the competition between species.
The idea that competition amongst early hominids might have been so fierce as to impact our genes and cause our brains to grow bigger shouldn't come as a huge surprise. Anthropologists have known for awhile now that violence in hunter and gatherer communities is hundreds of times greater than the violence we're accustomed to in modern society. Lawrence Keeley at Oxford has compiled data showing that if the rate of mortality from violence were as high today as it were in hunting and gathering societies, World War II would have resulted in the deaths of 2 billion people, not 100 million. (more) Yet humans were hunters and gatherers for over 3/4 of their existence. It's unlikely we were more peaceful then than contemporary hunters and gatherers are now. It's extremely likely that archaic hominids were less civilized than that.
One can easily imagine a scenario in which extreme, vicious inter- and intra-species conflict selects for those who aren't just brutal but who are exceptionally cunning as well. Not just those who are able to overcome others by means of brute strength, but who are good liars and artificers, too, ones who have a more developed sense of self and so who are able to hide things from others and to anticipate just a few more steps into the future. A fortuitous side-effect of such an ability is that we're able eventually to realize that such gifts are best used toward the ends of culture and contemplation, and that these things are impossible without peace and civil order. But the knife that carved those abilities might have been far more insidious and less accidental than climate change.