Out of all the recent articles and brewhahas in evolutionary biology, the idea that the climate turmoil of the past three million years has been a strong selective force for larger, more complex brains is the most interesting to me. Those pesky Himalayas growing to the point they started to have a major impact on global weather patterns, as well as other, sundry changes to a planet that had been growing colder and dryer since around the time of the dinosaurs meteoric downfall.
That's not to say it's the single factor that made us what we are. There probably isn't such a thing. Still, it looks like an interesting and potentially important factor in our evolution, both as primates and as omnivores.
Brains, after all, are a risky evolutionary gamble. They are hugely expensive in terms of calories in, require specific vitamins, and they generate a lot of waste heat, which can get an organism killed in the tropics. So the unprecedented behavioral flexibility and the ability to create large, non-kin based societies that our associative cortex and advanced social emotions grant us come with significant risks. That said, behavioral flexibility seems like it would beneficial trait in a world where species were being subjected to repeated ecological disruptions on an evolutionary time scale.