Last night I watched Nova's Becoming Human Part 1. An interesting theory was presented in it. Some scientists think our evolutionary ancestors developed as they did because of wild swings of the climate conditions in Africa. Some changes, like going from lush forest to dry desert, happened with 1,000 years. So the idea is that our ancestors had to adapt. They came up with this based on measuring the brain capacity of fossilized skulls. For millions of years the brains of our ancestors remained small, but during the turbulent climate times, their brains began to grow.
One fellow even said we didn't have to worry about the impending climate change since we adapted so much millions of years ago. Perhaps, if you're only concerned with the survival of the human race and now how many of them may die in the process.