1. Apeman to spaceman
About two million years ago, something extraordinary started happening in the Great Rift Valley in East Africa. Our ape ancestors began to evolve from animals with brain power close to that of a chimpanzee, to apes that would ultimately become human - able to talk, and construct complex tools, from spears to spaceships.
Recent scientific evidence suggests that this evolutionary leap was driven by the impact of climate change on the valley of the apes; remarkably, climate change caused by the way that the Earth moves through space, as it orbits the Sun.
3. How apes got smarter
Watch the video below to see Brian explain how climate change was amplified in the Rift Valley, and what the fossils of our ancestors can tell us.
Clip from Human Universe (BBC Two)
4. Human intelligence - other explanations
The theory that climate change drove the evolution of human intelligence is based on the latest evidence of how the landscape changed in the Great Rift Valley over hundreds of thousands of years. But it is not the only explanation of what made our ape ancestors get smarter.
None of the theories below are incompatible with the climate change theory. In fact, they may explain how our ancestors coped with a changing climate, and in particular how they survived periods of drought and famine.
Better tools and weapons
Apes with bigger brains were able to construct more advanced tools and weapons, giving them an advantage when hunting, or butchering meat. This could have created an evolutionary pressure for brains to get larger.
More complex social structures
Having a larger brain could have helped our ancestors form more complex social groups that were able to co-operate when times got tough.
Mastery of fire
It takes a smart ape to understand how to start and control fire. Fire allows cooking, and cooking makes food more digestible, so that it provides more energy. However, there is no archaeological evidence of controlled fires at the time of the first leap in brain size, 1.8 million years ago.
5. What stopped other animals becoming like us?
Other animals could have followed a similar evolutionary path to that of our ape ancestors. So what stopped them becoming as adaptable as us?