Did climate change make us intelligent?

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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.

2. Earth's climate can change rapidly

The wobbles and variations in the orbit of the Earth as it goes round the Sun have caused many periods of rapid and violent climate change.

Because the Earth isn't a perfect sphere, the gravitational pull of the Moon and the Sun causes its axis to wobble.

The Earth's axis is said to precess, moving around in a circle every 27,000 years.

Precession changes the point in the planet's orbit at which summer and winter occur. And that affects the amount of sunlight that falls on the Earth in summer and winter, and makes the climate change.

But Earth is also subjected to other gravitational pulls and tugs, mainly from the planet Jupiter. And that has the effect of changing the Earth's orbit itself. Sometimes the ellipse gets bigger, so that Earth moves further away from the Sun.

Sometimes the ellipse gets smaller, so that the Earth stays closer to the Sun.

Every 400,000 years, these variations in the Earth's orbit conspire to amplify the effects of precession, which makes climate change even more radically.

And in the Great Rift Valley in East Africa, the planet's geography amplifies them still further, to produce periods of rapid and extreme climate fluctuation.

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?

Dolphins

Image: John Downer Productions

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Dolphins

Image: Getty Images

Dolphins have advanced social and language skills. But without hands, it would have been hard for them to evolve to make tools or change their environment.

Elephants

Image: Getty Images

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Elephants

Image: Getty Images

Elephants are great at problem-solving and communication. But they evolved large bodies instead of fine motor skills, so their trunks have limited flexibility.

Birds

Image: Getty Images

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Birds

Image: Getty Images

Crows are great problem solvers, and can empathise with other crows. But they haven't evolved language, and having wings limits what they can do with tools.