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© Doug Allan/Getty
Letting off some steam.
It is one of the biggest mysteries in human evolution. Why did we humans evolve such big brains, making us the unrivalled rulers of the world?

Some 2.5 million years ago, our ancestors' brains expanded from a mere 600 cubic centimetres to about a litre. Two new studies suggest it is no fluke that this brain boom coincided with the onset of an ice age. Cooler heads, it seems, allowed ancient human brains to let off steam and grow.

For all its advantages, the modern human brain is a huge energy glutton, accounting for nearly half of our resting metabolic rate. About a decade ago, biologists David Schwartzman and George Middendorf of Howard University in Washington DC hypothesised that our modern brain could not have evolved until the Quaternary ice age started, about 2.5 million years ago. They reckoned such a large brain would have generated heat faster than it could dissipate it in the warmer climate of earlier times, but they lacked evidence to back their hypothesis.

Now hints of that evidence are beginning to emerge. Climate researcher Axel Kleidon of the Max Planck Institute for Biogeochemistry in Jena, Germany, modelled present-day temperature, humidity and wind conditions around the world using an Earth-systems computer model. He used these factors to predict the maximum rate at which a modern human brain can lose heat in different regions. He found that, even today, the ability to dissipate heat should restrict the activity of people in many tropical regions (Climatic Change, vol 95, p 405).

If keeping cool is a problem now, Kleidon says, it would have been even more challenging - perhaps too challenging - 2 or 3 million years ago when temperatures were a few degrees warmer than today and air-conditioning units were harder to come by.

A new study by Schwartzman and Middendorf suggests that a small drop in global temperatures may have made a big difference. The pair used basic equations of heat loss to estimate how fast the small-brained Homo habilis would have been able to cool off. Assuming overheating limited the size of H. habilis's brain, they then calculated what drop in air temperature would have been needed for Homo erectus to be able to support its bigger brain. They found that a drop in air temperature of just 1.5 °C would have done the trick (Climatic Change, vol 95, p 439).

Given the timescales involved, it may be near-impossible to match definitively the onset of an ice age with speciation, but a 1.5 °C drop is consistent with the cooling climate of the time, says Middendorf.

"In principle, I'm receptive to the hypothesis," says Dean Falk, a palaeoanthropologist at Florida State University in Tallahassee, "but I need the data." She says that if measurements showed that people living in tropical countries today have smaller brains relative to their body size than people in temperate climates, this would go against expectation and lend support to Kleidon's model.

Being able to cool bigger brains can only be part of the story, however. It would have lifted the brakes on expansion, says psychologist David Geary at the University of Missouri in Columbia, but there has to be something driving the increase.

Over the years, researchers have come up with three broad reasons why bigger brains might have been advantageous: to give their owners the ability to cope with changing climates by exploiting technologies such as shelter, fire and clothing; to deal with the cognitive demands of hunting and gathering; or to help people outsmart their neighbours.

To help narrow this down, Geary collected data from 175 fossil hominin skulls, from 1.9 million to 10,000 years old. Then he looked to see whether brain size was best correlated with climatic variability - a crude measure of biodiversity which could indicate the complexity of hunting and gathering - or the human population size at the time, which could reflect the complexity of social interactions.

Geary's analysis found that population size was the best predictor of brain size, suggesting that our ancestors' need to outcompete their neighbours in order to survive may have been the strongest driver of brain growth (Human Nature, vol 20, p 67).

The case is far from closed - Geary's study does not demonstrate cause and effect, for one thing - but the picture beginning to emerge suggests that an ice age set the stage for a socially driven brain boom. And from that time on, it was the brainiacs who stole the show.