Humans grew bigger brains as the climate they lived in got cooler, according to researchers at the University at Albany, New York.
The researchers concluded that humans got brainier because they had to adapt to a more challenging environment. They base this assertion on a plot of cranial capacity of 109 fossilised human skulls against the corresponding paleontological record of two million years of changing climate.
As well as a relationship between a cooling earth and growing skulls, the researchers report that where the skulls were found matters, too, because the further you get from the Equator, the more varied the weather becomes.
Gordon G Gallup Jr, a professor of evolutionary psychology at the university, and co-author of the work along with graduate student Jessica Ash, commented: "It became clear that seasonal variation in climate may also have been an important selective force behind the evolution of human cranial capacity. Specifically, we found that as the distance from the Equator increased, north or south, so did brain size."
Lower temperatures and seasonal variations threw up new challenges for the early human, such as fluctuations in the availability of food and the need for fire and clothes to keep warm, the researchers argue. More co-operation would have been needed to find, preserve, and store food; and the people would have needed more complex tools. Along with that, more intricate social structures would have evolved, which in turn would have required more grey matter.
The researchers suggest that having to adapt to the impact of lower temperatures could account for as much as 50 per cent of the increase in the size of our skulls.
The researchers don't mention whether or not the extra small human skull found on the island of Flores was included in the sample. ®