The study. . . suggests that human cranial capacity as an indicator of brain size grew dramatically during our evolution, and that variations in global temperature as well as progressive shifts toward global cooling account for as much as 50 percent of the variation in cranial capacity. The research utilized several measures of paleotemperatures and a sample of 109 fossilized hominid skulls (ranging) over the past 2 million years.

In addition to the impact of global cooling, "By paying close attention to the geographic origin of each of the fossilized skulls," said Gallup, "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."

The authors suggest that a key environmental trigger to the evolution of larger brains was the need to devise ways to keep warm and manage the fluctuations in food availability that resulted from cold weather.