A study, to be published by Jessica Ash, a graduate student in psychology, and professor of evolutionary psychology Gordon G. Gallup Jr. in the spring edition of Human Nature (Vol. 18, Issue 2, 2007: Transaction Publishers), 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 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. They suggest seasonal variation in climate may also have been an important selective force behind the evolution of human cranial capacity. Specifically, they found that as the distance from the equator increased, north or south, so did brain size.

Gallup and Ash suggest that while our understanding of brain evolution remains incomplete, the study provides evidence of the role of climate and migration away from the equator as selective forces in promoting human intelligence, and that the recent trend toward global warming may be reversing a trend that led to brain expansion in humans. => Source