Early humans developed larger brains as they adapted to colder climates, according to University at Albany researchers.

The 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 research utilized several measures of paleotemperatures and a sample of 109 fossilized hominid skulls collected 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.

In species other than humans, problems posed by cooler climates were solved by adaptations such as hibernation and migration, and by metabolic adaptations including fur and the development of fat deposits. During human evolution, however, the authors surmise that solutions to the problems of cold weather and a scarce food supply featured detailed and progressively more refined cognitive and intellectual strategies, such as the development of cooperative hunting techniques and more sophisticated tools and weapons. Increased brain capacity also brought with it the use of fire as a means to keep warm and cook, adaptations in clothing and shelter, and the development of more refined social skills.

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.