Wrangham said he reasoned that the nutritional effect of cooking should be evident from the fossil record. There was only one period that shows dramatic changes in tooth structure and body size, about 1.9 million years ago.
It is this time period that marks the appearance of the genus Homo. Wrangham said it was very encouraging to find that archaeologists placed the harnessing of fire around the same date.
Before this period, human ancestors inhabited the trees, and size differences between males and females were much more pronounced. Wrangham said that, anytime during the Homo period, after the harnessing of fire, if a human ancestor were to put on modern clothes, a hat, and strut down Manhattan, he would not be noticed.
"We believe that one of Anthropology's major challenges is to develop a hypothesis that explains this critical change," Wrangham said. "And we think our hypothesis might be right."