It might reasonably have been expected that the adoption of cooking by early humans would not have led to any changes in human digestive anatomy. After all, cooking makes food easier to eat, which means that no special adaptations are required to process cooked food. However, current evidence suggests that humans today are capable of living on raw food only under unusual circumstances, such as a relatively sedentary lifestyle in a well supported urban environment. Important theoretical obstacles to living on raw food in the wild today include both the low digestibility of much raw plant food, and the toughness of much raw meat. These points suggest that humans are so evolutionarily constrained to eating foods that are digestible and easily chewed that cooking is normally obligatory. Furthermore, the widespread assumption that cooking could not have had any impact on biological evolution because its practice is too recent appears to be wrong. (Various European and Middle Eastern sites that go back more than 250,000 years contain extensive evidence of hominid use of fire and apparent “earth ovens.”) The implication is that the adoption of cooked food created opportunities for humans to use diets of high caloric density more efficiently. Selection for such efficiency, we suggest, led to an inability to survive on raw-food diets in the wild.

Important questions therefore arise concerning what limits the ability of humans to utilize raw food. The principal effect of cooking considered to date has been a reduction in tooth and jaw size over evolutionary time. Human tooth and jaw size show signs of decreasing approximately 100,000 years ago; we suggest that this was a consequence of eating cooked food. Subsequent population variation in the extent and timing of dental reduction is broadly explicable by regional variation in the times when improvements in cooking technology were adopted. It is also possible that the earliest impact of cooking was the reduction of tooth and jaw size that accompanied the evolution of Homo ergaster approximately 1.9 million years ago. If so, the decrease in tooth and jaw size that started around 100,000 years ago may prove to result from later modifications in cooking technique, such as the adoption of boiling.

The evolution of soft parts of the digestive system is harder to reconstruct because they leave no fossil record. Human digestive anatomy differs from that of the other great apes in ways that have traditionally been explained as adaptations to a high raw-meat diet. Differences include the smaller gut volume, longer small intestine, and smaller colon. All such features are essentially adaptations to a diet of relatively high caloric density, however, and may therefore be at least as well explained by the adoption of cooking as by eating raw meat. Testing between the cooking and raw-meat models for understanding human digestive anatomy is therefore warranted.