An adaptive model for the changes in postcanine tooth size that have been observed during human evolution is presented. The argument is based on integrating the oral processing of food (largely mastication) into the kinetics of the mammalian digestive system. It is concluded that the influence of the mouth on the rate of digestion is probably underestimated. Several pieces of recent research support this, including the fact that the oral exposure of at least some food nutrients to taste receptors is important in the rate of uptake of those nutrients later on. These “cephalic phase responses”, long known in outline in physiology, thus have a very specific digestive function. The rate of oral processing must match that of the rest of the digestive system with a critical anatomical variable being postcanine tooth size. The chapter reviews digestive function in a broad sense and then applies its logic to selection on tooth size during the Palaeolithic. It is suggested that postcanine tooth size is regulated evolutionarily to provide a rate of digestive throughput appropriate to diet. Cooking and food processing techniques that developed during the Palaeolithic must have reduced the period of oral processing. Coupled with the advent of meals in the life of early humans, a reduction in tooth size would have reduced a potential “avalanche” of food being sent down from the mouth.