The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution

Abstract

Brain tissue is metabolically expensive, but there is no signifi- cant correlation between relative basal metabolic rate and rela- The present paper was sumitted in final form i5 VI 94. tive brain size in humans and other encephalized mammals. The expensive-tissue hypothesis suggests that the metabolic require- ments of relatively large brains are offset by a corresponding reduction of the gut. The splanchnic organs (liver and gastro- Much of the work that has been done on encephalization intestinal tract) are as metabolically expensive as brains, and the in humans and other primates has been oriented toward gut is the only one of the metabolically expensive organs in the why questions-why different primate taxa have differ- human body that is markedly small in relation to body size. Gut size is highly correlated with diet, and relatively small guts ent relative brain sizes or why the human line has un- are compatible only with high-quality, easy-to-digest food. The dergone such a phenomenal increase in brain size during often-cited relationship between diet and relative brain size is the past 2 million years. Hypotheses that have been put more properly viewed as a relationship between relative brain forward to answer these questions primarily invoke size and relative gut size, the latter being determined by dietary quality. No matter what is selecting for relatively large brains in socio-ecological factors such as group size (Aiello and humans and other primates, they cannot be achieved without a Dunbar I993), social (or Machiavellian) intelligence shift to a high-quality diet unless there is a rise in the metabolic (Byrne and Whiten I988), or complexity of foraging strat- rate. Therefore the incorporation of increasingly greater amounts egy (Milton I979, Parker and Gibson I979, Clutton- of animal products into the diet was essential in the evolution of Brock and Harvey I980, Gibson I986, MacNab and Eisen- the large human brain