No longer incidental is an adaptive effect recently suggested under the expensive-tissue hypothesis (Aiello and Wheeler, 1995): the idea that only animals with cheap guts can afford expensive (i.e., large) brains. It suggests that the metabolic requirements of a relatively large brain must be offset by a corresponding reduction of the gut. The enlarged human brain is a highly expensive organ in terms of the quantity of energy it consumes. It costs us roughly 16% of our total basal metabolic rate (BMR). To fuel (with substrate and oxygen) such a large brain and still maintain a normal BMR, another expensive metabolic tissue must be reduced. The key argument is that this has to be the gut.The abstract of Aiello and Weeler (1995) reads,
In fact, the increase in mass of the human brain appears to be balanced by an almost identical reduction in the size of the gastrointestinal tract. Now at 15% of total BMR (reduced, presumably, from a previous total of 25%), the gut is the only body part that could have possibly varied in size sufficiently to offset the metabolic cost of the encephalizing human brain. The reason for this is that other metabolically expensive organs - liver, heart, kidneys, and lungs (accounting for 19,11,8, and 4% of total body BMR, respectively) - could not be reduced since they are physiologically constrained in fairly strict proportion to body size. Gut size is only partly related to body size. Its size and proportions (in a species) are in large measure also determined by diet, that is, by behavior. It follows that the relatively large brains in humans (and to a lesser extent in other primates) could not have been achieved without a shift to a high-quality diet. In this respect, Aiello and Wheeler emphasize the consumption of greater quantities of meat relative to plant foods as a likely possibility. (Ofek 2001: 68-9)
Brain tissue is metabolically expensive, but there is no significant correlation between relative basal metabolic rate and relative brain size in humans and other encephalized mammals. The expensive-tissue hypothesis suggests that the metabolic requirements of relatively large brains are offset by a corresponding reduction of the gut. The splanchnic organs (liver and gastrointestinal tract) are as metabolically expensive as brains, and the gut is the only one of the metabolically expensive organs in the human body that is markedly small in relation to body size. Gut size is highly correlated with diet, and relatively small guts are compatible only with high-quality, easy-to-digest food. The often-cited relationship between diet and relative brain size is more properly viewed as a relationship between relative brain size and relative gut size, the latter being determined by dietary quality. No matter what is selecting for relatively large brains in humans and other primates, they cannot be achieved without a shift to a high-quality diet unless there is a rise in the metabolic rate. Therefore the incorporation of increasingly greater amounts of animal products into the diet was essential in the evolution of the large human brain.
- Aiello, L. C. and Weeler, P. (1995). The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution. Current Anthropology, 33: 199-221.