Compared to other primates and mammals of our size, humans allocate a much larger share of their daily energy budget to “feed their brains.” The disproportionately large allocation of our energy budget to brain metabolism has important implications for our dietary needs. To accommodate the high energy demands of our large brains, humans consume diets that are of much higher quality (i.e., more dense in energy and fat) than those of our primate kin (Leonard and Robertson, 1992, 1994). On average, we consume higher levels of dietary fat than other primates (Popovich et al., 1997), and much higher levels of key long-chain polyunsaturated fatty acids (LC-PUFAs) that are critical to brain development (Crawford et al., 1999; Cordain et al., 2001). Moreover, humans also appear to be distinctive in their developmental changes in body composition. We have higher levels of body fatness than other primate species, and these differences are particularly evident early in life.
The need for an energy-rich diet also appears to have shaped our ability to detect and metabolize high-fat foods. Humans show strong preferences for lipid-rich foods. Recent work in neuroscience has shown that these preferences are based on the smell, texture, and taste of fatty foods (Sclafani, 2001; Gaillard et al., 2008; Le Coutre and Schmitt, 2008), and that our brains have the ability to assess the energy content of foods with remarkable speed and accuracy (Toepel et al., 2009). Additionally, compared to large-bodied apes, humans have an enhanced capacity to digest and metabolize higher fat diets. Our gastrointestinal (GI) tract, with its expanded small intestine and reduced colon, is quite different from those of chimpanzees and gorillas and is consistent with the consumption of a high-quality diet with large amounts of animal food (Milton, 1987, ). Finch and Stanford (2004) have recently shown that the evolution of key “meat-adaptive” genes in hominid evolution were critical to promoting enhanced lipid metabolism necessary for subsisting on diets with greater levels of animal material.
This chapter draws on both analyses of living primate species and the human fossil record to explore the evolutionary importance of fat in the nutritional biology of our species. We begin by examining comparative dietary data for modern human groups and other primate species to evaluate the influence that variation in relative brain size has on dietary patterns among modern primates. We then turn to an examination of the human fossil record to consider when and under what conditions in our evolutionary past key changes in brain size and diet likely took place. Finally, we explore how the evolution of large human brains was likely accommodated by distinctive aspects of human growth and development that promote increased levels of body fatness from early in life.
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