Marcia L. Robertson

The Energetics of Encephalization in Early Hominids

Bioenergetics, the study of the use and transfer of energy, can provide important insights into the ecology and evolution of early hominids. Despite a relatively large brain with high metabolic demands, contemporary humans and other primates have resting metabolic rates (RMRs) that are similar to those of other mammals. As a result, a comparatively large proportion of their resting energy budget is spent on brain metabolism among humans (∼20–25%) and other primates (∼8–10%) compared to other mammals (∼3–5%). To understand this shift in energy budget, Aiello and Wheeler’s Expensive Tissue Hypothesis (ETH) posits a metabolic trade-off – a reduction in gut size with brain size increase – to explain this phenomenon. Here, we explore the interrelationships between brain size, body size, diet, and body composition using comparative data for humans, non-human primates, and other mammals. Among living primates, the relative proportion of energy allocated to brain metabolism is positively correlated with dietary quality. Contemporary humans fall at the positive end of this relationship, having both a high quality diet and a large brain. Thus, high costs associated with the large human brain are supported, in part, by energy-rich diets. Although contemporary humans display relatively small guts, primates as a group have gut sizes that are similar to non-primate mammals. In contrast, humans and other primates have significantly less skeletal muscle for their size compared to other mammals. These comparative analyses suggest that alterations in diet quality and body composition were necessary conditions for overcoming the constraints on encephalization. Fossil evidence indicates that brain expansion with the emergence of Homo erectus at about 1.8 million years ago was likely associated with important changes in diet, body composition, and body size.

Primate Bioenergetics: An Evolutionary Perspective

Energy dynamics represent an important interface between an organism and its environment. A variety of factors, including body mass, locomotor strategy, and foraging behavior, determine an animal’s energy demands. Body mass is the most important determinant in predicting metabolic costs both for resting metabolic rate (RMR; the amount of energy used by an inactive animal under thermoneutral conditions) (Kleiber, 1961) and total daily energy costs (TEE or FMR) (Nagy, 1987; Nagy et al., 1999). The Kleiber (1961) scaling relationship correlates RMR with adult body mass and demonstrates that RMR scales to the three-quarters power of body mass in mammals, from the very small (e.g., mice) to the very large (e.g., elephants). While most mammals have RMRs predicted by body size, certain groups (e.g., marsupials, edentates) deviate significantly from this relationship. Primates as a group do not significantly differ from the mammalian scaling relationship, though there exists a great deal of variation within the order. For example, strepsirrhines differ from other primates in having depressed RMRs from those predicted for their size based on the Kleiber

Comparative and Evolutionary Perspectives on Human Brain Growth

This chapter draws on comparative information from human and primate biology to explore the influence of brain size and metabolism on human growth and development. It begins by examining how variation in brain size influences metabolic demands and dietary/nutritional patterns among modern primates. Next, it briefly considers the ecological and nutritional factors that have promoted the evolution of human brain size. The chapter then examines patterns of early life brain growth in humans and how they differ from those of other primates. Finally, it explores how the high energy demands of brain growth in early childhood shape nutritional needs and patterns of growth and development in body size and composition.

Diet and brain evolution. Nutritional implications of large human brain size

The evolution of large human brain size has had important implications for the nutritional biology of our species. Relative to other primates and other mammals, humans spend much larger share of their resting energy budget on brain metabolism. The high energy costs of our brains are supported by the consumption of diets that are high in quality, that is, relatively rich in energy and fat. Among living primates, relative brain size is positively correlated with dietary quality. Humans fall at the positive extreme for both parameters, having the largest relative brain size and the highest quality diet. High levels of encephalization in humans also have important implications for early childhood growth and development. Human infants have much higher levels of body fatness than those of other mammals. These greater levels of adiposity provide a supply of stored energy to accommodate the high metabolic demands of rapid brain growth. Under conditions of nutritional stress, human infants preserve body fat for brain metabolism by reducing rates of linear growth and decreasing levels of fat oxidation. The human fossil record indicates that major changes in both brain size and diet quality occurred in association with the emergence of the genus Homo between 2.0 and 1.7 million years ago in Africa. With the evolution of Homo erectus at 1.8 million years ago, we find evidence of an important adaptive shift – major increases in brain size and body size, along with the emergence of the first hunting and gathering economy. While dietary change was not the prime force responsible for the evolution of large human brain size, improvements in dietary quality and increased consumption of dietary fat appear to have been a necessary condition for promoting encephalization in the human lineage. Further research is needed to better understand the nature of the dietary changes that took place with emergence of early human ancestors.