The Energetics of Encephalization in Early Hominids

Abstract

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.

Ecosystem variability and early human habitats in eastern Africa

“…Fossil evidence for pronounced aridification and faunal turnover in eastern Africa between about 2.0 and 1.8 Ma has sparked hypotheses linking the emergence and dispersal of the genus Homo to climate-driven ecosystem change (2-6). Fossil evidence for cranial expansion in premodern Homo (e.g., H. erectus sensu lato) has been linked to irregular resource distributions (67), and our carbon-isotopic data are consistent with enhanced ecosystem variability as a context for encephalization (Fig. 5).…”

“…During the early Pleistocene, strong ecosystem preferences are not apparent between transitional (e.g., H. habilis) and archaic (e.g., Paranthropus boisei) hominins (68); however, isotopic and fossil data suggest that transitional species accessed a broad spectrum of dietary resources compared with archaic species (68)(69)(70). Among primates, quality (i.e., energy density) of dietary resources correlates strongly with brain size (67). Assuming that dietary resources were primarily unrelated to technological innovations by transitional species (4, 68), we hypothesize that ecosystem variability favored hominin species with large brains that allowed for versatile foraging strategies and dietary diversity.…”

The evolutionary roles of nutrition selection and dietary quality in the human brain size and encephalization

“…To compensate for the increased energy expenditure of the enlarged human brain, approximately 19 kg of muscle, about 70% of the total, would have to be replaced by an equal amount of tissue with no metabolic cost at all [18]. Nonetheless, comparative analyses have shown that human and other primates are relatively "under-muscled" compared to other mammalian species [44].…”

How Our Ancestors Broke through the Gray Ceiling

“…The abandonment of the energetically very expensive climbing also freed these hominins from the anatomical compromise between climbing and walking (Isler and van Schaik 2006a). Apart from reducing costs of locomotion, this change in the locomotor habits may also have induced a reduction of maintenance costs during rest, as humans are reported to have relatively less muscle mass than great apes (Leonard et al 2003;Snodgrass, Leonard, and Robertson 2009). However, this seeming difference could arise because of the higher amount of fat stores in humans.…”