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Although humans have much bigger brains relative to body weight than do other primates, the total resting energy requirements of the human body are no greater than those of any other mammal of the same size, implying that we use a higher share of our daily energy budget to feed our voracious brains. How did such an energetically costly brain evolve?

One theory holds that bipedalism enabled hominids to cool their cranial blood, thereby freeing the heat-sensitive brain of the temperature constraints that had kept its size in check. But brain expansion almost certainly could not have occurred until hominids adopted a diet sufficiently rich in calories and nutrients to meet the associated costs. Across all primates, species with bigger brains dine on richer foods, and humans are the extreme example of this correlation, boasting the largest relative brain size and the choicest diet. Animal foods are far denser in calories and nutrients than most plant foods, and so it stands to reason that for early Homo, acquiring more gray matter meant seeking out more of the energy-dense fare.

Fossils, too, indicate that improvements to dietary quality accompanied evolutionary brain growth. The later, robust proto-humans—a dead-end branch of the human family tree that lived alongside members of our own genus—had heavily built mandibles and huge, thickly enameled molar teeth built for processing tough, low-quality plant foods, while early members of the genus Homo, which descended from the gracile proto-humans, had much more delicate jaws and, smaller molars despite being far larger in terms of overall body size than their predecessors.

Environmental change appears to have set the stage for this evolutionary change when the continued desiccation of the African landscape limited the amount and variety of edible plant foods available to hominids. Thus we often see an increase in animal bones at hominid sites during this period, along with evidence that these beasts were butchered using stone tools. While the robust proto-humans coped with this problem morphologically, evolving anatomical specializations that enabled them to subsist on more widely available, difficult-to-chew foods, Homo took a different path. As it turns out, the spread of grasslands also led to an increase in the relative abundance of grazing mammals such as antelope and gazelle, creating opportunities for hominids capable of exploiting them. Homo developed the first hunting-and-gathering economy in which game animals became a significant part of the diet and resources were shared among members of the foraging groups.

These changes in diet and foraging behavior did not turn our ancestors into strict carnivores, but the addition of modest amounts of animal foods to the menu, combined with the sharing of resources that is typical of hunter-gatherer groups, significantly increased the quality and stability of hominid diets, and after the initial spurt in brain growth, diet and brain expansion probably interacted synergistically: bigger brains produced more complex social behavior, which led to further shifts in foraging tactics and improved diet, which in turn fostered additional brain evolution.

31. The author is primarily concerned with______.

A) disproving the view that bipedalism alone can account for the human brain’s evolution

B) describing a relationship between the acquisition of an improved diet and the development of the human brain

C) contrasting the characteristics of Homo and his primate ancestors

D) analyzing the evolutionary basis for the development of the modern human diet

32. The author refers to the increase in animal bones in the fourth paragraph primarily in order to _____.

A) demonstrate the increase of the availability of grazing mammals during the desiccation of the African landscape

B) provide proof that environmental changes did indeed occur around a time of rapid evolution for Homo

C) explain the means by which Homo was able to make use of the appearance of the antelope and gazelle

D) offer evidence that with the desiccation of the African landscape, Homo’s diet changed from that of the australopithecine

33. It can be inferred from the passage that chimpanzees are characterized by ____.

A) brains that use less overall body energy than the percentage humans use

B) the possession of less total body weight than the average human weight

C) a total resting energy equal to that of human resting energy

D) a diet completely devoid of rich foods preferred by more developed species

34. The author mentions all of the following as evidence that dietary changes accompanied the expansion of Homo’s brain EXCEPT____.

A) the higher calorie density found in foods Homo ate during the expansion, and their extra nutrients

B) the fossil record of Homo, and what it indicates about Homo’s progressive adaptation

C) the fact Homo was larger in terms of overall body size than its predecessors

D) the development of a hunting-and-gathering economy beneficial to further brain growth

35. According to the passage, initial improvement in brain function in Homo’s ancestors resulted at least partially from which of the following?

A) the sharing of resources typical of hunter-gatherer groups to which Homo eventually came to belong

B) the freeing of Homo’s heat-sensitive brain from the temperature constraints that had kept its size in check

C) the availability of foods far denser in calories and nutrients than those previously available

D) morphological solutions of environmental change developed by the robust proto-humans

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