单选题：Although humans have much bigger brains relative to body wei

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
Line a higher share of our daily energy budget to feed our voracious brains. How did
(5) 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
(10) 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
(15) 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
(20) 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
(25) 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
(30) 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
(35) 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
(40) groups, significantly increased the quality and stab