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  3. Although humans have much bigger brains relative to body weight than do other primates, the total resting energy require

Although humans have much bigger brains relative to body weight than do other primates, the total resting energy require

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问题              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
       Hondo, 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 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.
According to the passage, initial improvement in brain function in Homo’s ancestors resulted at least partially from which of the following?
选项 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、a rise in Homo’s brain weight relative to his body weight
E、morphological solutions of environmental change developed by the robust proto-humans
答案C
解析
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