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  3. As a species, humans are incredibly smart. Our intelligence comes with a curious caveat: our babies are among the dumbest—or, ra

As a species, humans are incredibly smart. Our intelligence comes with a curious caveat: our babies are among the dumbest—or, ra

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问题     As a species, humans are incredibly smart. Our intelligence comes with a curious caveat: our babies are among the dumbest—or, rather, the most helpless—that exist. A baby giraffe can stand within an hour of birth, and can even potentially flee predators on its first day of life. A monkey can grasp its mother and hang on for protection and nourishment. A human infant can’t even hold up its own head.
    However, as Kidd and Piantadosi argue in their new paper, published in a June issue of PNAS, hu-mans become so intelligent because human infants are so incredibly helpless; the one necessitates the other. The theory is startling, but it isn’t entirely new. Researchers have been pondering the peculiarities of our birth and its evolutionary significance for quite some time. Humans belong to the subset of mammals, called viviparous mammals, that give live birth to their young. This means that infants must grow to a mature enough state inside the body to be born, but they can’t be so big that they are unable to come out.【B11】_____________The brain, therefore, must keep maturing, and the head must continue growing, long after birth. The more intelligent an animal will eventually be, the more relatively immature its brain is at birth.
    Researchers have long known about this trade-off, and about the connection between brain size and neural density and intelligence.【B12】_________________
    Kidd and Piantadosi’s new idea is that increased helplessness in newborns mandates increased intelligence in parents—and that a runaway selection dynamic can account for both. Natural selection favors humans with large brains, because those humans tend to be smarter.【B13】__________
    During their investigation, Kidd and Piantadosi realized something important that strengthened their theory. It turns out that another variable has an even higher correlation with intelligence  than brain size—time to maturity, or weaning time.【B14】___________________Orangutans  have smarter babies than baboons and they wean them longer. Baboon babies, in turn, are weaned longer, and are smarter, than lemur babies.
    Putting these facts together helped Kidd and Piantadosi develop their hypothesis. The connection be-tween head size and intelligence does create incentives for babies to arrive earlier. But it’s the connection between weaning time and intelligence that may really be driving the cycle.【B15】__________________And so the cycle continues.
    [A]  As we grew smarter, we were better able to take care of our infants, so they could be born more helpless and allow us to grow even smarter. This is why the cycle happens to humans and not to lemurs.
    [B]  You need to be smarter to care for more helpless creatures, which means you need a larger brain— which means that babies have to enter the world at an even more helpless stage of development, since there is a finite size to their brain at birth, mandated by the physiology of live birth.
    [C]  In other words, the time it takes to shepherd newborns through absolute helplessness to a point of relative self-sufficiency predicts primate intelligence more strongly than the best measure that has previously been proposed, namely, head circumference.
    [D]  For instance, Robin Dunbar found that the ratio of neocortical volume to brain size can predict the social-group size in a number of species, including bats, cetaceans, and primates, while Simon Read-er has demonstrated links in tool use and innovation to brain size in primates.
    [E]   This may create evolutionary incentives for babies that are born at an even earlier developmental stage, which require more intelligence to raise. This creates the dynamic: over time, helpless babies make parents more intelligent, which makes babies more helpless, which makes their parents more intelligent, and so on.
    [F]  This leads to a trade-off: the more intelligent an animal is, the larger its head generally is, but the birth canal imposes an upper limit on just how large that head can be before it gets stuck.
    [G]  This is a contradiction. Humans are born quite helpless, far more so than any other primate, but, fairly early on, we start becoming quite smart, again far more so than any other primate.
【B11】
选项
答案F
解析 空格前指出人类胎儿必须在母体内成长到足够成熟才能出生，可是胎儿又不能成长得太大，不然无法出世。空格后一句讲述在出生之后很长一段时间内，大脑必须不断成熟、长大，其中的therefore说明空格处是空格后所说的内容的缘由。F指出产道限制了婴儿头部的大小，这既是由空格前的内容导致的，也是空格后内容的原由，因此F用leads to来承接上文，同时使得下文的therefore衔接恰当，过渡连贯，自然顺畅。F中的This指代的是空格前提到的情形，而trade-off则与第三段的this trade-off相对应。故答案选F。
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