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  3. Why do animals grow old and die at characteristic ages? Even if maintained in peak condition and not eaten by your cat, your ham

Why do animals grow old and die at characteristic ages? Even if maintained in peak condition and not eaten by your cat, your ham

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问题    Why do animals grow old and die at characteristic ages? Even if maintained in peak condition and not eaten by your cat, your hamster is unlikely to make it much past its second birthday. And your cat might live for ten times that. Yet neither cat nor hamster will ever match the average healthy human for longevity.
   A study uses demographic data to reveal a lifespan that human beings cannot exceed. It’s like running. 【F1】Elite athletes might shave a few milliseconds off the world record for the 100-metre race, but they’ll never run the same distance in, say, five seconds, or two. Human beings are simply not made that way. The same is true for longevity. 【F2】The consequences of numerous factors related to our genetics, metabolism, reproduction and development, all shaped over millions of years of evolution, means that few humans will make it past their 120th birthdays.
   Maximum lifespan is a bald measure of years accumulated. It is not the same as life expectancy, which is an actuarial measure of how long one is expected to live from birth, or indeed from any given age. 【F3】Life expectancy at birth has increased in most countries over the past century, not because people have longer lifespans, but mainly because infectious disease does not kill as many infants as it once did.
   【F4】So if we owe our increases in life expectancy to better public health, nutrition, sanitation and vaccination, is it not fair to ask whether more-effective treatments for diseases such as cancer, Parkinson’s disease and Alzheimer’s might also yield dividends in maximum lifespan? Will 120th birthday parties become routine, outmatched by a small yet increasing number of sesquicentenarians? The demographic data say no. People are living longer, and the population as a whole is greying, but the rate of increase in the number of centenarians is slowing, and might even have peaked.
   Could it be possible, in some science-fictional future, to break free from the bonds of human life expectancy and increase lifespan indefinitely? Technological solutions might one day transcend the limitations of the human body. But the risks of transcendence are twofold. 【F5】First, it might be that to extend our lives beyond our normal span, we must somehow become other than human. After all, what would a 50-year-old hamster be like? Second, there is a risk that life wouldn’t really be that much longer—it would only feel like it.
【F5】
选项
答案首先，人类的寿命超出了正常阈值，或许人类就不再是人类了。
解析 ①本句是复合句。主句用it might be that结构中肯地引出第一个risk“风险”的所在。that后面的不定式to extend our lives beyond our normal span是表语从句中的状语，意为“使人类寿命超出正常阈值”。②表语从句中的must表示猜测，推断在to extend our lives beyond our normal span情况下的人类情况。③other than本义是“与……不同，非”，这里是指人类就不成其为人了。
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