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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.
【F3】
选项
答案在过去一百年，大多数国家的自出生算起的预期寿命都在增加，但这不是因为人类寿命变长了，而主要是因为婴儿没有像过去那样轻易被感染性疾病夺走性命。
解析 ①本句是复合句。主干是Life expectancy has increased not because…but mainly because…，意为“人们的预期寿命已经提高，不是因为……，而是因为……”。②主句主语中的at birth是后置定语，修饰Life expectancy，意为“从出生时算起的预期寿命”。in most countries和over the past century分别是地点状语和时间状语。说明主句所述情况出现的地点和时间。③后面的两个原因状语从句分别由not…but…连接，指出原因是后者而非前者。④第二个原因状语从句中的宾语包含as…as…的比较结构，将现在因感染性疾病死亡的婴儿数和过去相比，kill前的否定词not说明现在因感染性疾病死亡的婴儿数没有以前多。句末的did指代的是infectious disease killed infants。
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