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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.
【F1】
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答案顶级运动员在100米赛跑项目上或许还可以把世界纪录再削减几毫秒，但怎么也不可能跑出5秒或2秒的成绩。
解析 ①本句是由but连接的并列句。分句1的谓语为shave sth．off sth．结构，表示“略减，削减”。shave a few milliseconds off the world record意为“将世界纪录缩短几毫秒”；for the 100-metre race是后置定语，修饰前面的the world record，意为“百米赛跑的世界纪录”。②分句2由but连接，和前一分句是转折关系，说明顶级运动员虽然能将世界纪录再缩减几毫秒，但也不可能在几秒内跑完全程。③same distance指前半句的the 100-metre race，即100米赛跑项目。④say是插入语，表示“假如，例如”，常用于举例说明。two后面省略了seconds，即分句2的时间状语完整形式为in five seconds or two seconds。
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