Scientists estimate that person's natural life span is 120 years old, why?

科学家推算，人的自然寿命是120岁，为什么呢？

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But 20th century medicine did little to increase the natural life span of healthy humans.

然而，20世纪的医学对健康人的自然生命周期的增长几乎无能为力。

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Some are so slow growing they never would have caused medical problems in the person's natural life span.

一些肿瘤长的那么慢，它们从不会在一个人的自然生存期内导致问题。

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Sceptics point out that if big cats have been surviving, and breeding, in the wild for all this time, they only have a natural life span of some 20 years.

BBC: The Surrey puma: Fact or fiction?

Scientists have long speculated that the key to a person's "natural" life span lies in so-called diseases of aging, those that emerge only as the body begins to wear out.

WSJ: A Gene Gives a Hint Of How Long a Person Might Hope to Live

But bacteria and viruses breed fast, so natural selection has time, within the span of a human life, to make a difference.

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