Understanding this cycle is important, not only because it sustains life on Earth, but also because it is bound up with the rate of global warming.

理解这种循环很重要，不仅因为碳循环维持地球上的生命，而且因为它对于全球变暖的速度有密切关系。

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Early life-forms on Earth were confined to the oceans because water was the only available protection they had against ultraviolet radiation.

地球上早期的生命形式被限制在海洋里，因为水是帮助它们抵御紫外线辐射的唯一有效保护。

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The finding not only presents the possibility that alternative life forms can exist, or once existed, on Earth.

这一发现不仅显示了地球上可能存在其他类型的生命，或者曾经存在过。

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There's a short story by the science fiction writer Orson Scott Card, where the basic point of the story is that of all the life forms in the universe, we, here on Earth, are the only ones that are mortal.

科幻作家Orson，Scott，Card写了一个短故事，该故事的主旨是,在宇宙的所有生命形式中，我们，在地球上是唯一终有一死的生命。

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That might help evolutionary biologists answer the question of why, for billions of years, and in every form of life on earth, only two base pairs are used.

ECONOMIST: A new kind of DNA

Because water is considered essential for the development of the only life known to exist life on Earth scientists are focusing on wet areas, past and present.

MSN: Curiosity aims to unlock Mars mysteries - Technology & science - Space - Space.com | NBC News

Life on earth today is possible only because the key ratios are exactly right.

FORBES: Wealth and Fitness Secret - Ratios