(单词翻译:单击)
听力文本
Oxygen and water are crucial to most life on Earth, but what about nitrogen?
It's in every molecule of DNA in your body, and in all your proteins—you literally can't live without it.
But most of Earth's nitrogen exists as an inert atmospheric gas that organisms can't use.
Lightning strikes can convert some nitrogen into a bioavailable form.
But most of the biosphere's usable nitrogen is the result of bacteria employing an enzyme called nitrogenase to pull nitrogen out of the air.
Based on genetic evidence, scientists have thought that nitrogenase first evolved around 2 billion years ago.
Before that, life on Earth might have been confined to the oceans and been limited by the crucial substance's inaccessibility.
But researchers at the University of Washington now have evidence for the existence of nitrogenase in bacteria going back some 3.2 billion years.
The researchers base their argument on the ratios of light to heavy nitrogen isotopes in ancient rocks from Australia.
The study is in the journal Nature.
The finding indicates that the biosphere more than three billion years ago was much more complex than previously appreciated, and perhaps had already colonized land.
An earlier arrival for nitrogenase also may mean that the enzyme evolves more easily than was previously believed.
Which could increase the odds that, sooner or later, astrobiologists will find signs of another robust biosphere on some world far away.
中文翻译
氧气和水对地球上大多数生物而言不可或缺，那氮又如何呢?
它存在于你体内每个DNA分子结构以及所有的蛋白质里—毫不夸张地说，没有它你无法存活。
但地球上的绝大多数氮气都是以一种生物体无法使用的惰性气体形式存在。
雷电能将部分氮气转化成一种可用的生物形式。
但生物圈中的大部分可用氮都是细菌利用一种名为固氮酶的酶氮气从空气中分离出来的。
科学家们基于基因遗传证据认为约20亿年前固氮酶就已经完成了第一次进化。
在那之前，地球上的生命可能仍仅存在海洋当中并且关键物质的获取极为困难。
但华盛顿大学研究人员的证据表明早在约32亿年前固氮酶就存在于细菌中。
研究人员是根据澳大利亚古老岩石中重氮同位素光的比率所得出的结论。
这项研究已经在《自然》杂志上发表。
该研究表明30多亿年前的生物圈比我们目前推测的要复杂得多，而且也许已延伸到了陆地。
固氮酶存在时间的提前也可能揭示酶的发展比科学家早些推测的更为简单。
而这可能会增加天体生物学家在某个遥远星球上发现新的生机勃勃生物圈存在迹象的几率。
译文为可可英语翻译，未经授权请勿转载！
词语解释
1.crucial to a.关键性的，极其显要的
Talent, hard work and sheer tenacity are all crucial to career success.
事业要成功，才能、勤奋和顽强的意志都至关重要。
It is crucial that all documents presented are authentic and easily verifiable.
所有提交的文件必须真实且易于查证，这一点很关键。
2.confine to 限于…之内
Doctors confine themselves to prescribing rather than dispensing.
医生仅仅是开药而不配药。
Doctors are trying to confine the disease within the city.
医生正试图将此病控制在城内。
