For two billion years bacterial organisms were the only forms of life. 在20亿年时间里，细菌是惟一的生命形式。

　　They lived, they reproduced, they swarmed, but they didn't show any particular inclination to move on to another, more challenging level of existence. 它们活着，它们繁殖，它们数量增加，但没有表现出想发展展到另一个更富挑战性的生存层面的特别倾向。

　　At some point in the first billion years of life, cyanobacteria, or blue-green algae, 在生命的头10亿年的某个时候，藻青菌，或称蓝绿藻，

　　learned to tap into a freely available resource—the hydrogen that exists in spectacular abundance in water. 学会了利用大量存在的资源——存在于水中的特别丰富的氢。

　　They absorbed water molecules, supped on the hydrogen, and released the oxygen as waste, and in so doing invented photosynthesis. 它们吸叫水分子，吃掉了氢，排出了氧，在此过程中发明了光合作用。

　　As Margulis and Sagan note, photosynthesis is "undoubtedly the most important single metabolic innovation in the history of life on the planet" 正如马古利斯和萨根指出的，“光合作用无疑是本星球的生命史上所创造的最重要的新陈代谢方法”

　　and it was invented not by plants but by bacteria. 光合作用是由细菌而不是由植物发明的。

　　As cyanobacteria proliferated the world began to fill with O2 to the consternation of those organisms that found it poisonous—which in those days was all of them. 随着藻青菌的增多，世界开始充满氧气，发现氧是有毒的微生生深感吃惊——而在那个年代，那种微生物比比皆是。

　　In an anaerobic (or a non-oxygen-using) world, oxygen is extremely poisonous. 在一个厌氧的（或不使用氧的）世界里，氧是剧毒的。

　　Our white cells actually use oxygen to kill invading bacteria. 我们的白血球实际上就是氧来杀死入侵的细菌。

　　That oxygen is fundamentally toxic often comes as a surprise to those of us who find it so convivial to our well-being,  氧从根本上说是很毒的，我们听了往往会大吃一惊，因为许多人觉得呼吸氧是很舒服的事，

　　but that is only because we have evolved to exploit it. 但那只是因为我们已经逐步进化到了能利用氧。

　　To other things it is a terror. 对于别的东西来说，它是一种可怕的东西。

　　It is what turns butter rancid and makes iron rust.  它使黄油变质，使铁生锈。