Why has life flourished on Earth? This question has a two-part answer. First, Earth has been a cradle for life because of its position relative to the Sun. Second, once life began on Earth, simple early life-forms (photosynthetic bacteria) slowly but inexorably altered the environment in a manner that not only maintained life but also paved the way for later, complex life-forms. These changes allowed later organisms to evolve and thrive. Humans and other higher organisms owe their life-supporting environment to these early life-forms.

Earth’s earliest atmosphere contained several gases: hydrogen, water vapor, ammonia, nitrogen, methane, and carbon dioxide, but no oxygen. Gas mixtures emitted from present-day volcanoes resemble this early atmosphere, suggesting its origin from volcanic eruptions. In Earth’s earliest atmosphere, methane and carbon dioxide occurred at much higher levels than at present—a circumstance that was favorable for early life. Methane and carbon dioxide are greenhouse gases that warm atmospheres by retarding loss of heat to space. These two gases kept Earth warm during the Sun’s early history, when the Sun did not burn as brightly as it now does. (An early dim period, with later brightening, is normal for stars of our Sun’s type.)

Earth’s modern atmosphere, which is 78 percent nitrogen gas, 21 percent oxygen, and about 1 percent argon, water vapor, ozone, and carbon dioxide, differs dramatically from the earliest atmosphere just described. The modern atmosphere supports many forms of complex life that would not have been able to exist in Earth’s first atmosphere because the oxygen level was too low. Also, if atmospheric methane and carbon dioxide were as abundant now as they were in Earth’s earliest atmosphere, the planet’s temperature would likely be too hot for most species living today. How and when did the atmosphere change?

The answer to this riddle lies in the metabolic activity of early photosynthetic life-forms that slowly but surely transformed the chemical composition of Earth’s atmosphere. Some of these early organisms were photosynthetic relatives of modern cyanobacteria (blue-green bacteria). In the process of photosynthesis, carbon dioxide gas combined with water yields oxygen. In Earth’s early days, all over the planet countless photosynthetic bacteria performed photosynthesis. Together, these ancient bacteria removed massive amounts of carbon dioxide from Earth’s atmosphere by converting it to solid organic carbon. These ancient bacteria also released huge quantities of oxygen into the atmosphere. Other ancient bacteria consumed methane, greatly reducing its amount in the atmosphere. When our Sun later became hotter, the continued removal of atmospheric carbon dioxide and methane by early bacteria kept Earth’s climate from becoming too hot to sustain life. Modern cyanobacteria still provide these valuable services today.

The bacterial oxygen release improved conditions for life in two ways. First, oxygen is essential for the metabolic process known as cell respiration that allows cells to efficiently harvest energy from organic food. Second, oxygen in the upper atmosphere reacts to form a protective shield of ozone. Earth is constantly bombarded by harmful ultraviolet (UV) radiation from the Sun. Today, Earth’s upper-atmosphere ozone shield absorbs enough UV to allow diverse forms of life to survive. But because early Earth lacked oxygen in its atmosphere, it also lacked a protective ozone barrier. As a result, early life on Earth was confined to the oceans, where the water absorbed the UV radiation. Only after oxygen released by ancient bacteria drifted up into the upper atmosphere and reacted with other oxygen molecules to form a protective layer of ozone could life flourish at the surface and on the land. The absence of an oxygen atmosphere on Mars and other planets in our solar system means that these planets also lack an ozone shield that would protect surface-dwelling life from UV radiation. The surface of Mars is bombarded with deadly radiation; if any life exists on Mars, it would almost certainly be subterranean.

为什么地球上的生命会蓬勃发展？这个问题有两部分答案：首先，由于和太阳的位置关系，地球成了生命的摇篮；其次，曾经在地球上孕育的生命，是简单的早期生命形态（光合细菌），它们以一种缓慢的但不可阻挡的方式改变着环境，不仅维持着生命，也为未来复杂的生命形态的形成做好了准备。这些改变能让未来的有机体进化并更好地成长。人类和其他的有机体把支撑他们生命的环境归功于这些早期生命形态。 地球最早的大气包括：氢气、水蒸气、氨气、氮气、甲烷以及二氧化碳，但没有氧气。从现今的火山中喷出的混合气体与地球最早的大气成分相类似，这说明这种混合气体来自于火山喷发。在地球最早的大气中，甲烷和二氧化碳的含量比现今高得多——这种情况有利于早期生命的发展。甲烷和二氧化碳是温室气体，它们使空间里热量的消耗减少，从而使太阳早期历史阶段里的地球保持温暖，那时的太阳并没有像现今这般猛烈的燃烧。（早期的昏暗阶段对像太阳这样的恒星来说是正常的，但后来变得明亮了。） 地球如今的大气富含78%的氮气、21%的氧气以及大约1%的氩气、水蒸气、臭氧以及二氧化碳，这与刚刚描述的最早的大气成分截然不同。如今的大气环境养活了多种复杂的生命，而在地球最早的大气环境中，由于氧气含量极低，他们将不能生存。另外，如果现今大气中的甲烷和二氧化碳含量和在地球最早的大气中的含量一样丰富，那么地球的温度对大多数物种来说可能太高了。大气究竟是怎样又是何时变化的呢？ 这个谜题的答案在于早期光和生命形态的新陈代谢活动——这种活动缓慢但确实改变了地球大气中的化学成分。一些这种早期有机体是蓝藻细菌的光合作用的“亲属”。在光合作用过程中，二氧化碳气体与水结合产生了氧气。在地球的早期，无数的光合细菌进行着光合作用。这些古老的细菌通过把二氧化碳转化成固体有机碳的形式将其从地球大气中消耗掉，与此同时，它们也向大气中释放了大量的氧气。其他的古老的细菌消耗着甲烷，使其在大气中的含量急剧减少。当太阳变得越来越热，早期的细菌持续消耗着大气中的二氧化碳和甲烷，使得地球的温度不会变得太高，以此维持着生命。如今的蓝藻细菌也为地球提供着这样有价值的服务。 这种细菌释放的氧气以两种方式改善着生命的生存条件：首先，氧气是新陈代谢过程（也叫细胞呼吸）中必不可少的，它能使细胞从有机食物中有效地获取能量；其次，上层大气中的氧气发生作用产生一层保护膜——臭氧。地球经常受到来自太阳紫外（UV）线的辐射，而地球的上层气体臭氧保护层吸收掉了足够的紫外（UV）线，使多样的物种得以生存。但是，由于早期的地球大气层中没有氧气，也没有一层保护性的臭氧屏障，而水能吸紫外（UV）线，所以地球上的早期生命仅限于生存在海洋中。只有在古老的细菌释放出的氧气上升到了大气上层，从而与其他氧气分子发生反应形成一层臭氧保护层时，地球上的生命才能在地表和陆地上蓬勃发展。太阳系中的火星和其他行星上没有氧气层，这意味着这些行星也没有能保护地表上生存的生物免受紫外（UV）线辐射的一层臭氧屏障，所以火星表面的辐射十分强烈。如果火星上有生命存在，几乎肯定是生存在地下的。