Des Marais D J
Space Science Division, Ames Research Center, Moffett Field, CA, USA.
Gravit Space Biol Bull. 1998 May;11(2):23-30.
Understanding our own early biosphere is essential to our search for life elsewhere, because life arose on Earth very early and rocky planets shared similar early histories. The biosphere arose before 3.8 Ga ago, was exclusively unicellular and was dominated by hyperthermophiles that utilized chemical sources of energy and employed a range of metabolic pathways for CO2 assimilation. Photosynthesis also arose very early. Oxygenic photosynthesis arose later but still prior to 2.7 Ga. The transition toward the modern global environment was paced by a decline in volcanic and hydrothermal activity. These developments allowed atmospheric O2 levels to increase. The O2 increase created new niches for aerobic life, most notably the more advanced Eukarya that eventually spawned the megascopic fauna and flora of our modern biosphere.
了解我们自己早期的生物圈对于我们寻找其他地方的生命至关重要，因为生命在地球早期就已出现，且岩石行星有着相似的早期历史。生物圈在38亿年前就已出现，最初完全由单细胞生物构成，且以嗜热微生物为主，这些微生物利用化学能源，并采用一系列代谢途径进行二氧化碳同化。光合作用也很早就出现了。产氧光合作用出现得较晚，但仍在27亿年前之前。向现代全球环境的转变是由火山和热液活动的减少所推动的。这些发展使得大气中的氧气水平得以增加。氧气的增加为需氧生物创造了新的生态位，最显著的是更高级的真核生物，它们最终孕育出了我们现代生物圈中的宏观动植物。
