In Earth's early days, all over the planet countless photosynthetic bacteria performed photosynthesis.
在地球早期，整个星球上有无数的光合细菌在进行光合作用。
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Photosynthetic life is impossible there because sunlight is completely absent, but life such as the microbes that flourish deep within earth may still be possible.
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.