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.
这个谜题的答案在于早期光合生命形式的新陈代谢活动，这种活动缓慢但一定改变了地球大气的化学成分。
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An interesting thing is that our brain will release a pleasure-related chemical when it has been given new information, making seeking information much like eating—another pleasant activity.