大概10亿年前，一些简单的绿色植物开始生成氧气，后来就逐渐变成我们现在大气层的一个主要部分。
About a billion years ago, simple green plants started generating oxygen, which soon became a major part of our atmosphere as well.
大概10亿年前，一些简单的绿色植物开始生成氧气，后来就逐渐变成我们现在大气层的一个主要部分。
About a billion years ago, simple green plants started generating oxygen, which soon became a major part of our atmosphere as well.
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