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460 million years ago, the plates have been moving, again.
4亿6000万年前板块再度移动，
A new continent now exists, Gondwana.
底下有一块新大陆叫冈瓦那。
It's 90 degrees and oxygen levels are close to what they are today.
温度约32摄氏度，氧气浓度与现在的大气相仿，
The land should be covered with plants and crawling with creatures.
陆地应该满是动植物，
But there's not much here,
然而除了几块藻，
besides a few patches of algae.
这里什么也没有。
There's only one explanation, the Sun.
只有一种解释，就是太阳。
It blasts the surface with deadly radiation,
它用致命辐射照射地面，
so the complex life in the ocean doesn't stand a chance on land.
我们在海里看到的复杂生命，无法在陆地上存活。
But 30 miles up, where the rays enter the Earth's atmosphere, something is happening.
不过往上约50公里，也就是阳光进入地球大气之处，却出现奇怪的现象，
When oxygen meets the sun's radiation,
碰到阳光辐射之后，
it turns into another kind of gas, called ozone.
氧变成名为臭氧的气体。
This gas forms a blanket around the planet and absorbs the lethal radiation.
这种气体入毯子般包住地球，臭氧层吸收致命辐射。
