- But for now, vertebrates are only at the beginning
但是现在脊椎动物仍处于 - of a very, very long journey.
漫长旅程的起始阶段 - Twenty million years later, life faced one of its greatest challenges...
2000万年后 生命面临又一个巨大的挑战 - ...as the climate suddenly began to cool...
气候突然变冷 - ...a development that was to have dire consequences.
这将带来可怕的后果 - Little by little, the sea started to freeze.
海洋逐渐开始冰冻 - For the inhabitants of Earth's once tropical waters,
过去地球上热带水域的“居民们”
...as the climate suddenly began to cool... 气候突然变冷
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