there have been more than 50 cycles of ice buildup and melt... 已经发生50多次结冰融化的循环
But there is one more chapter in the mammal story still to be told. 但是哺乳动物的故事 还有一章仍待讲述
But there is one obvious opportunity. 但有一个明显的机会
there rose from the ashes a life-form that had previously only survived 一种此前只在边缘地带幸存的生物
there was a limit to how far they could rise. 它们的崛起受到限制