Various theories seek to explain this mass extinction.

对于这次大灭绝，有很多种解释。

But the Mesozoic, too, began with a mass extinction.

但中生代本身便是以一次大灭绝开始的。

Before the mass extinction, the top ocean predators were primitive sharks.

在大规模灭绝之前，高等海洋肉食生物是远古的鲨鱼。

Yet sheer Numbers do not totally explain the current mass extinction.

然而，当前物种的大规模灭绝不光是人口数目激增造成的。

First, that there was not one mass extinction at the end of the Permian, but several.

第一，在二叠纪结束的时候并不是一个大规模灭绝，而是好几个。