请问为什么要用having been separated?而不是用to be separated?

问题描述:

请问为什么要用having been separated?而不是用to be separated?
Having been separated from other countinents for millions of years,Australia has many plants and animals not found in any other country in the world.

因为已经发生的事实“be separated from other continents for millions of years" 才造成了现在”Australia has many plants and animals not found in any other country in the world“ 这个事实,所以是过去发生...