there're almost 7 billion people on earth is in large part due to the roles the dogs have played in our evolutionary existence.我想地球上有几乎70亿的人口的原因很大程度上也是基于狗狗在我们进化过程中所扮演的角色

due to:因为，由于

Some students quit school due to poverty.

一些学生因贫困而辍学。

Mr.Black bowed out of politics due to poor health.

布莱克先生由于健康不佳而退出政治生涯。

play a role in:在 ... 中起作用； 扮演角色

He was invited to play a role in this TV play.

他被邀请在这个电视剧里扮演一个角色。

we can have good relationship with wide variety of animals historically our relationship with dogs人类历史上和很多种类的动物有很好的关系

variety of:各种的

A great variety of flowers were shown in the park.

公园里展出了很多品种的花。

Personally I don't think it's any coincidence the dogs is refered to man's best friend.以我个人之见我认为狗狗能成为人类最好的朋友并不仅是一种偶然的巧合

refer to:涉及；指的是；提及；参考；适用于

The new law does not refer to land used for farming.

这条新的法律不适用于农用土地。

Don't refer to this matter again, please.

请不要再提这件事了。