Verbal | Re: What is dash meant for..?

Hi Brett,

Please help me understand the hyphen here

Some anthropologists believe that the genetic homogeneity evident in the world's people is the result of a "population bottleneck"—at some time in the past our ancestors suffered an event, greatly reducing their numbers and thus our genetic variation.

(B) that at some time in the past our ancestors suffered an event that greatly reduced their numbers

This is correct answer choice but i really didnt understand

Thanks for ur