题目选项分析

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

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.