GMAT Sentence Correction (SC) | Re: Some anthropologists believe that the genetic homogeneity

I have a doubt regarding your explanation of answer 'A'. According to my understanding, Greatly reducing is a -ing modifier followed by comma

generally modifies the entire clause or is used to indicate the result of preceding statement. So in this case the modifier modifies the entire clause that at some time ancestors suffered an event and this led to great reduction in their number. So, according to me the modifier usage