Wow, an actual reasoned response. OK, that makes sense. Believe it or not, I have no problem with that part. A certain trait, already present, becomes dominant among one group when they are able to procreate more successfully than the other group. Just for the sake of info, how do these researchers know when this took place? It seems to me that knowing when a particular gene moderator became the dominant expressed trait in any group, has to be, well... questionable?
By looking at the degree of variation between different copies of the mutated gene, microbiologists can estimate the age of its earliest appearance, as certain mutation rates are reasonably constant. At the present time, this kind of dating is still only approximate, as you'll notice with the date used in this article which has an accuracy of +/- 2000 years.