According to this article in The Atlantic back in May 2002,
The mathematical study of genealogy indicates that everyone in the world is descended from Nefertiti and Confucius, and everyone of European ancestry is descended from Muhammad and Charlemagne
The article further states (in paragraph 8):
The idea that virtually anyone with a European ancestor descends from English royalty seems bizarre, but it accords perfectly with some recent research done by Joseph Chang, a statistician at Yale University. The mathematics of our ancestry is exceedingly complex, because the number of our ancestors increases exponentially, not linearly.
and in paragraph 9:
In other words, all Europeans alive today have among their ancestors the same man or woman who lived around 1400.
and in paragraph 12:
This constant churning of people makes it possible to apply Chang's analysis to the world as a whole. For example, almost everyone in the New World must be descended from English royalty—even people of predominantly African or Native American ancestry, because of the long history of intermarriage in the Americas. Similarly, everyone of European ancestry must descend from Muhammad. The line of descent for which records exist is through the daughter of the Emir of Seville, who is reported to have converted from Islam to Catholicism in about 1200. But many other, unrecorded descents must also exist.
and in paragraph 13:
Chang's model has even more dramatic implications. Because people are always migrating from continent to continent, networks of descent quickly interconnect. This means that the most recent common ancestor of all six billion people on earth today probably lived just a couple of thousand years ago. And not long before that the majority of the people on the planet were the direct ancestors of everyone alive today. Confucius, Nefertiti, and just about any other ancient historical figure who was even moderately prolific must today be counted among everyone's ancestors.
Has this study or reasoning since been rejected?