All she's done is demonstrate a particular case of the obvious, we're all cousins to some degree and it's easy to demonstrate with a relatively simple analysis. Everybody has one pair of parents, two pairs of grandparents, four pairs of great grandparents, the numbers doubling at each step back. Given the nature of geometric progressions, the number of ancestors soon exceeds the number of people who've ever lived, and the explanation is that people start showing up on both sides of a family tree and get counted multiple times. First cousins who marry, for instance, share a set of grandparents, so their children will have only three pairs of great grandparents. The farther back you go the more widespread that is in a family tree. If you go back in time far enough (and it's not really very far, only a few thousand years I think) everybody alive at that point can be put into one of two categories: ancestors of everybody currently alive, or ancestors of nobody currently alive. Richard Dawkins laid out a detailed analysis in his 1995 book A River Out of Eden.