A lot of readers have commented on my recent post about a study that suggests we all share a common ancestor who lived 2,300 years ago. Some people doubted that isolated groups could share such a recent ancestry. One of the study's authors, Steve Olson (also the author of the book Mapping Human History) sent me the following email yesterday: "Ensuring a recent common ancestor doesn't take long-range migrations (although contact between the Polynesians and South Americans certainly speeds things up). All it really requires is that a person from one village occasionally mates with a person from an adjoining village; after that the power of exponential growth, and the dynamics of small worlds networks, take over. As for counterexamples, I've been looking for five years for examples of populations that were completely isolated, and I've decided that they're rare to the point of nonexistence. The Tasmanians are a possibility, but it's only 60 miles from Tasmania to Australia -- that no one made that trip in 9,000 years seems counterintuitive to me. And of course it only takes one person to link two genealogical networks, even though the amount of gene flow represented by that one person may be negligible (though I also think that gene flow in the past has been much more extensive and much more continuous than most people imagine)."