Back in March, I wrote a blog entry discussing recent research on the number of descendants of Genghis Khan who still walk the earth today; the entry also touched on the question of just how far back we need to go in history to find the first common ancestor of everyone alive today.
At The Loom, Carl Zimmer has written two entries on the subject. (The first is the more detailed and interesting.) Here's an excerpt:
It turns out that those awkward cousins are what keeps our family tree--along with everyone else's--from exploding into exponential absurdity. Go back far enough, and all the branches on your family converge on common ancestors. Go back further, and all family trees meet. How far back you have to go is the subject of a report in today's issue of Nature. The researchers put together a statistical model of today's population and worked their way back. To give the model some historical realism, they didn't let their virtual people randomly join together to have kids. Instead, they included some geographical structure. People in Indonesia, for example, have historically had children with other Indonesians. But they've also had a small amount of interchange with people in other regions--even with people as far away as Madagascar, which some Indonesians colonized 1600 years ago. (Douglas Rohde, the lead author, has posted a draft manuscript of a longer paper on this work on his MIT web site.)
Once the model was ready, Rohde and his colleagues used it to go back in time, in order to find a common ancestor of all living humans. They estimate that that person lived just 2,300 years ago. Of course, this doesn't mean that this person gave rise to all humans single-handedly. All it means is that, by one route or another, all family trees lead back to him or her. Those trees may belong to Australian aborigines, Arctic Inuits, or the residents of Easter Island.
Go back 5,000 years (160 generations) and things even weirder. According to the model, 80% of all people alive at the time are ancestors of every living person.
One of these days I'll have the chance to read the Nature article or Rohde's longer draft manuscript. Until then, here's another short summary of the research from news@nature.Posted by Ed at October 13, 2004 09:24 AM