Our common forefather?

Captain Ed links to an interesting article from the Daily Telegraph -- all 6 billion of us may be descended from one man who lived only 3500 years ago.

Using a computer model, researchers from the Massachusetts Institute of Technology attempted to trace back the most recent common ancestor using estimated patterns of migration throughout history.

They calculated that the ancestor's location in eastern Asia allowed his or her descendants to spread to Europe, Asia, remote Pacific Islands and the Americas. Going back a few thousand years more, the researchers found a time when a large fraction of people in the world were the common ancestors of everybody alive today - while the rest were ancestors of no one alive. That date was 5,353BC, the team reports in Nature.

The researchers, led by Dr Steve Olson, stressed that the date was an estimate.

"Nevertheless, our results suggest that the most recent common ancestor for the world's current population lived in the relatively recent past - perhaps within the last few thousand years," he said.

He added: "No matter the languages we speak or the colour of our skin, we share ancestors who planted rice on the banks of the Yangtze, who domesticated horses on the steppes of the Ukraine, who hunted giant sloths in the forest of north and south America and who laboured to build the Great Pyramid of Khufu."

Now the phrase "most recent common ancestor" doesn't imply that he was the proverbial last man on earth. It just means that like every other person on earth, I have this guy as one of my many, many great¹⁰⁰grandpas.

One generation before you, you have two ancestors, mom and dad. Two generations, it's 2², four grandparents. At 100 generations, if there were no overlap, if every one of your ancestors married someone without a common ancestor, you would have 2¹⁰⁰ = 1267650600228229401496703205376 ancestors -- that's 1.2 nonillion. Since the world's population has never been higher than it is right now, there has to have been overlap -- cousins marrying cousins, even if very distant ones.