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Billions of people, one common ancestor

By ASSOCIATED PRESS
Published July 2, 2006

Whoever it was probably lived a few thousand years ago, somewhere in East Asia - Taiwan, Malaysia and Siberia all are likely locations. He - or she - did nothing more remarkable than be born, live, have children and die.

Yet this was the ancestor of every person now living on Earth - the last person in history whose family tree branches out to touch all 6.5-billion people on the planet today.

That means everybody on Earth descends from somebody who was around as recently as the reign of Tutankhamen, maybe even during the Golden Age of ancient Greece. There's even a chance that our last shared ancestor lived at the time of Christ.

"It's a mathematical certainty that that person existed," said Steve Olson, whose book Mapping Human History traces the history of the species since its origins in Africa more than 100,000 years ago.

Few people realize just how intricately the web is that connects them not just to people living on the planet today, but to everyone who ever lived.

With the help of a statistician, a computer scientist and a supercomputer, Olson has calculated that you would have to go back in time only 2,000 to 5,000 years to find somebody who could count every person alive today as a descendant.

Furthermore, Olson and his colleagues have found that if you go back 5,000 to 7,000 years, everybody living today has exactly the same set of ancestors.

That means all of us have ancestors of every color and creed. Every Palestinian suicide bomber has Jews in his past. And every Ku Klux Klan member's family has African roots.

It's simple math. Every person has two parents, four grandparents and eight great-grandparents. Keep doubling back through the generations - 16, 32, 64, 128 - and within a few hundred years you have thousands of ancestors.

It's nothing more than exponential growth combined with the facts of life. By the 15th century you've got 1-million ancestors. By the 13th you've got 1-billion. Sometime around the 9th century - just 40 generations ago - the number tops 1-trillion.

But how could anybody - much less everybody - alive today have had 1-trillion ancestors living during the 9th century?

The answer is, they didn't. Imagine there was a man living 1,200 years ago whose daughter was your mother's 36th great-grandmother, and whose son was your father's 36th great-grandfather. That would put him on two branches on your family tree.

Most of the people who lived 1,200 years ago appear not twice, but thousands of times on our family trees, because there were only 200-million people on Earth back then.

But many of the people who were alive in the year 800 never had children. Meanwhile, more prolific members of society would show up many more than 5,000 times on a lot of people's trees.

Keep going back in time, and there are fewer and fewer people available to put on more and more branches of the 6.5-billion family trees of people living today. It is mathematically inevitable that at some point, there will be a person who appears at least once on everybody's tree.

[Last modified July 2, 2006, 02:36:29]