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Roots of human family tree are shallow

By MATT CRENSON, AP National Writer Sat Jul 1, 5:17 PM ET

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 2002 book "Mapping Human History" traces the history of the species since its origins in Africa more than 100,000 years ago.

It is human nature to wonder about our ancestors — who they were, where they lived, what they were like. People trace their genealogy, collect antiques and visit historical sites hoping to capture just a glimpse of those who came before, to locate themselves in the sweep of history and position themselves in the web of human existence.

But few people realize just how intricately that web 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 just how interconnected the human family tree is. You would have to go back in time only 2,000 to 5,000 years — and probably on the low side of that range — 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 a little farther — about 5,000 to 7,000 years ago — everybody living today has exactly the same set of ancestors. In other words, every person who was alive at that time is either an ancestor to all 6 billion people living today, or their line died out and they have no remaining descendants.

That revelation is "especially startling," statistician Jotun Hein of England's Oxford University wrote in a commentary on the research published by the journal Nature.

"Had you entered any village on Earth in around 3,000 B.C., the first person you would have met would probably be your ancestor," Hein marveled.

It also means that all of us have ancestors of every color and creed. Every Palestinian suicide bomber has Jews in his past. Every Sunni Muslim in Iraq is descended from at least one Shiite. And every Klansman's family has African roots.

How can this be?

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 a million ancestors. By the 13th you've got a billion. Sometime around the 9th century — just 40 generations ago — the number tops a trillion.

But wait. How could anybody — much less everybody — alive today have had a 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, one on your mother's side and one on your father's.

In fact, 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. Simple division — a trillion divided by 200 million — shows that on average each person back then would appear 5,000 times on the family tree of every single individual living today.

But things are never average. Many of the people who were alive in the year 800 never had children; they don't appear on anybody's family tree. 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.

But don't stop there; keep going back. As the number of potential ancestors dwindles and the number of branches explodes there comes a time when every single person on Earth is an ancestor to all of us, except the ones who never had children or whose lines eventually died out.

And it wasn't all that long ago. When you walk through an exhibit of Ancient Egyptian art from the time of the pyramids, everything there was very likely created by one of your ancestors — every statue, every hieroglyph, every gold necklace. If there is a mummy lying in the center of the room, that person was almost certainly your ancestor, too.

It means when Muslims, Jews or Christians claim to be children of Abraham, they are all bound to be right.

. . .

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Originally posted by beskeptigal

I don't buy this. For example Aboriginals in Australia left Africa 40-60,000 years ago. How could they have an ancestor in common from only 6,000 years ago? So these people have no pure descendants left? Is that the idea?

I can't see that we've all mixed that much in the last few millenniums. I understand the statistical premise but it doesn't account for isolation of groups.

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Originally posted by Dave W.

Well, we know already from the poorly named "Mitochondrial Eve" studies that all living humans today are related through a single woman. The difference between the current research and the "ME" research is that the "ME" lived some 80,000 years ago, a much more credible figure given isolationist peoples around the world. I mean, the idea that our last common ancestor might have existed only 2,000 years ago is based on the premise that as soon as "contact" between some small tribe and the rest of the world, the tribespeople almost instantly are swapping European genes like rabbits. I would find that hard to believe. I would find it hard to believe that there do not exist any native Americans who've avoided European genes.

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Originally posted by Dave W.

Well, we know already from the poorly named "Mitochondrial Eve" studies that all living humans today are related through a single woman. The difference between the current research and the "ME" research is that the "ME" lived some 80,000 years ago, a much more credible figure given isolationist peoples around the world. I mean, the idea that our last common ancestor might have existed only 2,000 years ago is based on the premise that as soon as "contact" between some small tribe and the rest of the world, the tribespeople almost instantly are swapping European genes like rabbits. I would find that hard to believe. I would find it hard to believe that there do not exist any native Americans who've avoided European genes.

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Originally posted by Zebra

Interesting. Mathematically it seems like it makes sense, but it's hard to picture people looking so different, and living so many different places, yet being so inter-related.

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Depending on the survival of isolated lineages without admixture from Modern migrations and taking into account long-isolated peoples, such as historical tribes in central Africa, Australia and remote islands in the South Pacific, the human MRCA is generally assumed to have lived in the Paleolithic period.

However, Rohde, Olson, and Chang (2004), using a non-genetic model, estimated that the MRCA of all living humans may have lived within historical times (3rd millennium BC to 1st millennium AD). Rohde (2005) refined the simulation with parameters from estimated historical human migrations and of population densities. For conservative parameters, he pushes back the date for the MRCA to the 6th millennium BC (p. 20), but still concludes with a "surprisingly recent" estimate of a MRCA living in the second or first millennium BC (p. 27). An explanation of this result is that, while humanity's MRCA was indeed a Paleolithic individual up to Early Modern times, the European explorers of the 16th and 17th centuries would have fathered enough offspring so that some "mainland" ancestry by today pervades even remote habitats. The possibility remains, however, that a single isolated population with no recent "mainland" admixture persists somewhere, which would immediately push back the date of humanity's MRCA by many millennia. While simulations help estimate probabilities, the question can only be resolved authoritatively by genetically testing every living human individual.

Other models reported in Rohde, Olson, and Chang (2004) suggest that the MRCA of Western Europeans lived as recently as AD 1000. The same article provides surprisingly recent estimates for the identical ancestors point, the most recent time when each person then living was either an ancestor of all the persons alive today or an ancestor of none of them. The estimates for this are similarly uncertain, but date to considerably earlier than the MRCA, according to Rohde (2005) roughly to between 15,000 and 5,000 years ago.

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Originally posted by JohnOAS

[quote]The maths in the article posted by HalfMooner is simply flawed in a few assumptions and the attached calculations. I can imagine the YEC crowd claiming that evolution supports their stance any moment now. It's also worth that "Mitochondrial Eve" (ME) is a shifting title, and that there would have been a different ME at the time our ME was alive.

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Originally posted by HalfMooner

Math hard. Brain hurts. Can I go home now?

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