Monday, July 03, 2006

One big happy family: the universal ancestor

Associated Press has a somewhat tortuous article following up on a recent Nature publication on human geneology. I think it's a bit easier to read chopped into pieces:
Roots of human family tree are shallow - Yahoo! News

... 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.

... 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.

... 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.

... 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.

... 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.

"No matter the languages we speak or the color of our skin, we share ancestors who planted rice on the banks of the Yangtze, who first domesticated horses on the steppes of the Ukraine, who hunted giant sloths in the forests of North and South America, and who labored to build the Great Pyramid of Khufu," Olson and his colleagues wrote in the journal Nature.

... Seven years ago one of Olson's colleagues, a Yale University statistician named Joseph Chang, started thinking about how to estimate when the last common ancestor of everybody on Earth today lived...

... A few years later Chang was contacted by Olson, who had started thinking about the world's interrelatedness while writing his book. They started corresponding by e-mail, and soon included in their deliberations Douglas Rohde, a Massachusetts Institute of Technology neuroscientist and computer expert who now works for Google.

The researchers knew they would have to account for geography to get a better picture of how the family tree converges as it reaches deeper into the past. They decided to build a massive computer simulation that would essentially re-enact the history of humanity as people were born, moved from one place to another, reproduced and died.

Rohde created a program that put an initial population on a map of the world at some date in the past, ranging from 7,000 to 20,000 years ago. Then the program allowed those initial inhabitants to go about their business. He allowed them to expand in number according to accepted estimates of past population growth, but had to cap the expansion at 55 million people due to computing limitations. Although unrealistic in some respects — 55 million is a lot less than the 6.5 billion people who actually live on Earth today — he found through trial and error that the limitation did not significantly change the outcome with regard to common ancestry.

The model also had to allow for migration based on what historians, anthropologists and archaeologists know about how frequently past populations moved both within and between continents. Rohde, Chang and Olson chose a range of migration rates, from a low level where almost nobody left their native home to a much higher one where up to 20 percent of the population reproduced in a town other than the one where they were born, and one person in 400 moved to a foreign country.

Allowing very little migration, Rohde's simulation produced a date of about 5,000 B.C. for humanity's most recent common ancestor. Assuming a higher, but still realistic, migration rate produced a shockingly recent date of around 1 A.D.

... One ancestral link to another cultural group among your millions of forbears, and you share ancestors with everyone in that group. So everyone who reproduced with somebody who was born far from their own natal home — every sailor blown off course, every young man who set off to seek his fortune, every woman who left home with a trader from a foreign land — as long as they had children, they helped weave the tight web of brotherhood we all share.
Could there be exceptions? Australian aborigines were isolated a long time ago...

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