We all have the same ancestors, researchers say

One big meet market
Chang’s calculations essentially treated the world like one big meet market where any given guy was equally likely to pair up with any woman, whether she lived in the next village or halfway around the world. Chang was fully aware of the inaccuracy — people have to select their partners from the pool of individuals they have actually met, unless they are entering into an arranged marriage. But even then, they are much more likely to mate with partners who live nearby. And that means that geography can’t be ignored if you are going to determine the relatedness of the world’s population.

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.

Some people even suspect that the most recent common ancestor could have lived later than that.

“A number of people have written to me making the argument that the simulations were too conservative,” Rohde said.

Not so close to home
Migration is the key. When a people have offspring far from their birthplaces, they essentially introduce their entire family lines into their adopted populations, giving their immediate offspring and all who come after them a set of ancestors from far away.

People tend to think of preindustrial societies as places where this sort of thing rarely happened, where virtually everyone lived and died within a few miles of the place where they were born. But history is full of examples that belie that notion.

Take Alexander the Great, who conquered every country between Greece and northern India, siring two sons along the way by Persian mothers. Consider Prince Abd Al-Rahman, son of a Syrian father and a Berber mother, who escaped Damascus after the overthrow of his family’s dynasty and started a new one in Spain. The Vikings, the Mongols, and the Huns all traveled thousands of miles to burn, pillage and — most pertinent to genealogical considerations — rape more settled populations.

Though people like to think of culture, language and religion as barriers between groups, history is full of religious conversions, intermarriages, illegitimate births and adoptions across those lines. Some historical times and places were especially active melting pots — medieval Spain, ancient Rome and the Egypt of the pharaohs, for example.

“And the thing is, you only need one,” said Mark Humphrys, an amateur anthropologist and professor of computer science at Dublin City University.

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.

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