Yale Bulletin and Calendar

October 8, 2004|Volume 33, Number 6

Model shows most recent common
ancestor of today's humans lived
less than 1,000 years ago

In the Sept. 30 issue of Nature, a Yale mathematician presents models showing that the most recent person who was a direct ancestor of all humans currently alive may have lived just a few thousand years ago.

"While we may not all be 'brothers,' the models suggest we are all hundredth cousins or so," says Joseph T. Chang, professor in the Department of Statistics and senior author on the paper.

Chang established the basis of this research in a previous publication with an intentionally simplified model that ignored such complexities as geography and migration. Those precise mathematical results showed that in a world obeying the simplified assumptions, the most recent common ancestor would have lived less than 1,000 years ago. He also introduced the "identical ancestors point," i.e., the most recent time -- less than 2,000 years ago in the simplified model -- when each person was an ancestor to all or ancestor to none of the people alive today.

The current paper presents more realistic mathematical and computer models. It incorporates factors such as socially driven mating, physical barriers of geography and migration, and recorded historical events. Although such complexities make pure mathematical analysis difficult, it was possible to integrate them into an elaborate computer simulation model, says Chang. The computer repeatedly simulated history under varying assumptions, tracking the lives, movements and reproduction of all people who lived within the last 20,000 years.

These more realistic models estimate that the most recent common ancestor of mankind lived as recently as about 3,000 years ago, and the identical ancestors point was as recent as several thousand years ago. The paper suggests, "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."

This model can also be used to calculate the percentage of ancestry current individuals received from different parts of the world.

"And, surprisingly, having a distant common ancestor does not necessarily mean that individuals have many or any genes in common," says Chang. "Biologically, this suggests that genes, rather than individuals, determine fitness and survival."

The model can also be used to predict the future. According to Chang, "Within two thousand years, it is likely that everyone on earth will be descended from most of us."

Other authors are Douglas L.T. Rhode of the Massachusetts Institute of Technology and Steve Olson of Bethesda, Maryland. The National Institutes of Health supported this research.

-- By Janet Rettig Emanuel

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