Human populations are tightly interwoven
Family tree shows our common ancestor lived just 3,500 years ago.
The most recent common ancestor of all humanity lived just a few thousand years ago, according to a computer model of our family tree. Researchers have calculated that the mystery person, from whom everyone alive today is directly descended, probably lived around 1,500 BC in eastern Asia.
Douglas Rohde of the Massachusetts Institute of Technology in Cambridge and his colleagues devised the computer program to simulate the migration and breeding of humans across the world. By estimating how different groups intermingle, the researchers built up a picture of how tightly the world's ancestral lines are linked.
The figure of 1,500 BC might sound surprisingly recent. But think how wide your own family tree would be if you extended it back that far. Lurking somewhere in your many hundreds of ancestors at that date is likely to be somebody who crops up in the corresponding family tree for anyone alive in 2004.
In fact, if it were not for the fact that oceans helped to keep populations apart, the human race would have mingled even more freely, the researchers argue. "The most recent common ancestor for a randomly mating population would have lived in the very recent past," they write in this week's Nature1.
To work out how much different groups of humans mingled, Rohde's team simulated the rates at which a few pioneering people made journeys across the world to meet and breed with other populations. Their model gave each individual a certain probability of quitting their home town, country or continent and striking out for pastures new.
They were then able to name a time and place at which our most recent common ancestor lived. But who was this person? He or she must have had a flourishing family, says Rohde. "Maybe it was someone who happened to have 40 children or some such astronomical number," he says. "But it could equally have been someone with above-average productivity for a few generations." Instead of two kids, Rohde suggests, maybe the person and his or her direct descendants had three.
The fact that the person probably lived in Asia is down to its prime position along the most commonly used migration routes, Rohde suggests. "East Asia is at a crossroads," he says. "It's close to the Bering Strait and the Pacific."
Rohde's simulation aims to include everyone alive today, and therefore relies on the assumption that no population has remained completely isolated for any significant length of time. Rohde is confident that this is the case; even Tasmania, once thought to be isolated by choppy seas, contains no people with purely Tasmanian blood.
If we discount those living in the world's remotest places, the common ancestor becomes more recent still, says Mark Humphrys, who studies human family trees at Dublin City University in Ireland. "Looking at the whole sweep of the Americas, Europe, Asia, right across to Japan, I wouldn't be surprised if we had a common ancestor in the AD years," he says.
A single prolific parent can have a vast influence once their descendants begin to multiply, Humphrys says. "The entire Western world is descended from Charlemagne, for example," he says. "There's really no doubt."
All or nothing
Besides dating our most recent common ancestor, Rohde's team also calculates that in 5,400 BC everyone alive was either an ancestor of all of humanity, or of nobody alive today. The researchers call this the 'identical ancestors' point: the time before which all the family trees of people today are composed of exactly the same individuals.
This recent date is not really surprising either, Rohde says. Anyone whose lineage survived for a few generations was likely to have descendants spread all over the world. At the identical ancestors point, then, our ancestors came from every corner of the globe, although those from far afield are unlikely to have made a significant contribution to our genetic make-up.
Nonetheless, the results show that we are one big family, Rohde says. As he and his colleagues write: "No matter the languages we speak or the colour of our skin, we share ancestors with those 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 laboured to build the Great Pyramid of Khufu."
- Rohde D. L. T, Olson S. & Chang J. T. Nature, 431, 562- 565. (2004).