Long journey (Image: Ton Koene/Corbis)

Humanity’s dramatic race across the Old World after it left its African cradle has been told countless times. But for a true sense of the rapidity of events, look no further than the Y chromosome. The most comprehensive analysis of the Y yet shows that within 150 years, an evolutionary blink of an eye, the first migrants to make it into Eurasia split into three distinct groups that can still be identified today.

Men inherit their Y chromosome from their fathers. Like all other chromosomes, the Y mutates over time so the more distant the relationship between two men, the more genetically distinct their Y chromosomes. In 2000, biologists used this fact to construct a family tree of all men, which shows how human populations around the world are related.

But this tree was built using the information from a small samples of DNA on the Y chromosome, which revealed only around 100 sites of genetic variation that could help establish familial relations. Advances in gene technology mean it is now possible to build a more comprehensive tree based on information from the entire Y chromosome.


69 dudes

David Poznik and Carlos Bustamante at Stanford University in California, together with their colleagues, analysed the entire Y chromosome of 69 men from Africa, Eurasia and Central America. They found more than 11,000 sites of variation, allowing them to build a much more detailed tree.

For ideas on how he could use the new tree, Poznik talked to Peter Underhill, another researcher at Stanford. Underhill has been studying the Y chromosome for years and helped build the first Y tree in 2000.

“He’d always had in the back of his mind this problematic area in the tree that he’d never been able to crack,” says Poznik.

That area of the Y tree describes the crucial period when modern humans began going global, sometime after they left Africa between 70,000 and 50,000 years ago. The earlier family tree showed that the three major Eurasian genetic groups that survive today emerged from those pioneering migrants, who were probably living in the Arabian peninsula. But exactly how the ancestors of those three groups relate to one another, and how quickly they diverged after leaving Arabia was not resolved.

The new analysis reveals that just a single genetic mutation differentiated the three when they first arose. “The fact that we have exactly one variant separating the three indicates a timescale on the order of 150 years for the split,” says Poznik.

A mountain high enough

Those near-identical genetics imply that the early Eurasians split into two populations, and then in little more than a century – before those two populations could accrue more than one new mutation – one of them split in two again to create three in total. Although these populations probably appeared around 50,000 years ago, cultural or geographical barriers like mountain chains have kept them recognisably distinct to this day.

The speed of these events make sense, says Ornella Semino at the University of Pavia in Italy. “When modern humans spread out of Africa, they encountered multiple different environments that stimulated the fragmentation of the first migrant group,” she says.

The first Eurasians were significant for another reason: their pioneering exploration brought them into contact with Neanderthals for the first time. Studies within the last few years have shown that interbreeding did occur between the two species, although there is no evidence a Y chromosome from a male Neanderthal passed into the Homo sapiens gene pool.

Or rather, there is no evidence so far. “In theory there may be a Y chromosome from a Neanderthal still present among humans,” says Bustamante – especially since a different analysis earlier this year suggested, remarkably, that a Y chromosome from another now-extinct species of human might be circulating in today’s male population.

Michael Hammer at the University of Arizona in Tucson and his colleagues identified a Y chromosome unlike any other in a small group of people who can trace their roots back to Cameroon.

It is so different from any other Y chromosome so far found that it may actually have entered the modern gene pool via an ancient human that lived in western Africa, says Bustamante. “It might be our first glimpse of archaic interbreeding within Africa.”

Hammer, who was not involved in the new study, says the unusual Y chromosome he found is too rare and too new to science to have featured in the latest analysis. But Bustamante says he would “absolutely love” to add it to the data set in future.

Journal reference: Science, DOI: 10.1126/science.1237619