Did pioneer farmers fail to spread their seed?
European immigrants may have passed on agricultural skills, but not their genes.
A group of travellers brought farming to Europe about 7,500 years ago. But did their children thrive and hand down the skill? Researchers studying ancient DNA say instead that the idea was stolen by more successful locals, as the farmers failed to leave their mark on Europe's genes.
The finding adds to a debate about Europeans' origins that "has been raging for the past ten years", says Peter Forster, a geneticist at the University of Cambridge. Scientists have turned to archaeology and analyses of modern DNA to try and settle the question, but they come up with contradictory results1,2.
Some say that today's Europeans arose mostly from a population of hunter-gatherers who appeared on the continent up to 40,000 years ago, and who later won a genetic war against incoming farmers.
Others say that the agricultural arrivals, who came west from the fertile lands encompassing modern countries such as Egypt, Turkey and Iran, contributed substantially to the genetic make-up of the continent's population.
Forster and his colleagues weigh in on the side of the hunter-gatherers with their analysis of ancient DNA, reported in Science this week3.
Going, going, gone
The researchers, led by Joachim Burger at Johannes Gutenberg University in Mainz, extracted DNA from the bone and teeth of farmers buried in archaeological sites scattered throughout modern-day Germany, Hungary and Austria. Pottery and other artefacts identified the sites as farming land.
The investigators sequenced pieces of mitochondrial DNA in part because the genetic material in a corpse disintegrates over time: with thousands of mitochondria crammed into each cell, enough DNA often survives for analysis. Of the 57 individuals tested, 24 yielded usable DNA despite their age.
University of Cambridge
"The farmers were really a pioneer group and didn't leave much of a mark," concludes Forster.
It could be a statistical fluke that the researchers stumbled on a large percentage of people with type N1a in their relatively small sample, admits Forster. But the fact that they came from three different areas adds to the likelihood that N1a really was more common among the farmers than it is today.
The researchers also ran a computer simulation suggesting that N1a type was probably not lost over time by chance.
What's your type?
"The study does not resolve the puzzle, but their interpretation is in line with what a lot of people, including me, have been suggesting for quite some time," says Antonio Torroni, a geneticist from the University of Pavia, Italy.
But Terry Brown, a geneticist from the University of Manchester, UK, calls the conclusions "a little bit tenuous". Brown points out that the study examines events in only one geographic region.
"During the 3,000 years in which agriculture spread into Europe, many different things were happening," says Brown. In some areas, hunter-gatherers probably picked up farming quickly; in others, farmers may have overrun the locals. "I don't think you can generalize," he says.
It is also difficult to explain the observation that N1a is not prevalent today in countries such as Iran, although the data for this are sparse. To facilitate a better analysis of current N1a prevalence and origins, Forster and his colleagues have started a project called Genetic Ancestor, through which members of the public can pay to have their DNA analyzed; the customers get information about their ancestry and the researchers build a genetic database.
Researchers on both sides say the debate could be clarified by data from the bones of the hunter-gatherers. But viable DNA from such bones is in very short supply.
- Gibbons A. Science, 290. 1080 - 1081 (2000).
- Chikhi L. PNAS, 99. 11008 - 11013 (2002).
- Haak W., et al. Science, 310. 1016 - 1018 (2005).