Mixing with native foragers led to modern genetic signature.
Geneticists analysing DNA from Neolithic burial sites in Sweden have made a surprising discovery. The genetic make-up of one individual — a female farmer known as Gök4 — bears a startling similarity to that of modern-day Mediterraneans. And the woman's genome provides clues as to how agriculture spread across Europe.
Modern Europeans’ genetic profile may have been partly cultivated by early Mediterranean farmers who moved to what’s now Scandinavia, where they paired up with resident hunter-gatherers.
DNA taken from 5,000-year-old skeletons previously excavated in Sweden unveils a scenario in which agricultural newcomers from the south interbred with northern hunter-gatherers, say evolutionary genetics graduate student Pontus Skoglund of Uppsala University in Sweden and his colleagues. Their findings feed into a picture of many early migrations of farmers into Europe, which often would have included interactions with local hunter-gatherers.
Pieces of DNA extracted from an ancient farmer’s remains buried in southern Sweden display gene variants most like those found in people now living in Greece and Cyprus, the scientists report in the April 27 Science. DNA retrieved from the bones of three hunter-gatherers interred on an island off Sweden’s coast contains distinctive gene variants that most resemble those of native Finns.
Most Europeans today possess genetic arrangements in between those of the long-dead farmer and his hunter-gatherer neighbors, Skoglund’s team finds. Breeding between culturally discrete cultivators and foragers contributed to Europeans’ current genetic makeup, the researchers propose.
“Our data suggest that northern European farmers originated in Mediterranean Europe and were genetically distinct from northern hunter-gatherers some 5,000 years ago,” says study coauthor and evolutionary geneticist Mattias Jakobsson, also of Uppsala University.
Ancient DNA in the new analysis came from cell nuclei, a form of genetic material inherited from both parents. Researchers isolated and studied from 1 percent to 3 percent of the nuclear genome for each excavated individual.
Jakobsson says the new nuclear DNA evidence challenges a 2010 investigation that traced genes extracted from the remains of members of a 7,000-year-old farming culture in Central Europe to current residents of Turkey and areas just to its east. That study, led by human paleobiologist Wolfgang Haak of the University of Adelaide in Australia, examined maternally inherited DNA from cell structures called mitochondria and the paternally inherited Y chromosome.
Archaeological finds point to several routes into Europe for early Middle Eastern farmers, Haak says. A Mediterranean-based farming group may have reached Scandinavia 1,000 to 2,000 years after an initial expansion of farmers from further east into Central Europe, he suggests.
Farming’s rise “was by no means a uniform process across Europe,” Haak says.
Investigators generally agree that farming originated about 11,000 years ago in the Middle East and reached Europe by around 7,000 years ago. Debate has long revolved around whether waves of advancing farmers chased off European hunter-gatherers or traded cultural practices with native groups that then took up agriculture.
DNA links between the ancient Swedish farmer and nearby hunter-gatherers show that agriculture spread across Europe with the aid of genetic as well as cultural exchanges, Jakobsson says.
The result is strong evidence for the migration hypothesis. "If farming had spread solely as a cultural process, we would not expect to see a farmer in the north with such genetic affinity to southern populations,” says Skoglund.
But Wolfgang Haak, a human palaeobiologist at the University of Adelaide in Australia and a co-author of some of the mitochondrial studies, says that “one farmer from Scandinavia is certainly not enough to explain the spread of farming in places like Spain or Bulgaria and we definitely need more farmers from other parts of Europe”.
Another ancient DNA study, published this week in PLoS One, compares the mitochondrial DNA of Neolithic farmers and hunter-gatherers in northern Spain. The genetic variation amongst the farmers indicates that there was a complex pattern of settlement, with waves of colonization bringing small and different Neolithic groups into the region.
“Given the genetic evidence from studies of both modern-day and ancient DNA, I am convinced that farmers expanded in many different ways and under many different demographic scenarios,” says Haak. But Skoglund's study adds “an important piece to the big puzzle” of how farming became commonplace across Europe, Haak adds.
“The technical advances in the genomic era now provide us with the necessary tools to tackle the different modes of the Neolithization in every corner of Europe with much higher precision,” says Haak.
There is also the tantalizing prospect that as understanding of the genetic basis of behaviour improves, it might be possible to comb fragments of ancient DNA for signs of behavioural differences between prehistoric populations. “In the future we might know a few genes that affect behaviour and we could go back and look into the hunter gatherers and see what variants of those genes are present,” says Mattias Jakobsson, a population geneticist at the University of Uppsala and a co-author of the Science paper. “But that’s not going to happen in the next couple of weeks at least.”
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