DNA recovered from underwater British site may rewrite history of farming in Europe
By Michael Balter – Science Magazine Feb. 26, 2015
Hunter-gatherers may have brought agricultural products to the British Isles by trading wheat and other grains with early farmers from the European mainland. That’s the intriguing conclusion of a new study of ancient DNA from a now submerged hunter-gatherer camp off the British coast. If true, the find suggests that wheat made its way to the far edge of Western Europe 2000 years before farming was thought to have taken hold in Britain.
The work confronts archaeologists “with the challenge of fitting this into our worldview,” says Dorian Fuller, an archaeobotanist at University College London who was not involved in the work.
For decades, archaeologists had thought that incoming farmers from the Middle East moved into Europe beginning about 10,500 years ago and replaced or transformed hunter-gatherer populations as they moved west, not reaching Britain until about 6000 years ago. But that worldview had already undergone some modifications. Recent discoveries, for example, have shown some incoming farmers coexisted with the hunter-gatherers already living in Europe rather than quickly replacing them. In 2013, researchers reported that, beginning about 6000 years ago, farmers and hunter-gatherers had both buried their dead in the same cave in Germany and continued to do so for 800 years, suggesting that the two groups were in close contact. More controversially, researchers claimed that about 6500 years ago hunter-gatherers in Germany and Scandinavia may have acquired domesticated pigs from nearby farmers.
The new findings promise to further upset the scenario that farming steadily marched from east to west. A team led by Robin Allaby, a plant geneticist at the University of Warwick in the United Kingdom, was looking for the earliest evidence of domesticated plants in the British Isles. The researchers decided to take a gander at an underwater site called Bouldnor Cliff, 250 meters offshore from the hamlet of Bouldnor in the northwest corner of the Isle of Wight. (The island is in the English Channel just off Britain’s southern coast.)
Bouldnor Cliff, located 11 meters below the water’s surface, was discovered in 1999, when, as the United Kingdom’s Maritime Archaeology Trust puts it on its website, “a lobster was seen throwing Stone Age worked flints from its burrow.” Archaeologists have been working there ever since. The site was clearly occupied by hunter-gatherers, who may have built wooden boats. Allaby’s team took four core samples of sediments from a section of the site littered with burnt hazelnut shells apparently left by the hunter-gatherers and subjected the samples to both radiocarbon dating and ancient DNA analysis. The samples’ wood and plants were dated to between 8020 and 7980 years ago, after which the site was inundated by the rising seas that created the English Channel and separated Britain from France.
For the ancient DNA analysis, the team used methods pioneered by paleogeneticist Eske Willerslev of the University of Copenhagen to recover and sequence genetic material left behind in sediments even after the plants that originally contained it have disintegrated. As might be expected, Allaby and his colleagues found DNA from a wide variety of trees and plants known to have populated southern Britain 8000 years ago, including oak, poplar, and beech, along with various grasses and herbs. But the team also got a big surprise: Among the DNA samples were two types of domesticated wheat that originated in the Middle East and that have no wild ancestors in northern Europe. That meant they must have been associated with the original spread of farming from the Middle East, beginning about 10,500 years ago, rather than domesticated locally. Yet many archaeologists assume that by 8000 years ago farming was no further west than the Balkans region and modern Hungary.
The researchers performed a number of tests to eliminate the possibility of contamination from modern wheat, including trying to sequence DNA from the chemical solutions it used in the experiments, but no plant sequences were detected. The only possible conclusion was that the domesticated wheat had actually come from the hunter-gatherer site at Bouldnor Cliff, the team reports online today in Science.
“The paper is methodologically impressive,” Fuller says. Willerslev agrees: “The study is quite convincing,” he says, adding that loose DNA from sediments will provide “some of the earliest detectable evidence for farming” because cereal grains themselves are less likely to be preserved.
So how did domesticated wheat get to Britain 2000 years before people began to farm there? Allaby’s team does not think the hunter-gatherers cultivated wheat themselves, because no wheat pollen was found in the samples—as should have been expected if the cereal had been allowed to go through its entire life cycle, including flowering.
The team proposes that farming might have spread to western France earlier than had been thought, up to 7600 years ago, and thus only a 400-year gap would have to be explained. But Peter Rowley-Conwy, an archaeologist at the Durham University in the United Kingdom, rejects that suggestion. “The authors do not do justice to the chronology of the spread of agriculture,” he complains, noting that “thousands of directly radiocarbon-dated cereal grains” argue against farming in Western Europe that early. “One DNA study of this kind is just not enough to overturn all this.”
Another possibility, Allaby says, is that the nomadic hunter-gatherers of southern Britain roamed much farther into the European mainland than previously realized, picked up wheat or wheat products from farmers to the east, and brought them back to Britain. He also suggests that the conventional dating of the spread of agriculture, based on clearly detectable cereal grains, might be missing earlier samples.
Allaby may well be right, says Greger Larson, an evolutionary biologist at the University of Oxford in the United Kingdom. “Are we underestimating the degree to which there were exchange networks between farmers and hunter-gatherers which extended far across time and space? Maybe the only way to pick them up is from DNA signatures.”
Yet Fuller says that the new finds do not necessarily indicate that the spread of farming needs to be radically redated. Rather, he suggests, small-scale pioneers of both farmers and hunter-gatherers may have been “operating beyond the frontier of farming” as it spread west in a wave of advance. The wheat might have been part of trade or cultural exchanges between them. Just as rare spices from the east are regarded as valuable commodities today, Fuller says, the wheat at Bouldnor Cliff might have been symbolically charged and seen as “rare, exotic, and valuable,” rather than something to be eaten daily.