Neolithic Britain: where did the first farmers come from?
The introduction of farming across the world changed the course of human history. In Britain, the island's entire culture changed, incorporating new pottery, tools and funerary practices.
But where did farming come from? And what happened to the hunter-gatherers already living in Britain? Genome sequencing at the Museum has been providing some answers.
The culture of farming arrived in Britain some 6,000 years ago, marking the beginning of the Neolithic period (New Stone Age).
Previously, in the Mesolithic period (Middle Stone Age) Britain had been home to a population of hunter-fisher-gatherers.
This transition to farming marked a huge shift in cultural life in the region. It has long been debated whether the arrival of Neolithic farming cultures represented the native population adopting these new practices or the arrival of migrant farmers from continental Europe.
To answer this, Museum postdoctoral researcher Dr Tom Booth and his colleagues have been delving in the genetics of ancient Britons. Their results are published in Nature Ecology and Evolution.
'We looked at the genetic ancestry of human remains from both before and after 6,000 years ago - so some dating to the Mesolithic and some to the Neolithic - to see if we can characterise any changes,' explains Tom.
'As soon as these Neolithic cultures start to arrive, we see a big change in the ancestry of the British population. It looks like the development of farming and these Neolithic cultures was mainly driven by the migration of people from mainland Europe.'
The spread of farming
Farming is thought to have originated in the Near East and made its way to the Aegean coast in Turkey. From there, farming and the specific culture that came with it (such as new funerary rites and pottery) spread across much of Western Europe.
How this spread occurred has been debated for the last century, but new analysis techniques are revealing more clues than ever before.
'There has been a big debate in archaeology about whether these new cultures appearing in Europe represent the arrival of new people or just the spread of ideas,' explains Tom. 'With the revolution in ancient DNA in the last few years, it means that we can actually address these questions.'
The analysis of ancient human DNA has shown that every time the Neolithic culture arrived in a region of Europe, it appeared alongside new genetic ancestry that came from areas around the Aegean Sea. This suggests that it was not just farming cultures that swept across the continent, but farmers too.
Importantly, this DNA evidence also shows that as these new farmers were moving through the unfamiliar forests and grasslands of Europe, they were also mixing with the local hunter-gatherers who had already made a living there.
'As this Neolithic population moved west, we can track cumulatively increasing levels of the local hunter-gatherer signatures in the genetics,' says Tom. 'So this wasn't just one population wiping the other out. Instead, they were mixing.
'The Aegean ancestry nearly always dominated because farming allowed these people to maintain much larger population sizes.
'This means that even though they were mixing continuously, the hunter-gatherers were always a more minor component in the overall genetics.'
As the farmers moved east to west, by the time they reached Iberia these accumulations mean that about 40% of their ancestry could be traced back to the original European hunter-gatherer populations that they mixed with as they moved across the continent.
Crossing the Channel
When the early farmers reached the English Channel, however, something happened.
'Neolithic cultures arrived in adjacent regions of northern Europe - northern France, Belgium and Germany - around 1,000 years before they arrived in Britain,' explains Tom.
'In Britain the hunter-gatherers carried on with the Mesolithic cultures for 1,000 years, until around 6,000 years ago, when there was a very sudden change in Britain as these Neolithic practices started appearing.'
The reason for this delay is unclear. We know people at that time could build boats and cross the English Channel, so the physical barrier was probably not the problem.
One argument suggests that hunter-gatherer people from Britain were making forays to the continent and gradually coming around to the idea of farming before taking it up wholesale 6,000 years ago. Another argument is that there was an influx of Neolithic farmers at this time.
By studying the ancient DNA of six Mesolithic Britons and 67 Neolithic Britons, Tom and his team were able to see what genetic changes occurred, if any, at this turning point.
This is the first time the full genome of a Mesolithic Briton has been sequenced - that of Britain’s oldest mostly-complete skeleton, Cheddar Man.
The team found that most of the hunter-gatherer population was replaced by those carrying ancestry originating in the Aegean. But DNA analysis doesn't necessarily give the full picture, nor does it explain why changes in genetics happened.
'The best explanation at the moment is that the hunter-gatherer Mesolithic population of Britain just wasn't very high. So the newly arrived farmers could have mixed entirely with the native population but because this was quite small, the hunter-gatherers left little genetic legacy overall.'
The southern route
The intrigue doesn't stop there. When the original Neolithic farmers left the Aegean and began spreading out across Europe, the population very quickly split into two rough groups that developed slightly different cultures.
One of these went north along the Danube and mixed with the hunter-gatherer populations of Central Europe, while the other took a more southerly route along the Mediterranean before reaching Iberia.
While we know that the hunter-gatherers of Britain share close ties with those from Scandinavia, the Neolithic culture shows a mix of both these Central European and Mediterranean traditions. It is difficult to fully understand where the Neolithic farmers came from.
While it might make more sense for them to have crossed over from Central Europe group, the genetics show that the new influx of Neolithic farmers came instead from the Iberian contingent that travelled first along the Mediterranean and then up the Atlantic coast.
'To some extent, this is quite surprising,' says Tom. 'Culturally the Neolithic Britons looks like a mix, but genetically they are very much more Iberian and Mediterranean then they are central European.
'What seems to happen is that populations moved up from southern France and maybe Iberia to northern France, where they then mixed slightly with the central European population, before moving into Britain.'
The genetics can also give hints as to what these early people may have looked like. The genetic variants found in the Mesolithic, including that of Cheddar Man and at least one of the Neolithic humans, show that they likely had darker skin than we associate with northern Europeans today.
They indicate that these early populations of people were far more variable in their skin pigmentation than has been thought.
'In higher latitudes where sunlight is lower, paler skin allows people to absorb UV light from the Sun more efficiently, which our bodies use to synthesise vitamin D,' explains Tom.
'Certainly natural selection is acting on genetic variants linked to lighter skin pigmentation in prehistoric populations, but it looks like the development of paler skin in northern Europe generally is not straightforward.
'Key genetic variants linked to lighter skin pigmentation are introduced by later migrations of new populations. Diets rich in vitamin D may have weakened selective pressures on light pigmentation variants.'