Did hunters farm or disappear? dna may have the answer
Generations of archaeologists have puzzled over one of the key moments in Britain’s long story – the arrival of farming. Was it an indigenous initiative, or was it brought by immigrants? A new DNA study has solved the issue in a way few would have predicted. Tom Booth explains
Did migrant farmers introduce cropgrowing and the rearing of domestic animals to Europe? Or were the ideas and materials transmitted from one society to another? When farming finally arrived in Britain around 6,000 years ago, was it brought here by new settlers, or was it copied and adapted by local hunter-gatherers? These questions have plagued and inspired archaeologists in equal measure for over a century.
Tackling the issue from the cultural changes we see in the archaeological record – structures, artefacts and plant and animal remains from settlements –
has always been tricky. Such traditions can evolve without substantial changes among the people themselves; archaeology on its own is likely to offer a poor or at least inconsistent proxy of human history. Recent major advances in the study of ancient dna (adna), however, make possible more direct study of population change.
The new possibilities for analysing ancient genome-wide data (sequences of adna distributed across an individual’s whole genome, comprising 23 pairs of chromosomes and the mitochondrial dna), have brought great leaps forward in our understanding of the population history of prehistoric Europe. In most areas that have been studied, Neolithic farming lifestyles first appear at the same time as people with ancestors who lived to the east around the Aegean Sea some 8,000 years ago. This suggests that the development of these societies across Europe was heavily influenced by periodic migrations of established farmers over several millennia. The adna data show people dispersing out of the Aegean in two directions: west along the Mediterranean, and north-west into central Europe (the so-called “Danubian” route), producing distinct genetic and cultural signatures.
There is ample evidence that the descendants of local Mesolithic hunter-gatherers and people of Aegean ancestry mixed, though the scale and timing of such interactions vary across Europe: in some regions it takes up to two millennia for major admixture between these two groups to occur. Only a minority of the ancestry of most European Neolithic populations is derived from Mesolithic people, however. This is most simply explained by population size: the descendants of migrating groups of farmers greatly outnumbered local populations of hunter-fisher-gatherers.
Native or immigrant?
There have long been conflicting interpretations of the archaeological evidence for the role of migration and acculturation in the development of the British Neolithic. Most archaeologists would agree that there was a significant cultural shift around 4000bc. Domesticates, extensive forest clearance, pottery and, most prominently, megalithic tombs and other monumental funerary structures, all first appear then in south-east England. Within a few generations they occur widely across the rest of Britain and Ireland (see feature Jul/Aug 2011/119).
For some archaeologists, this suggests continental migrants bringing all the features of a Neolithic world with them. However, this happened around a millennium after Neolithic cultures developed in adjacent areas of continental Europe. Other archaeologists have argued that features associated with Neolithic behaviour can already be seen in Britain at this earlier time, indicating that local hunter-fisher-gatherer groups experimented with new lifestyles after contact with farmers across the Channel. Conversely, evidence is claimed for continuity of Mesolithic practices into the British Neolithic, such as hunting, particular forms of flint technology, deposition of the dead in caves, and Neolithic monuments built at older Mesolithic sites. Thus, it is said, indigenous people underwent slow social and ideological changes before committing fully to a Neolithic way of life.
Neither scenario is absolute. Archaeologists promoting an acculturation model allow that social networks extending across the Channel would have encouraged some limited gene flow in both directions. Most archaeologists who favour significant migrations would not deny that local groups probably joined immigrant farming communities, or even occasionally adopted some Neolithic habits themselves. And there is a middle way: local hunter-gatherers could have taken on Neolithic
practices from small groups of influential pioneer farmers from continental Europe.
Nonetheless, there remains a large hypothetical element in the debate when the evidence is restricted to hard-to-interpret artefacts and monuments. The question remains: who were the prime movers in the adoption of farming in Britain, local hunter-fisher-gatherers or established farmers travelling to Britain from continental Europe?
Here come the farmers
For a new approach to this question, we analysed genome-wide dna data extracted from 73 individuals from Britain dating to before and after 4000bc. An archaeologist used to working with ancient sites and artefacts may find this a small sample for resolving such a significant and complex issue, but this would be misguided. Genome-wide data from a single individual provide rich information not just on that person’s ancestry, but also the ancestry of their ancestors. This is information on potentially thousands of people, most of whose remains are unlikely to have survived, let alone be found. We can treat each ancient individual as a population of ancestors, allowing robust inferences about ancient human histories.
Mesolithic human remains from Britain are scarce, and we were able to include only six such individuals in our study, all predating 4000bc. Importantly, however, we did successfully obtain adna from a woman whose cranial bone has been radiocarbon dated to the late fifth millennium bc. She was buried in the Cnoc Coig shell midden on the island of Oronsay in the Inner Hebrides, just before or possibly contemporary with the first appearance of Neolithic farming practices in Britain.
These six people were all genetically similar to other populations who lived in Mesolithic Europe. The Cnoc Coig woman has none of the Aegean ancestry which characterises Neolithic farmers from continental Europe. There seem to have been no substantial biological interactions between Mesolithic populations of Britain and farming communities inhabiting continental Europe over the thousand years preceding the arrival of farming here.
By contrast Aegean ancestry was the majority ancestry component in all
analysed 67 individuals associated with Neolithic material culture or who post-dated 4000bc. There is therefore a strong association between the transformation of the archaeological record, and transformation of ancestry in the British Early Neolithic. The coarseness of our data means that this change could have occurred over several hundred years. But even when we allow for this, the shift in ancestry we see is too large, too quick, and too persistent to be explained without invoking movements of people into Britain from continental Europe.
Furthermore, our results are also inconsistent with a scenario where small groups of influential farmers kickstarted the development of Neolithic practices among local communities. If that had happened, we would expect British Neolithic populations to maintain significant ancestry from people who inhabited Britain during the Mesolithic. Our results strongly favour the introduction of Neolithic practices to Britain by movements of established farmers from continental Europe.
Where did they come from? The current lack of Neolithic adna from the continent close to Britain, particularly from modern-day France, means we cannot yet identify the precise origins of the uk’s earliest Neolithic farmers. However, British
Neolithic populations are genetically more similar to Neolithic populations who inhabited Iberia – Spain and Portugal – than those who lived in central Europe. This suggests that British Neolithic peoples mainly descended from groups who dispersed out of the Aegean along the Mediterranean.
On the face of it, this is surprising. Parallels have long been recognised between the archaeology of Neolithic Britain and Neolithic cultures of central Europe such as the Linearbandkeramik ( lbk), rather than with the south. However, British Neolithic farmers do show a small proportion of central European ancestry. This can be explained as the result of successive movements: the Neolithic populations who dispersed west along the Mediterranean moved into northern France from Iberia or southern France, and, before crossing the Channel, mixed to a limited degree with Neolithic populations carrying ancestry similar to that we see in central Europe. This is consistent with the archaeological evidence, which suggests cultural connections between Early Neolithic Britain and adjacent continental Europe, particular parts of northern France. It is also consistent with the development of Neolithic megalith-building societies along the Atlantic fringe. It seems the lbk’s legacy in Britain was more cultural than genetic.
Meanwhile, a substantial minority ancestral component of British Neolithic populations is derived from the people who inhabited Europe in Mesolithic times. However, most of this ancestry can be explained by prior admixture with local groups descended from Mesolithic populations in continental Europe. The amount of detectable ancestry from British Mesolithic populations that is carried through to the Neolithic is small, and in some cases not significantly greater than zero. Individuals from Wales and the west of England show next to no detectable ancestry derived from local Mesolithic people.
We see the highest levels of Britishspecific admixture in Scotland. Two
Early Neolithic individuals from western Scotland had ancestors within the last three generations whose ancestry was predominantly derived from British Mesolithic populations. Though Mesolithic populations had only a small long-term genetic legacy, the two populations there did mix in the early days.
From east and west
In the past the British Neolithic has been characterised as odd compared to the rest of Europe, in terms of the prime role of acculturation attributed to its development. What we have shown in our adna study is that the Neolithic transition in Britain is indeed strange when compared to other parts of Europe, but in the opposite way: genetic evidence for the adoption of farming by local groups of hunter-fisher-gatherers is unusually scarce in Britain compared to other studied areas of Europe.
Why do British Mesolithic populations have such a small genetic legacy in the Neolithic? It is unlikely these two populations were naturally averse to mixing, on the evidence for contact between similar populations in other parts of Europe, as well as the direct evidence for mixing from western Scotland. The persistent predominance of Aegean ancestry in European Neolithic populations is most easily explained by farming lifestyles facilitating more rapid population growth, generating growing disparities in population size between incoming farmers and their descendants compared to local Mesolithic hunter-fisher-gatherers. In these cases Mesolithic populations would have only a small long-term genetic legacy even if the two groups mixed completely. An extreme version of this scenario would explain the low levels of British Mesolithic ancestry present in Neolithic Britain. On the one hand Neolithic populations entering Britain may have been particularly adept at exploiting this new territory, and grew unusually rapidly. On the other Late Mesolithic populations of Britain may have been particularly small and distributed at low density.
In British Neolithic populations, ancestry derived from people who inhabited Europe during the Mesolithic follows a positive southwest to north-east cline: Mesolithic dna is more apparent in the east than the west. Taken at face value this pattern could be interpreted as showing that continental Neolithic populations entered south-west Britain and gradually mixed with local groups as they moved north and east. However, as we know that little of the Mesolithic European ancestry we see in British Neolithic populations is actually derived from people who had lived in Britain, this cline must reflect different levels of pre-existing European Mesolithic ancestry in migrating populations.
This cline of ancestry from the European Mesolithic suggests that at least two different populations entered the east and west of Britain from different parts of continental Europe around the same time. The idea that more than one population came to Britain from continental Europe is also supported by regional variation in the small levels of central European ancestry we see. Central European ancestry is highest in Neolithic populations from Scotland, lower in those from England and absent from Wales. The east-west divide in ancestry broadly corresponds with regional variability in Neolithic material culture, suggesting that these differences may be partially influenced by different source populations with diverse cultural traditions having settled in particular regions of Britain.
Why did groups of farmers living in at least two different areas of continental Europe simultaneously decide to move into Britain around 4000bc? They had after all reached the shores of the Channel some thousand years earlier. Continental Neolithic farmers first start appearing in Britain soon after the development of maritime exchange networks that connect and influence megalithbuilding Neolithic societies around the Atlantic facade. Given the British Neolithic belongs to this loose group of megalith-building societies, it may be that the development of these maritime networks enabled movements of people into Britain.
Further analysis of the ancient human genomes we have already obtained, as well as sequencing of adna from more skeletons from Britain and continental Europe, will undoubtedly add nuance to this story, and provide more details on the origins of migrating farmers and their interactions with local groups. However, at the very least the simple question of whether there were influential migrations into Britain from continental Europe associated with the development of the Neolithic has been resolved. Britani’s Neolithic era was founded on migrations that changed the island’s population beyond recognition.
See “Ancient genomes indicate population replacement in Early Neolithic Britain,” by Selina Brace, Yoan Diekmann, Tom Booth & 24 others, Nature Ecology & Evolution (2019). Tom Booth is a senior research scientist at the Francis Crick Institute, London