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The genetic landscape of England and Wales was shaken up thousands of years ago as new arrivals redefined its people.
Scientists analysing the remains of ancient Europeans identified a new wave of people coming to Britain who would become responsible for half the genetic ancestry of future generations.
A British fondness for milk came about around a millennium earlier than in the rest of Europe, as one of many genetic changes revealed in the wake of ancient migrations.
Europeans moving into Britain at the beginning of the Bronze Age around 4,500 years ago caused large shifts in the genetics of its inhabitants, as well as cultural changes.
A new study published in Nature has revealed a previously unknown second wave of arrivals around 1,500 years later, which may have helped the introduction of early Celtic languages to Britain among other changes it brought.
Researchers hope the work will help them to understand more about the early history of humans in Britain.
Dr Selina Brace, who co-authored the paper and is an ancient DNA specialist at the Museum, says, 'When archaeologists examine ancient human remains, they have an idea of who they are, based on the associated culture, which includes the manner in which they were buried, the artefacts that they're buried with and the time of their death.
'By looking at the DNA from these individuals we can analyse their genetic signature, which can tell us many things about the individual but also about groups and movements of people. For instance, when there is a change in culture, we can analyse the DNA from individuals before and after the change to see if this is also associated with a change in genetic signature.
'This is something we have seen at the beginning of the Bronze Age, where a huge shift in genetic ancestry signifies a movement of people into Britain at this time. We’ve now identified a second shift, suggesting a second migration of people, likely from France, into Britain during the middle to late Bronze Age.'
The first humans in Britain arrived at least 950,000 years ago, leaving footprints preserved in sediment at Happisburgh in Norfolk. The exact species isn't known but it could have been Homo antecessor, remains of which found in Spain have been dated to a similar time.
At this point in history our ancestors could have walked to Britain and Ireland, rather than travel over water. However, from around 450,000 years ago the British Isles gradually became harder to get to as the channel started to form.
During those 400,000 years, there are brief periods in which Neanderthals and Homo sapiens moved into Britain, but not permanently. Modern humans would not consistently occupy the islands until around 12,000 years ago.
Around 6,000 years ago, the first farmers, descended from ancestors who lived in Anatolia (modern-day Turkey), arrived in Britain.
This was followed by the arrival of people associated with the Bell Beaker culture 1,500 years later, which once again led to a replacement of the genetic ancestry in Britain. Their arrival may have been associated with a famine in western Europe at the time, providing an opportunity for these humans to move in.
However, despite this influx from Asia, people living in southern England and Wales have inherited a much higher proportion of their genetic ancestry from their Anatolian ancestors than those who live in northern England and Scotland.
In order to unwrap this mystery, scientists extracted DNA from the remains of hundreds of ancient humans from across the UK and Europe.
The findings point to the arrival of people with higher levels of early European farmer ancestry into southern Britain in the later years of the Bronze Age. While this event didn't change the dominant culture at the time, it does see the amount of Anatolian heritage rise significantly from the Bronze Age to the Iron Age.
While it can't yet be confirmed, the genetic evidence from the remains of 800 individuals studied by the researchers suggests these new residents most likely moved from France.
The genetic analysis of these ancient humans also uncovered an unexpected change associated with digesting milk. While all mammals are able to digest milk as juveniles, the lactase gene which allows this often deactivates when they grow into adults.
The researchers found that the lactase persistence gene, which allows humans to drink milk in adulthood, increased sharply in frequency until it was found in about half of Britons in the Iron Age, as opposed to around 7% in the rest of Europe. The impact of this is still felt today, with those of British genetic ancestry much more likely to have this mutation than those from elsewhere.
'It's easy to assume that the ability to drink milk would happen at the same time as people started farming but it didn't,' Selina says. 'It's been a real wild card, but we see a massive increase in the frequency of this genetic variant in Britain in the Iron Age which doesn't seem to coincide with a specific migratory event.'
While researchers don't know exactly why this happened, such strong selection suggests it must have provided an advantage to those who had a copy of the gene.
The timing of the migration adds evidence to theories about the arrival of early Celtic languages in Britain.
'There have been ongoing debates about the origins and development of Celtic languages in different parts of Britain, and many theories have involved migration from continental Europe as a possible influential factor,' said Dr Tom Booth, co-author and former postdoctoral researcher at the Museum.
'Our results by no means resolve these questions absolutely, but by finding evidence for substantial migrations in the Middle-Late Bronze Age rather than the Iron Age, our results do increase the chances that it was in this earlier period when Celtic languages were actually introduced to Britain, and that these languages originated in southern present-day France.'
The researchers now hope to increase their knowledge of the genetics of ancient French and Irish residents to try and decipher more about these influxes and how they changed the peoples of the world thousands of years ago.