Kents Cavern in Torquay had produced human remains from 30–40,000 years ago. Dartmoor is thought to have been occupied by Mesolithic hunter-gatherer peoples from about 6000 BC. The Romans held the area under military occupation for around 350 years. Later, the area began to experience Saxon incursions from the east around 600 AD, firstly as small bands of settlers along the coasts of Lyme Bay and southern estuaries and later as more organised bands pushing in from the east. Devon became a frontier between Brittonic and Anglo-Saxon Wessex, and it was largely absorbed into Wessex by the mid 9th century. A genetic study carried out by the University of Oxford & University College London discovered separate genetic groups in Cornwall and Devon, not only were there differences on either side of the Tamar, with a division almost exactly along the modern county boundary dating back to the 6th Century but also between Devon and the rest of Southern England, and similarities with the modern northern France, including Brittany. This suggests the Anglo-Saxon migration into Devon was limited rather than a mass movement of people.