If fewer Anglo-Saxons arrived, it is proposed that they formed a ruling elite, with acculturation of the local population. Thus some "Saxon" graves may be of Britons, though many scholars disagree. Two genetic studies published in 2016, using data from ancient burials found in Cambridgeshire, Yorkshire and Durham, found that the ancestry of the modern-day English population contained substantial contributions from both Anglo-Saxon and Celtic peoples. Various dates have been proposed to mark the end of Roman Britain, including the end of Roman currency coinage importation in 402, Constantine III's rebellion in 407, the rebellion mentioned by Zosimus in 409, and the Rescript of Honorius in 410. Unlike modern decolonisation, the dating of the end of Roman Britain is complex, and the exact process is unknown. There is some controversy as to why Roman rule ended in Britain. The view first advocated by Theodor Mommsen was that Rome left Britain. This argument was substantiated over time, most recently by A.S. Esmonde-Cleary. According to this argument, internal turmoil in the Roman Empire and the need to withdraw troops to fight off barbarian armies led Rome to abandon Britain. It was the collapse of the imperial system that led to the end of imperial rule in Britain. However, Michael Jones has advanced an alternative thesis that argues that Rome did not leave Britain, but that Britain left Rome. He highlights the numerous usurpers who came from Britain in the late 4th and early 5th centuries, and points out that the supply of coinage to Britain had dried up by the early 5th century, so that administrators and troops were not getting paid. All of this, he argues, led the British people to rebel against Rome. Both of these arguments are open to criticism, and the question is still open. It was a violent period, and there was probably widespread tension, alluded to in all the written sources. This may have led to the deaths of many Britons. There are also references to plagues. Laycock ("Britannia the Failed State", 2008) suggests tribal conflict, possibly even starting before 410, may have sliced up much of Britain and helped destroy the economy. The evidence from land use suggests a decline in production, which might be a sign of population decline. It is clear that some British people migrated to Europe, and Armorica in northwest Gaul became known as Brittany. There is also evidence of British migration to Gallaecia, in Hispania. The dates of these migrations are uncertain, but recent studies suggest that the migration from south western Britain to Brittany may have begun as early as 300 and had largely ended by 500.