Mitochondrial DNA diversity and origin of human communities from 4th- 11th century Britain.

Abstract

Neither the archaeological nor the historical data have yet allowed a full understanding of the nature of the Germanic settlement in England. Analysis of the genetic structure of past history has mostly been carried out by inference from extant populations. However, genetic flow through migration over time is likely to have altered the genetic composition of modem samples. Analysis of the genetic composition of ancient populations (provided the authenticity of their DNA is obtained) gives a direct sight into the past. Thus, mitochondrial DNA from pre-Saxon (4th century), early Saxon (5th -7th century) and late Saxon (9th – 11th century) settlements has been analysed to obtain a better understanding of the population history of Britain. A methodology has been optimised, by which, ancient DNA from 1,000-1,800 year old archaeological material was extracted and ~200-bp fragments of the HVS-1, amplified and sequenced. Rigorous controls for work in human ancient DNA were undertaken to prevent and recognise contamination. Established authenticity criteria were followed, including expected ancient DNA behaviour, internal replication of sequences and confirmation by independent labs. The sample size obtained has enabled a population-level study of communities of ancient Britain. In addition, an extensive database of >6500 mitochondrial DNA sequences was compiled for comparisons. Several estimates of haplotype and nucleotide genetic diversity were computed for modem and ancient populations. Counter-intuitively, the modem population of England, encompassing all successive waves of migration to the island, has a lower diversity than the ancient population, suggesting that diversity has been lost over the last millennium. In addition, mtDNA genetic continuity between ancient and modem England seems to have been intermpted. Founder analyses of early (5th -7th century) and late (9th -11th century) periods indicate that, whereas the late period seems to have had Viking genetic influences, the early period has no close relationship with Germanic populations. Instead, the females of the early Anglo-Saxon period seem to represent the native British population. The female contribution of the Anglo-Saxon invasion would have therefore been minor, at least at that time and at these sites. The close genetic affinity between the ancient British population and the northern most populations of Europe suggests they might have shared a common past during pre-history. It is proposed that, after post-glacial times, inhabitants of areas now submerged expanded to northern territories. The early settlements analysed reflect that very early expansion. Some time since then, reduction in diversity seem to have occurred (possibly due to variation in family size after repeated epidemics) leading to the present day mtDNA composition of England.