By Judith Burns
Science and environment reporter, BBC News
Field voles re-occupied the British isles after the last ice age
DNA tests on British populations of small mammals show a genetically distinct "Celtic Fringe", say scientists at The University of York.
Voles, shrews, mice and stoats in northern and western areas have different DNA from their counterparts in other parts of the British Isles.
The paper, in Proceedings B journal, says the different populations arrived at the end of the last ice age.
The authors say the work sheds light on the origins of the Celtic people.
The traditional view is that the ancestors of British Celts spread from central Europe during the Iron Age and were later displaced by the arrival of the Anglo Saxons.
However, recent genetic studies have challenged this theory, suggesting a much earlier origin, dating back to the end of the last ice age, 19,000 years ago.
This paper suggests that the study of small mammal populations could help resolve the controversy.
The team compared the mitrochondrial DNA of seven small mammal species, this being the best genetic marker for studying colonisation history.
The seven species were three types of vole: bank vole, water vole and field vole; two types of shrew: common shrew and pygmy shrew; the house mouse and the stoat.
Lead author, Professor Jeremy Searle, told BBC News: "We found this extraordinary pattern that within the British Isles you get these two genetic types for a number of species. One having peripheral distribution and one having a central and eastern distribution.
"Seeing that distribution and trying to think how it came about was interesting in itself - but also the fact that it has this similarity with the distribution of cultural and genetic types in humans."
Professor Searle said that the mitrochondrial DNA in the two groups was sufficiently different that "they could not have originated one from the other within Britain but must have originated outside Britain and come in as two separate entities".
The authors say that both types of each species must have arrived before Britain was cut off from Europe by rising sea levels 8,000 years ago.
They believe that the type now confined to the Celtic periphery colonised Britain first, sometime after the end of the last ice age, and spread throughout the British Isles.
The global climate remained mild until 12,900 years ago when a sudden drop in temperature brought permafrost back to the British Isles for more than 1,000 years. This caused a drop in the small mammal population.
A new wave
When temperatures rose again a new wave of small mammals colonised Britain and replaced the now sparse populations of the first type everywhere except on the northern and western edges.
Professor Searle suggests that a similar pattern is likely to apply to humans as there is archaeological evidence of humans in Britain after the last ice age, 19,000 years ago.
He said: "It could be that, like the small mammals, this first wave of humans did not do well during the cold spell but managed to hang on. Once it was over, another group came in and replaced the first type everywhere apart from the peripheral areas.
"And so one could suggest that the human Celtic Fringe was set up by exactly the same sort of events that set up the animal Celtic Fringe.
He suggests that the Celtic Fringe has since been reinforced culturally by different sets of people occupying the fringe areas and also what is now England, for instance the Romans, Anglo Saxons and Normans.
"This fits well with studies that others are doing on human genetic patterns and with the growing idea that the Celtic genetic type has been there a very long time."
Dr John Stewart of The Natural History Museum said he appreciated the paper's realistic approach to the issue: "It is only by looking at a variety of different organisms that we can see genetic biogeographic patterns emerging."
"It is really important to treat humans as part of a suite of animals that were being pushed and pulled around the landscape by changes in climate."