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DNA is used to uncover or suggest migration patterns in the past. How this works is as follows. A population settles in an area, and is reasonably isolated geographically from other population groups. Then small mutations in the DNA, given enough time, become widespread in that population, and become a distinctive marker for that population. If the population later migrates, they carry with them the distinctive DNA, which can be used to trace the ancestry and migration patterns. This is the underlying theory, which is well supported by the available DNA evidence.
In the Channel Islands, it seems clear that we can fairly accurate discern migration and settlement patters with large percentages of Celtic and Invader DNA. What is not so sure, and where a note of caution should, I think be sounded, is in the interpretation of the smaller percentages.
There are strands of DNA relating in Channel Islanders relating to the Mesolithic period, and three distinct types of Neolithic DNA. But these are very small percentages, and the population samples from which they were taken were also small, so I think that there are at least two plausible interpretations of how they came into the gene pool.
One is that they were present in the Channel Islanders, who then intermarried with larger immigrant populations so that the original gene pool was diluted to its present extent. But the other, which is equally possible, is that they were already part of the gene pool present in Celtic and Invader DNA, so that when those peoples settled on the Islands, they brought with them these traces of their own ancestry. The problem is that the percentages are so small, that when one considers the range of standard error, it is impossible to be certain that another sample might not produce a different pattern. This is unlike the Celtic / Invader DNA, which forms a much larger percentage, and where we might expect variation, but not a wholesale alteration of the basic pattern.
So what does this very ancient DNA tell us? It tells a story of separate waves of migration, where the differing groups intermarried at a later age. But it does not tell us that this happened in the Channel Islands, or that there were three distinct groups here. That part of the DNA journey could have happened beforehand, and the DNA simply carried into the Islands by the later settlers.
One way to picture what might have happened is to tell this as a story.
Imagine two bus journeys. The Celtic Bus route leaves Gorey to come into St Helier. At the start of its journey, it has a few passengers get on in the early morning - Neolithic times, but the bulk of the passengers get on much later closer to town, perhaps at Longueville - the Viking place. The Celtic Bus route leaves Corbiere with a few passengers on board - Neolithic times, but picks up most passengers from First Tower. So each set of passengers is distinct, because each has come a quite different route, but there are some passengers from further out in each case, who have also come along in the same bus. At St Helier, the passengers mix together, and give today’s mix of DNA.