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third-generation sequence nearby. In these cases I thought it was reasonable to regard that fourth
mutation as having happened in Ireland. Counting up these home-grown mutations and factoring in
the mutation rate as usual gave me a corrected date for the clan in Ireland of a little over 7,000
years – 7,300 to be precise. This was much more reasonable than the 50,000 years which counted
all the Ursulan mutations as if they had all happened in Ireland.
Genetic dates, like the 7,300 years for the Irish Ursulans, are not very accurate. They are
estimates. We find this concept particularly difficult to grasp because we are accustomed to dates
being very precise. The 7,300-year date for the arrival of the Irish Ursulans is an estimate. The
date could vary a thousand years either way and still fall within the scope of the estimate.
Forgetting the inaccuracies for the moment, what does this date mean? It is an estimate for the
length of time it would have taken for all the Ursulan mutations to have accumulated within Ireland.
If the ancestors of all 91 Irish Ursulans had arrived at the same time and their mDNA mutations had
accumulated since then, the genetic estimate for their arrival would have been 7,300 years ago. Of
course, it is very unlikely indeed that they all arrived at once. Some would have come more
recently, but in that case, to achieve the average figure, others must have arrived more than 7,300
years ago to balance out the more recent arrivals.
I went through the same procedure with all the other Irish maternal clans, checking to see in
each one how many looked as though they had mutated to their final form in Ireland and not
elsewhere. From there I calculated the clan arrival times in the same way as I had for the Ursulans.
All of them came out between 7,500 and 4,500 years ago. Ursula was still the oldest clan in Ireland
and, in common with the rest of Europe, Jasmine was the youngest. It was Jasmine’s clan that
Martin Richards and I had linked to the arrival of Neolithic farmers in Europe from the Middle
East. The others clustered around the 5,000–6,000-year average. Even bearing in mind the
approximate nature of these genetic dates for the settlement of Ireland by the various maternal
clans, they are all way before the time, around 200 BC, when the Iron Age Celts were supposed to
have arrived. It was beginning to look as if the ancestors of today’s Irish had been there for a lot
longer than anybody thought.
But within Ireland, when I looked at the maternal clans in the different provinces of Ulster,
Leinster, Munster and Connacht, there was very little noticeable difference between any of them,
though the numbers in each were too low to be sure of statistical significance.
If women had been in Ireland for a very long time, what about the men? Just as mitochondrial
DNA traces our maternal ancestry, so the Y-chromosome follows paternal genealogies. Like
mitochondrial DNA, Y-chromosomes also experience random mutations over the course of time.
The precise nature of the mutations might be different between mDNA and Y-chromosomes, as we
shall see, but the principles are the same. The accumulation of mutations along paternal
genealogies over a very long time means that there are now tens of thousands of slightly different
Y-chromosomes which can be distinguished by genetic tests. If two men have the same Y-
chromosome fingerprint, then they have usually inherited it from a common patrilineal ancestor.
That’s exactly the same principle as saying that if two people have the same mitochondrial DNA
sequence they have inherited it from a common maternal ancestor. Although Y-chromosomes are
quite different from mitochondrial DNA in the way they change genetically, that doesn’t matter so
much when it comes to interpreting the signals they are bringing us from the past.
Just as each of us belongs to one of a small number of maternal clans, so men can be assigned