Viking ancestry, surnames, and medieval genetics

There's a fun paper in last month's Molecular Biology and Evolution by Georgina Bowden and colleagues, investigating whether the Viking influence on surnames in England is mirrored by Y chromosomes. This is one of those chapters in English history that people should know more about:

A useful test case to evaluate the potential of surname-based ascertainment is provided by the 1,000-year-old Viking settlement of the British Isles. The Viking age of raiding, exploration, trading, and colonization began in the late eighth century, with a series of attacks on the coasts of Britain, Ireland, and France. In England, a shift from raiding to permanent settlement began after AD 865. The modern consensus has moved from the earlier view of a mass migration of Viking settlers to a more geographically variable and gradual process, with much assimilation of local culture, under the administration of a Scandinavian elite (Richards 2004). Even though Viking rule in England came to an end nearly 1,000 years ago and the settlers were soon integrated linguistically and culturally, abundant evidence of Scandinavian influence remains today. Important archaeological findings exist, but the most striking evidence is linguistic and onomastic -- many words in Standard English and in local dialects, and many place-names, are of Scandinavian origin. In some areas of English counties that formed part of the Danelaw (fig. 1), the region under the administrative control of the Vikings from the late ninth century, up to 70% of major place-names are in this category, with endings such as "-by" and "-thorp(e)" (Bowden 2008:301-302).

What a word: "onomastic"!

The paper has a nice bit of this history. The general idea of the study was to assess whether a genetic sampling focusing on surnames that were present in the past can give an accurate picture of the genetic structure of the population in the past. To this end, they were able to exploit historical records that include lists of surnames present in northwest England more than 500 years ago.

Gene lineages erode out of populations over time. Every individual has a substantial chance (> 15%) of not having any children at all, and every time a person has no children, any unique gene lineages he carries will become extinct. Immigration can replace a population's ancient lineages with genes from elsewhere. So the genes that did characterize an ancient population disappear, while the genes that characterized other ancient populations may come to roost. Taken over enough time, everyone in every population is a descendant of immigrants, because human populations have emerged through mixture and dispersal. Anthropologists try to work out where and when the important dispersals and contacts happened. To do this, they sometimes try to reconstruct the ancient genetic structure of a population, attempting to see how it differs from the present genetic structure.

Information about the ancient structure of a population is hard to come by. By far the best source of information is ancient skeletons. Sample the DNA sequences from the ancient bones themselves and you can determine the genetic structure of their owners. But that's expensive and takes some luck -- you have to have enough ancient bones that preserve DNA and the right to grind them up.

If you had some kind of accurate indication of which people descend primarily from immigrants and which ones have mostly local ancestors, you might be able to sample the mainly local people's genes to get an idea of the genetic structure of the ancient population. This idea won't work well for most genes, because immigrants don't remain a separate, inbred population for long -- over the years (in this case, hundreds of years) they intermarry and blend into the population. People with immigrant surnames come to have the same genes as everyone else.

These authors propose that patrilineal English surnames let them reconstruct the genetic structure of Y chromosomes in the men of northwestern England more than 450 years ago. Over that time span, surnames have been conservative enough (and paternity confidence high enough) that most men who share a rare surname today also share closely related Y chromosomes. Hence, they may serve as a proxy for men who carried the same surname in the past. If you have a record of the surname proportions, you can substitute in today's Y chromosomes to estimate the past population structure.

They find that around 50 percent of northwest English men once descended from Viking patrilines. That's a surplus over today's value, which is under 40%:

The medieval sample from West Lancashire shows an increased proportion (51 ± 4%) of Scandinavian ancestry compared with its modern counterpart (38 ± 4%); the equivalent values for Wirral are 47 ± 5% and 38 ± 3%. These differences, revealed by our different sampling strategies, are likely to reflect a change in haplotype frequencies due to postmedieval immigration and are supported by genetic distances (FST) between the Norwegian sample and the Wirral and West Lancashire samples. FST between Norwegians and the West Lancashire modern sample is 0.130, whereas the value for the medieval sample is only 0.069; corresponding values for the Wirral samples are 0.162 and 0.096.

I don't want to oversell this study. The surname approach may work reasonably well for Y chromosomal diversity across a few hundred years, in strongly patrilineal societies. The idea starts to show weaknesses over longer time periods, as evidenced by the problems documenting the "Cohen modal haplotype" -- another Y chromosome-surname association. And it won't work at all for other genes, which are inherited from the mother as well as the father. The last paragraph of the study reflects on the limitations:

The use of patrilineal surnames in ascertainment provides both the power of our approach and its major limitation because any analysis of the structure or history of populations based on a single genetic locus, the Y chromosome, has inherently low power and population statistics calculated from Y data have high variance (Wilson et al. 2001). Furthermore, we can learn nothing about changes in the maternal genetic landscape, which is unfortunate, because the role of women in the Viking colonizations was as important as that of men (Jesch 1991). Nonetheless, in the absence of a time machine, the link between a haplotype and a cultural marker, the surname, may provide the only practical means to access the genetic composition of populations in the past. As well as allowing us to investigate the influence of migration and drift over the last few centuries in changing the population structure of Britain, the method should be applicable to other regions where surnames are patrilineal and suitable historical records survive.

Still, I wish that there were more papers like this, incorporating history with genetic information. Too many geneticists actively shun historical and archaeological evidence, preferring to rely on genes alone. Sure, at some superficial level they may tie their findings in to a "what everybody knows" version of history, but they don't report any history or archaeology in their papers, and they shelter their students from needing to learn any. That's a strategy for some pretty boring research, and these geneticists are leaving on the table some of the strongest evidence about recent human events.

References:

Bowden GR and 13 others. 2008. Excavating past population structures by surname-based sampling: The genetic legacy of the Vikings in northwest England. Mol Biol Evol 25:301-309. doi:10.1093/molbev/msm255