In your July issue you published two articles which claimed to show that it is possible to pinpoint your "genetic homeland" a thousand or more years ago by taking a simple DNA test. It would indeed be wonderful if a DNA test could give us this information but sadly it is not possible.
The first story “DNA: find your ancestral home” (p9) referred to a new autosomal DNA test developed by a company called Prosapia Genetics. They claim that they are able to pinpoint someone’s genetic homeland one thousand years ago and that their test is “accurate to home village with a time resolution of the past 1,000 years”. The underlying research on which this test is based was published in a scientific journal (Elhaik et el 2014, Nature Communications 5: 3513). However, the researchers were only able to place 50% of people within 450 kilometres of their country of origin, which is hardly the level of precision claimed. Furthermore the research focused solely on the present-day country of origin and made no attempt to determine an ancestral origin one thousand years ago. Such a method is best thought of as human provenancing – finding the location where an individual’s genotype is most likely to be found – not a method for inferring ancestry. Indeed, if you go back one thousand years you have in theory about 35 billion ancestors, although you actually inherited DNA from only a small, random subset of those ancestors. Identifying a single location as the “genetic homeland” of either all your pedigree ancestors or just the DNA ancestors would be a meaningless exercise. The International Society of Genetic Genealogy received a number of reports from dissatisfied customers of Prosapia Genetics, many of whom had received bizarre results placing their "genetic homeland" in the middle of a river or ocean. Fortunately they were able to get refunds from PayPal.
Dr Tyrone Bowes, the author of the second article "Routes to roots" (pp14-18), claims to be able to tell his customers when their ancestors arrived in Britain and where they came from based on recurring surname matches received as part of a commercial Y-chromosome DNA test. DNA testing is a very useful tool for the genealogist, and Y-DNA matches can often provide clues about our recent origins. However, Y-DNA results should always be interpreted in combination with genealogical and historical records. While a Y-DNA test is very good at indicating whether or not two people share a recent male-line ancestor, it is much more difficult to determine precisely when or where that ancestor might have lived. For example, a match on 34 out of 37 markers could indicate a shared ancestor who lived 200 years ago or 2000 years ago, and there is no way of determining the precise timeframe. Furthermore, surnames did not become common in Britain one thousand years ago, as is claimed. They were introduced into Britain with the Norman Conquest but the adoption of surnames was a gradual process. While most English people had acquired surnames by the fifteenth century, surnames were not adopted until the nineteenth century in some parts of Wales. In the Highlands of Scotland the clan system survived until the eighteenth century, and people adopted the name of the clan rather than using an hereditary surname. Even when surnames are passed on through the fatherline the link between the Y-chromosome and the surname is often broken as a result of illegitimacy, cuckoldry, adoption or name changes. In addition, there is an inherent American bias in the commercial databases. Consequently Y-DNA matches will often tell us more about non-paternity events in Colonial America in the last 400 years, rather than a person’s origins in the British Isles. All these factors need to be taken into account when interpreting DNA results.
The mutations that determine haplogroups (the deep-rooted branches of the human Y-DNA tree) did indeed occur at a specific time and place several thousand years ago, but determining when and where that happened is a different matter entirely. We are reliant on making inferences from the DNA of living people. However, the current distribution of haplogroups differs from each of the distributions at different times in the past. The changes occur due to migration and the randomness of genetic drift. As more ancient DNA samples become available it might one day be possible to provide some answers, but we are not there yet and it is unlikely that we will ever be able to "precisely reconstruct our ancestral journey".
Readers wishing to understand more about the legitimate uses of DNA testing for genealogy and the limited inferences that can be made from deep ancestry tests might like to refer to the new “Debunking genetic astrology” website that I have worked on with my colleagues at University College London: www.ucl.ac.uk/mace-lab/debunking. In particular readers might like to look at the page on dubious commercial claims (www.ucl.ac.uk/mace-lab/debunking/companies) where the genetic homeland stories are discussed in greater detail.