Professor Mark Thomas may have a point in his critique of genetic ancestry testing companies. The line between popularising science and misleading the public is a very difficult one to draw, as responses to the recent BBC TV programme Meet the Izzards, in which Eddie Izzard traces the migration of his ancestors out of Africa and into Europe, illustrate. But Thomas's portrayal of what he calls "interpretative phylogeography" – the principal means by which those of us who study mitochondrial DNA and the Y chromosome draw conclusions about the dispersal history of modern humans – as "genetic astrology" has profound implications. Here he is taking on not just the commercial use of the scientific work, but the science itself – work that has been published in peer-reviewed articles in leading scientific journals. Can he really be right?
Phylogeographers compare the genealogical relationships among genetic lineages with their geographical source, to try to work out when lineages moved from one place to another. It relies on the principle that every mutation in the DNA arises at a specific point in space and time, and that a plot pinpointing these locations is effectively an outline of the movement of people across the landscape and around the world.
According to Prof Thomas, this "has never been scientifically shown to work". Well, apart from the fact that the phylogeography of mitochondrial DNA predicted that modern humans had a recent African origin, about 200,000 years ago. This was in 1987, when most anthropologists believed that Homo sapiens had evolved in tandem across the Old World. Improved fossil dating methods soon began to confirm the new picture, and a new consensus was reached. This essentially still stands, even though the recovery of ancient Neanderthal sequences now suggests that there was some interbreeding with archaic groups along the way.
Then there's the fact that the mitochondrial picture almost exactly matches the archaeological evidence for the colonisation of the Remote Pacific, about 3,000 years ago. The same is true for the expansion of Bantu speakers in Africa, from the tropics into the far south, a little later. And the first settlement of the Americas from Asia, genetically dated to about 15,000 years ago, matches the archaeology pretty well too.
Hopefully, the point is made. There are enough archaeologically well-known cases of colonisation to show that phylogeographic interpretations can work in principle.
Why is this denied by Prof Thomas? In some cases, there is a genuine and laudable desire to improve the analytical tools with which we work, which is all well and good. But among people in this small (but vocal) group of detractors, as across science more generally, there is a tendency to want to say that "what we do is science and what you do isn't" – and to draw sharp dividing lines between "rigorous hypothesis testing" and "storytelling". You would think that some awareness of the history and philosophy of science would guard against making such distinctions, but it's a sad fact that some scientists still prefer to ignore the wider picture in favour of a gung-ho advocacy of their favoured bit of scientific methodology.
It's very easy for someone to claim there's "no evidence" for something. Creationists, after all, insist there's no evidence for evolution. What can we do in such circumstances? We can't force people to acknowledge the evidence. All we can do is present it as clearly as possible and hope they will engage.
The problem is perhaps most acute in historical subjects like this where the evidence is especially challenging to interpret, and alternative views always seem possible. Ironically, in areas of science where researchers believe that a new piece of evidence might decisively prove them wrong, they are perhaps more circumspect with their suggestions. Perhaps recent advances in the recovery of ancient DNA sequences – where we can directly see what some of the ancestral lineages were like – might help to chasten all sides of the debate. But contrary to Prof Thomas's apparent view that there should be no interpretations, only "facts", we suspect that "interpretative phylogeography" will still have a central role to play, even as prevailing models are challenged once again.
There are of course plenty of issues for discussion relating to genetic ancestry testing companies. Prof Thomas focuses on the "very little" information that testing the mitochondrial DNA or Y chromosome can provide. We agree, but the fact is that some people are genuinely interested in testing even a small fraction of their ancestry. We'd have thought that, rather than worrying that testing companies might not be giving the "full" picture, it might be more useful to question the wisdom of trying to track down our individual "origins" in the first place. The idea is surely mythical – except in the mundane sense of where we were born and grew up.
But the great thing about phylogeography, like archaeology and history, is that it gives us the bigger picture of our place in the world – a story, but one based on evidence. In a way, some of the better testing companies are, however imperfectly, trying to do that – to make a genuine contribution to the public understanding of science. And that was also what the BBC programme about "meeting the Izzards" was trying to do. We don't think they did it that badly.