THE question “why are we what we are, where we are?” has intrigued humans since at least the time of Herodotus, who is often acknowledged as the first anthropologist. But answering it is far from easy. The geographical dispersal of traits such as blonde hair or blue eyes, for instance, is of little help because millennia of migrations, persecution, slavery, wars and invasions have massively muddied the waters of racial “purity”. As Charles Darwin observed in The Descent of Man, “it may be doubted whether any character can be named which is distinctive of a race and is constant.” The character of DNA, however, is another matter—and in the past decade the availability of a mass of genetic big data is making it easier to figure out where we all came from. From the start of the DNA-sequencing era, computer modelling has established a strong relationship between genetic and geographical locations, although cultural isolation and admixture (when formerly separated populations interbreed) muddy these models too. As a result, existing biogeographic algorithms are still essentially based on racial definitions, and have proved fairly vague. If applied to Europe, for example, the best can trace individuals to within 700km of their ancestral origins (outside Europe they are even fuzzier), and correctly identify country of origin less than half of the time.

But a multinational research team led by Dr Eran Elhaik of Britain’s University of Sheffield is taking a different tack. Patterns of genetic diversity in human populations are often described as genome-based estimates of several ancestries that sum to 100%. In other words, admixture is everywhere. So Dr Elhaik figured that a better starting point for tracing ancestry might be the hypothesis that everyone is to some extent genetically mixed.

To test this theory, Dr Elhaik and his team proposed an admixture-based model to develop what he calls a Geographic Population Structure (GPS) method for predicting individuals’ biogeographical origin. He identified 130,000 ancestry-informative markers (AIMs) in the human genome, then used 40,000-60,000 of the most relevant ones to divide the world’s population into nine gene pools, each with a distinctive genetic signature. (Dr Elhaik is at pains to note that this is only a computer model—reality is more complex.) GPS then mapped a variety of populations to discover, for each one, to which gene pools it belonged. All populations belonged to at least two pools; some to as many as nine (Bermudians and Puerto Ricans).

A paper just published in Nature Communications suggests that this model can be remarkably accurate. The team applied GPS to some 1650 individuals from around the world, gleaned both from highly heterogeneous populations such as Bermuda and Puerto Rico, and from communities that had deep roots in a single country, such as indigenous highland Peruvians and Sardinians. GPS correctly assigned 83% of the sample individuals to their ancestral country of origin, and 50% of them within 87km of their point of true origin. In Sardinia, an island settled 14,000 years ago that is genetically isolated and has experienced little geographic matrimonial movement, GPS was able to place a half of individuals to within 15km of their point of origin—and a quarter to their specific village of origin. Currently, GPS can accurately track origins back about 1,000 years. With the help of ancient DNA data, it could work much further back in time.

The potential uses of GPS are many. It could help adoptees find their true homes, enable missing children to be returned to their country of origin, and be employed in the fight against human trafficking. It could give evolutionary biologists greater insight into species and speciation, and assist archaeologists in mapping the biogeography of ancient populations. Dr Elhaik’s co-author, Dr Tatiana Tatarinova of the University of Southern California, has also created a website where individuals who have had their DNA sequenced (which costs about $100) can for a small fee upload the data and discover that, although they live in Islington, their roots are in İzmir.