Tracing ancestry, researchers produce a genetic atlas of human mixing events

The rise and fall of empires, the march of armies, the flow of trade routes, the practice of slavery — all these events have led to a mixing of populations around the world. Such episodes have left a record in the human genome, but one that has so far been too complex to decipher on a global scale. Now, geneticists applying new statistical approaches have taken a first shot at both identifying and dating the major population mixture events of the last 4,000 years, with the goal of providing a new source of information for historians. Some of the hundred or so major mixing events they describe have plausible historical explanations, while many others remain to be accounted for. For instance, many populations of the southern Mediterranean and Middle East have segments of African origin in their genomes that were inserted at times between AD 650 and 1900, according to the geneticists' calculations. This could reflect the activity of the Arab slave trade, which originated in the seventh century, and the absorption of slaves into their host populations. The lowest amount of African admixture occurs in the Druse, a religious group of the Middle East that prohibited slavery and has been closed to converts since AD 1043. Another mixing event is the injection of European-type DNA into the Kalash, a people of Pakistan, at some time between 990 and 210 BC This could reflect the invasion of India by Alexander the Great in 326 BC The Kalash claim to be descended from Alexander's soldiers, as do several other groups in the region. The genetic atlas of human mixing events was published on Thursday in the journal Science by a team led by Simon Myers of Oxford University, Garrett Hellenthal of University College London and Daniel Falush of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Having sampled genomes from around the world, they found they could detect about 95 distinguishable populations. Though all humans have the same set of genes, their genomes are studded with mutations, which are differences in the sequence of DNA units in the genome. These mutations occur in patterns because whole sets of mutations are passed down from parent to child and hence will be common in a particular population. Based on these patterns, geneticists can scan a person's genome and assign the ancestry of each segment to a particular race or population. The team led by Dr Myers has developed a statistical technique for identifying the chromosomal segments with particular precision. This enables them to perform a second feat, that of assigning a date to the one or more mixing events that have affected a population. The dating system is based on measuring the length of chromosome segments of a particular ancestry that occur in a population. When people of two different populations intermarry, their children's genomes carry large chunks of DNA of one parent's ancestry interspersed with large chunks from the other's. In each successive generation, the average size of the chunks becomes smaller because when DNA is swapped between the parents' genomes in making the eggs or sperm, the cuts needed to generate the swapped sections are made in different places. Therefore, from the average size of the chunks in a person's genome, the geneticists can calculate the number of generations since the mixing event. "We are among the first to try to date ancestry events, and we have more ability to determine the source populations," Dr Myers said. One of the most widespread events his group has detected is the injection of Mongol ancestry into populations within the Mongol empire, such as the Hazara of Afghanistan and the Uighur Turks of Central Asia. The event occurred 22 generations ago, according to genetic dating, which corresponds to the beginning of the 14th century, fitting well with the period of the Mongol empire. In another example, the European colonization of America is recorded in the genomes of the Maya and Pima Indians. And Cambodian genomes mark the fall of the Khmer empire in the form of ancestral DNA from the invading Tai people. Dr Myers and his colleagues have detected European ancestry that entered the Tu people of central China between the 11th and 14th centuries; this, they surmise, could be from traders traveling the Silk Road. They find among Northern Italians an insertion of Middle Eastern DNA that occurred between 776 BC and AD 550, and may represent the Etruscans, a mysterious people said by the ancient Greek historian Herodotus to have emigrated from Lydia in Turkey. The Myers group has posted its results on a web page that records the degree of admixture in each population. The English, however, known to be a rich medley of Celts with invaders such as the Angles, Saxons, Jutes, Danes and Norwegians, carry the notation "No strong evidence of admixture." Dr Myers said his method cannot yet detect genetic mixing between very similar populations, as was the case with the English and their invaders from Scandinavia and Northern Germany. He said he hoped to distinguish all these groups in a separate project on British ancestry. Dr Hellenthal said, "We're fairly confident that increasing our sample size will help us follow local migrations." John Novembre, a geneticist at the University of Chicago, described the new genetic atlas as a "landmark study" because of its scale and the fact that the authors had been able to extract complex signatures from the data. "The detailed historical interpretations may need further questioning and testing," he said. Dr Myers and Dr Hellenthal said that they hoped historians would find their work useful, but that they had not collaborated with historians. "In some sense we don't want to talk to historians," Dr Falush said. "There's a great virtue in being objective: You put the data in and get the history out. We do think this is a way of reconstructing history by just using DNA."