Tracing human roots

The Americas may have been initially settled in two different migrations, a new method for tracing human ancestry reveals. The analysis also suggests populations in the Orkney Islands, a string of islands north of Scotland, share substantial common ancestry with northern Siberian populations.

The new work by researchers in the United Kingdom and Ireland supports most previous findings, including the “Out of Africa” hypothesis that all humans share common ancestors who first spread from Africa about 50,000 years ago. “The conclusions aren’t all that novel, but the way that they’ve reached the conclusions is extremely novel,” says Donald Conrad, a population geneticist at the Wellcome Trust Sanger Institute in Cambridge, England, who was not involved in the study. “It’s a benchmark of what population genetic research can look like in the coming years.”

A few surprising results emerged from the analysis, published May 23 in PLoS Genetics. Native Americans from North and South America may have had different predecessors. While the North American Pima are genetically similar to Colombian populations, the Pima also had connections to present-day Mongolians. The genetic link to Mongolians suggests that two waves of migrants crossed the Bering Strait into North America. The first group probably made it all the way to South America, while a second group mixed with the first but never made it past North America, says Garrett Hellenthal, a statistician at OxfordUniversity in England who was involved in the study.

The study also revealed that Orcadians from the Orkney Islands share a common lineage with the Yakuts of northern Siberia and with the Han of northern China. One hypothesis for this mix is that forebears of modern-day Orcadians may have ventured across the Arctic.

To determine the genetic relationships, Daniel Falush of the Environmental Research Institute in Cork, Ireland, and his colleagues analyzed a data set from the Human Genome Diversity Panel, which contained more than 2,500 single gene variations from 927 people of 53 different ethnicities. Similar to past methods, the technique tracks variations in the nucleotides — the building blocks of DNA — that each person carries. Unlike other methods, however, the length of similar strips of the genetic code is taken into account. “What it specifically does, compared to other methods, is look for sharing of not just individual nucleotides but of stretches of chromosomes,” Falush says.

The new technique may provide more insight than past methods because it better captures the way DNA is passed on from parents to children. Around the time of fertilization, chromosomes from the mother and father crisscross and recombine in random places on each chromosome. So a brother and sister may inherit the same chunk of DNA, while their children and grandchildren have a greater chance of that chunk being broken up during recombination. Thus, people who share a recent common ancestor are likely to have long stretches of identical DNA, while more distantly related people would be expected to have shorter strips of identical gene sequences.

One limitation of the current study is that it can’t date migration events or place them in chronological order, says Hellenthal. In addition, the researchers needed to assume certain facts about human history to interpret the model, which could skew the results, Conrad says.

“Unfortunately there is some circularity,” Conrad adds. “They are seeing patterns in their result that match with other more simple analyses, but the real test of this method is for somebody with no background on human population to fit the results and try to interpret them.”

A LONG AND WINDING ROAD

The current model, which takes into account the way chromosomes recombine, was used to reconstruct a migration history for people around the world. The new approach found that the Americas were peopled in two separate waves. Video courtesy of D. Falush, G. Hellenthal, and A. Auton.