If you travel the meandering Sepik River of New Guinea, it quickly becomes apparent that from one bend to the next the people along the banks speak distinct languages. The island’s remarkable linguistic diversity reflects real genetic differences, a research team reports this week in Science. More unexpected, the team concludes that this genetic variation dates back just 10,000 to 20,000 years, rather than to 50,000 years ago or so, when humans first arrived.
In the highlands, people split into three very distinct clusters of social groups within the past 10,000 years, soon after they began cultivating plants. In the lowlands, two main clusters arose in the north and south.
The best explanation for this pattern, says [says team member Anders Bergström, a graduate student at the Wellcome Trust Sanger Institute in Hinxton, U.K. In Europe], is that once people began cultivating crops, they spread their genes across the island along with the technology. But soon afterward, their descendants apparently stopped mixing as much and evolved distinct local genotypes. Although researchers have long thought that the island’s mountainous terrain kept highland groups isolated, this study finds that the clusters formed in both the highlands and lowlands—where the terrain is flatter. Cultural factors, such as warfare or within-group marriage, were more important than geographic barriers in preventing mixing, Bergström suggests.
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