The team found a DNA match for the mysterious human cousin in the islands of Southeast Asia, thousands of years removed from the Siberian Population. It wasn’t through fossil records, but through the DNA of their human descendants.
Denisovan DNA was compared with modern human populations, matching with the Melanesian people of Papua New Guinea and the Aborigines of Australia. Some—but not all—indigenous populations in the Philippines show evidence of Denisovan ancestry. Melanesians and Aborigines share 5 percent of their DNA with the Denisovan hominin; Filipino indigenous groups like the Mamanwa and the Manobo share as much, if not more. Comparatively, European humans share only between 1 percent and 4 percent of DNA with the Neanderthals. The genetic evidence showed the Siberian Denisovan was distantly related to the Denisovan group that interbred with the island populations of Southeast Asia. The lines between the two Denisovans diverged 280,000 to 400,000 years ago.
The concentration was less in continental Asia. Mainland Asia shows just a fraction—around one-twenty-fifth—of Denisovan ancestry compared to islanders. While the Siberian Denisovan populations lived near the border of modern day China and Mongolia, their genetics didn’t hang around for long. Around the time the Siberian Denisovan was alive 40,000 years ago, the species had already dispersed far and wide, and already interbred with human populations. The Siberian Denisovan first identified in 2010 was a distant relative to her cousins in Southeast Asia.
“What’s quite clear is that 40,000 years ago, Southeast Asia was already a patchwork of peoples with and without Denisovan ancestry,” Reich said. The limited interbreeding that took place on the main Asian continent still shows up in populations there today as a small piece of the Denisovan hominin puzzle.
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In tracing the steps of the Denisovan, we find evidence that the species migrated to Southeast Asia in a concentrated enough amount to impart a high degree of hybridization with groups in the region. From the location of the Denisova Cave, we can trace a path from modern day Russia into Southeast Asia and Australia.
What’s perhaps more surprising is that there is a low rate of interbreeding in China, Mongolia, Nepal and other countries on the main continent. If the Denisovans were in the area for long, they certainly didn’t interact with Homo sapiens in quite the same way as they did in Southeast Asia. But the fractions of a percent of shared DNA seen in modern Asian populations have imparted beneficial adaptations to some groups there—even if it’s just from a great-to-an-unknown-power grandfather. And all this adds up to more clues, however small, explaining the migration of the Denisovan.
DNA mixture in mainland Asia isn’t entirely absent. Some groups still have the markers of Denisovans, however small. For native Tibetans, ancient hominin interbreeding—however small a portion of their overall genome—may have impacted their ability to live in climates and altitudes hostile to other groups. Rasmus Nielsen, a faculty member of the Center for Theoretical Evolutionary Genomics, previously worked on tracing how Tibetans can withstand the effects of hypoxia in low-oxygen environments. In 2010, his team published a paper indicating the EPAS1 gene as the culprit behind this beneficial mutation. The gene regulates the body’s reaction to low oxygen environments, allowing Tibetans to produce fewer red blood cells and less hemoglobin.