Genetic Evidence Suggests a Denisovan Presence in the Pacific Islands
A new genetic study has provided important data to evolutionary scientists seeking to trace the migratory movements and cultural interactions of the people who settled the South Pacific islands of Oceania. Most intriguing is a discovery that seems to link people living in the highlands of Papua New Guinea (PNG) with the famed Denisovans, the long-extinct cousins of the Neanderthals who were believed to have resided exclusively in East Asia. While many Pacific Islands show traces of Denisovan DNA from encounters that occurred before their ancestors migrated to their current homes, the latest evidence suggests more recent interbreeding, dating to the post-island settlement era.
Growing Pacific Islands Evidence Indicates Denisovans
In a study published in April in the journal Nature, a team of evolutionary geneticists primarily affiliated with the Pasteur Institute in Paris introduced the findings of their extensive genomic analysis , during which they sequenced the DNA of 317 individuals from 20 different Pacific island populations.
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A wide range of interesting revelations emerged from this research project, which sought to untangle the complex genetic, evolutionary, and cultural relationships between these groups. The most notable finding was that some highland residents of Papua New Guinea were carrying strands of Denisovan DNA in their genomes that their ancestors had picked up approximately 25,000 years ago. This was long after their ancestors had migrated to Near Oceania from the East Asian mainland.
This discovery was eye-opening and could have profound implications. Adding weight to its authenticity is another study, published in 2019 , that found essentially the same thing.
A Tamuniaï village house on stilts off the coast of New Britain Island, Papua New Guinea. ( Marc Dozier )
In that project, an international team of researchers analyzed the genomes of 161 people from 14 groups living in Indonesia and Papua New Guinea. Among a group of Papua New Guineans, they found evidence of three distinct types of Denisovan DNA. Two were linked to earlier interbreeding periods, but one had apparently entered the PNG genome sometime within the last 15,000 to 30,000 years.
Remarkably, this finding represented only the second link in the chain suggesting that Denisovan descendants had mixed with the local population in Papua New Guinea relatively recently. The first study that implied such a revelation was carried out by genetic scientists from the University of Washington, and was first introduced in an article in the March 2018 edition of the journal Cell.
This genetic survey concluded that Denisovan DNA found in Papua New Guineans was somehow distinct and separate from that found in East Asians. The University of Washington researchers didn’t connect this to any particular timelines, but their work clearly dovetails with the more recent studies that have revealed the anomalous character of the Papua New Guinea variation.
Since no Denisovan remains have ever been found anywhere on any of the Pacific Islands , scientists previously had no reason to suspect they were ocean travelers. It was assumed that all the interbreeding between Denisovans and East Asian peoples and their Pacific Island descendants had occurred tens of thousands of years earlier, as older genetic studies had suggested.
But the latest results have called these past assumptions into question, creating a need for new explanations that cover all the available evidence.
Papua New Guinea, Southern Highlands Province, Lake Kutubu, Kutubu island. ( Marc Dozier )
Competing Theories Emerge, But Clear Answers Elusive
Three theories have so far been offered to explain these mysterious findings.
The first is that a Denisovan group was indeed living on the island of Papua New Guinea approximately 25,000 years ago, despite the lack of any fossils or any other type of physical evidence that could confirm their existence.
It should be noted that Denisovan fossil finds on the Asian mainland have been extremely rare and if their population numbers were relatively low in Papua New Guinea they may have left behind few if any physical traces for archaeologists to unearth.
The second theory asserts that the Papua New Guineans may not have interbred with Denisovans at all but may have instead shared their genetic material with another species of now-extinct hominin that was descended from the Denisovans.
A reassembled skeleton (or most of it) of a Homo floresiensis individual. (Emőke Dénes / CC BY-SA 4.0 )
The two candidates in this scenario are Homo luzonensis and Homo floresiensis , two species that were known to have lived on the islands of Oceania before the arrival of modern humans. If one of these ancient species were still holding on 25,000 years ago, their interactions with the Papua New Guineans could have provided a pathway for Denisovan DNA to enter the highland group’s genome.
As of now, all attempts to extract DNA samples from Homo luzonensis and Homo floresiensis fossils have failed, which makes this theory currently untestable.
The third theory comes from sources who are skeptical of the idea that Denisovans could have been living in the Pacific Islands at any point, let alone less than 30,000 years ago.
This theory, as elucidated by paleogeneticist Cosimo Posth from the Max Planck Institute for the Science of Human History in Germany, posits that two groups of modern humans living in the Pacific Islands 25,000 years ago had originally been one group, before diverging (physically and evolutionarily) at some time in the past. They eventually reconnected in the highlands of Papua New Guinea, but because of their previous split each was now carrying a slightly different set of Denisovan genes, which had evolved from Denisovan genetic material already present in their genomes before their divergence.
Interbreeding at that point would have resulted in new Denisovan genes being introduced into the genetic codes of the existing group of highlanders, making it appear like they had been mating with actual Denisovans.
As of now, all three theories are at least partially grounded in speculation, making it impossible for a definitive judgment to be rendered.
The spread and evolution of Denisovans from Africa to Indonesia and then somehow, according to the latest research, on to the Pacific Islands. (John D. Croft at English Wikipedia / CC BY-SA 3.0 )
On the Trail of the Ghostly Denisovans
Even with immense storehouses of genetic data at their disposal, scientists trying to reconstruct the migratory and breeding histories of hominin species face an extraordinarily difficult task. New and surprising connections are constantly being discovered, creating additional puzzle pieces that must be fitted into a complicated whole.
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The challenge to be as thorough as possible, and to ultimately “get it right” even if previously held concepts that seemed set in stone must be significantly altered or abandoned. This mission is particularly complicated when it involves the Denisovans, who left scientists with little to go on outside of a few fossilized bone scraps and a pervasive ghostly presence in the DNA of the people of East Asia and the Pacific Islands.
The newest evidence implies the Denisovans had some fairly extensive interactions with at least some modern humans , specifically with a remote Pacific Island population that wouldn’t have been traveling to the East Asian mainland to meet them.
But until further evidence accumulates, there will be no way to know for sure if Denisovans really were present in Papua New Guinea only 25,000 years ago.
Top image: A poetic illustration of a canoe leaving a trail of DNA in its wake. The seafarers navigate towards the Pleiades, thought to be used as coordinates by Polynesians of the Pacific islands during sea voyaging. Source: Annette Günzel / Nature
By Nathan Falde