March of the Denisovans: Evidence of Archaic Human Gene Now Detected In the Orient
The Denisovan story keeps evolving. This group of ancient hominins has been something of a mystery ever since they were first discovered in 2010 as an extinct sister group to the Neanderthals. Bits and pieces of their tale keep coming to light and almost everything is a surprise that makes international headlines. And of course, many people want to know more about the results of the mating of Denisovan and other hominin species. Of particular interest is how Denisovan DNA has lived on in our species.
The latest in Denisovan-related discoveries comes from an analysis of a 160,000-year-old archaic human molar fossil that was discovered decades ago in China. Science Daily reports that the study “offers the first morphological evidence of interbreeding between archaic humans and Homo sapiens in Asia.”
Science Daily explains that the study, which is published in the journal Proceedings of the National Academy of Sciences, is focused on “a three-rooted lower molar - a rare trait primarily found in modern Asians - that was previously thought to have evolved after H. sapiens dispersed from Africa.”
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The three-rooted lower molar anomaly in a recent Asian individual. Left: tooth sockets showing position of accessory root; right: three-rooted lower first molar tooth. (Christine Lee)
It seems a different evolutionary path is emerging.
"The trait's presence in the fossil suggests both that it is older than previously understood and that some modern Asian groups obtained the trait through interbreeding with a sister group of Neanderthals, the Denisovans," explains Shara Bailey, a professor of anthropology at New York University and the paper's lead author, in a NYU press release.
Denisovans on the Tibetan Plateau
In a previous study, which was published in Nature in May of this year, Bailey and her colleagues concluded that Denisovans were inhabiting the Tibetan Plateau well before Homo sapiens migrated to the region.
That work, along with the new PNAS analysis, focused on a hominin lower mandible found on the Tibetan Plateau in Baishiya Karst Cave in Xiahe, China in 1980. It was also an important discovery because it is the first time researchers have identified the physical remains of a Denisovan outside the cave that gave them their name, Denisova Cave, in Siberia.
Excavating in the Baishiya Karst Cave. ( Dongju Zhang, Lanzhou University )
The Denisovan jaw also has an interesting discovery story: an anonymous monk found the prehistoric hominin jaw fragment and then gifted it to the 6th Gung-Thang Living Buddha, who later gave it to Lanzhou University. The jaw sat in the university collection for 30-plus years before study co-author Dongju Zhang of Lanzhou University decided it was worth a look in 2016.
Heavy carbonate crust was attached to the mandible and U-series dating told researchers that the jaw is at least 160,000-years-old. According to a press release by the Max-Planck Institute, this is the earliest hominin fossil found to date on the Tibetan Plateau and that means that the people living in the area “had already adapted to living in this high-altitude low-oxygen environment long before Homo sapiens even arrived in the region.”
However, as National Geographic notes “While the jaw was found where oxygen levels are low, without the DNA itself, scientists can't be sure the jaw’s owner carried the adaptation to survive in that thin air.”
The Xiahe mandible, only represented by its right half, was found in 1980 in Baishiya Karst Cave. ( Dongju Zhang, Lanzhou University )
Denisovans and Archaic Humans in Asia
The same jaw has now been analyzed by NYU anthropologist Susan Antón and Jean-Jacques Hublin, director of the Department of Human Evolution at the Max Planck Institute for Evolutionary Anthropology. Their study centered on the molar and was focused on exploring what it can tell us about “the relationship between archaic humans who occupied Asia more than 160,000 years ago and modern Asians,” as the press release states. Bailey says that the results of the study show that:
"In Asia, there have long been claims for continuity between archaic and modern humans because of some shared traits. But many of those traits are primitive or are not unique to Asians. However, the three-rooted lower molar trait is unique to Asian groups. Its presence in a 160,000-year-old archaic human in Asia strongly suggests the trait was transferred to H. sapiens in the region through interbreeding with archaic humans in Asia."
Adding to Our Knowledge of Denisovans Interbreeding with Other Species
There’s been quite a lot of interest in Denisovans mating with other hominin groups lately. Another recent study shows that their genes live on in populations in “the Philippines and New Guinea to China and Tibet” where people “have inherited 3% to 5% of their DNA from Denisovans.”
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Proportion of the genome inferred to be Denisovan in ancestry in diverse non-Africans. The color scale is not linear to allow saturation of the high Denisova proportions in Oceania (bright red) and better visualization of the peak of Denisova proportion in South Asia. (Sankararaman et al./Current Biology 2016)
Even more exciting is that the genes had only entered the human genome as late as 15,000 years ago – suggesting modern humans and Denisovans were mating. And Andrew Collins has stated, “Moreover, that it was from them that these modern human groups inherited roughly 400 genes including an immune gene variant (TNFAIP3) and a gene involved in diet (WDFY2).”
And that’s not all, for some time now researchers have supposed that Denisovans were mating with Neanderthals. Proof for their offspring came in 2018, when a genetic analysis of a bone fragment found in Denisova cave was shown to have the DNA of a Denisovan mother and Neanderthal father.
Drawing of a Neandertal mother and a Denisovan father with their child, a girl, at Denisova Cave in Russia. ( Credit: Petra Korlević )
Svante Pääbo, Director of the Department of Evolutionary Genetics at the MPI-EVA and lead author of that study said, “Neanderthals and Denisovans may not have had many opportunities to meet. But when they did, they must have mated frequently -- much more so than we previously thought.”
Top Image: Representative image of a Denisovan. Several studies have recently explored the spread of Denisovan genes. Source: ginettigino /Adobe Stock