High atop the Tibetan Plateau, a Buddhist monk ventured to Baishiya Karst Cave, a sacred Buddhist sanctuary, to pray and meditate. During his sojourn there, the monk, whose name has since been lost to the passage of time, found a portion of a fossilized jawbone, yet another “holy bone” that the people from the nearby town of Xiahe would collect and grind into powder to produce medicines. But after his decent from Dalijiashan Mountain, the monk instead gave the mandible to the Sixth Gung-Thang Living Buddha, of whom in turn handed it over to scientists studying at China’s Lanzhou University.
But it wasn’t until thirty-nine years later that this partial jawbone was discovered to have belonged to a member of the enigmatic Denisovans, an ancient human subspecies that we know only though a few fragments of tooth and bone found in Denisova Cave, located 2,000 kilometers (1,240 miles) northwest of Baishiya Karst Cave, in Siberia’s Altai Mountains. Genetic testing performed on the Denisova Cave fossils, unearthed in 2010, revealed that the Denisovans shared a common ancestor with Neanderthals, and at one point had interbred with Homo sapiens.
Unfortunately, the mandible unearthed from Baishiya Karst Cave yielded no useable DNA that would allow researchers to determine this individual’s place in the human family tree; however the fossil had a heavy coating of carbonate crust that was used to date the individual, at 160,000 years old, using uranium dating methods.
Although there was no DNA to be found in the fossil, the teeth, including a pair of robust molars, held specific proteins that could be used as a fingerprint to identify the species that the individual belonged to; after the application of a new protein analysis technique, that fingerprint revealed a uniquely Denisovan signature. MPI anthropologist and study co-author Frido Welker said that the “preserved coding sequences were most similar to Denisovans compared to anything else. We concluded that the mandible belongs to Denisovans.”
Although this fossil is a mere fragment of the individual that it was originally part of, researchers were able to tell that it belonged to an adolescent, due to a molar that hadn’t erupted past the gums. It also exhibits some more primitive human features, such as the robust teeth still present in the jawbone.
But one of the more important revelations that this fossil provides is the resolution to a genetic puzzle the original fossils recovered in Siberia presented: the presence of a gene variant that made the Denisovans resistant to a high altitude, low-oxygen environment, despite living in a Siberian cave a mere 2,300 feet (700 meters) above sea level, where such an adaptation wasn’t needed. But Baishiya Karst Cave, high on the Tibetan Plateau, is located at an elevation of 10,760 feet (3,280 meters), where such an adaptation would be invaluable. Indeed, this genetic variant, the EPAS1 allele, is present in the genome of present-day Himalayans, a genetic trait that may have been handed down directly from the Denisovans.
160,000 years ago, during the late Middle Pleistocene, the Earth was experiencing a weak interglacial period that allowed humans, including Denisovans and Neanderthals, “to disperse widely, even in uninviting parts of Asia, followed by the penultimate Ice Age, which saw a return to colder conditions and the rather surprising presence of Denisovans in Siberia and, as we know now, the Tibetan plateau,” exclaims Katerina Douka, an archaeologist at the University of Oxford.
Lower global temperatures would have made for a much harsher environment on the Tibetan Plateau than what is found there today, but the Denisovans managed to make a home for themselves there, long enough to develop a genetic resistance to hypoxia, with some of their population later migrating to the mountains of southern Siberia.