Denisovans, Our Grand Ancestors
In remote Southeastern Siberia in the large Denisova Cave, archaeologists have found evidence of multiple human ancestors, including bones of Neanderthals and artifacts of early modern human culture. But there was also evidence that yet another species, known only as Denisovans, occupied the cave too.
The Denisova Cave is named after Denis, a Russian hermit who lived there in the 18th century. In 2008, the finger bone of a juvenile hominin was found, originally referred to as the "X woman" (referring to the maternal descent of mtDNA), or the Denisova hominin. Artifacts, including a bracelet, excavated in the cave at the same level were dated to around 40,000 BP and an intermittent presence going back 125,000 years has been shown by excavations since.
Beside the finger bone only a large molar, identified as a wisdom tooth, was found in the cave. But In 2010, researchers found a second wisdom tooth, buried deep in the back of the cave. The dental analysis fell to Bence Viola, an anthropologist of the University of Toronto who had examined the first Denisovan wisdom tooth and initially mistook it for the tooth of a cave bear, given its size and huge, splayed roots.
Viola found that the two teeth were consistent with one another and different from those of modern humans and Neanderthals—strongly suggesting for the first time that large teeth were part of the Denisovan package. Though it’s difficult to say what large-toothed Denisovans would have looked like—wisdom teeth have notoriously variable shapes—there’s little doubt that “large teeth with massive roots would probably require massive jaws,” says Viola. Some even speculate on a body height of more than two meters, true giants compared to most Asians today.
In 2017, a Chinese-U.S. team of archeologists presented 105,000- to 125,000-year-old skulls they called “archaic Homo.” Although the investigators weren't able to extract DNA from the skulls, they do fit what you’d expect from a Denisovan. The skulls lack faces and jaws but they include enough undistorted pieces to note a close resemblance to Neanderthals. One cranium has a huge brain volume of 1800 cubic centimeters—on the upper end for both Neanderthals and moderns with a volume of about 1450 cubic centimetres—plus a Neanderthal-like hollow in a bone on the back of its skull.
However, the crania also differ from the western Neanderthals of Europe and the Middle East. They have thinner brow ridges and less robust skull bones, similar to early modern humans and some other Asian fossils. The skulls are also too lightly built and their brains are too big to be late-occurring representatives of other archaic humans such as Homo erectus or Homo heidelbergensis, two species that were ancestral to Neanderthals and modern humans.
DNA analysis has shown that Denisovans left their mark on modern humans, contributing about five percent of the genome of modern Melanesians, who live in Papua New Guinea and other parts of the Pacific. In Tibetans, certain Denisovan genes assist with adaptation to low oxygen levels at high altitude which lends some credibility to the suggestion that the legends of the Yeti derive from populations of archaic humans who persisted in the uplands of the heart of Eurasia.
MtDNA analysis also suggests that the new hominin species was the result of an earlier migration out of Africa, distinct from the later out-of-Africa migrations associated with modern humans, but also distinct from the even earlier African exodus of Homo erectus. The Denisovans were actually a sister group to the Neanderthals, branching off from the human lineage 550,000 years ago, and diverging from Neanderthals, probably in the Middle East, 300,000 years ago.
The extent to which the species overlapped within Denisova Cave has long been a subject of speculation, but there's no doubt that there was extensive interbreeding between Neanderthals and Denisovans and additional discoveries suggest that Denisovans and Neanderthals may have overlapped in the cave for up to 50,000 years.
To make matters even more complicated, Denisovan remains also contain evidence of yet another, even more ancient species that is still unknown with which they must have interbred in the distant past.