Mysterious Denisovans emerged from the shadows in 2019

Denisovans’ days of Stone Age obscurity appear numbered. The mysterious “ghost clan” floated into view over a decade ago, when a bit of a girl’s pinkie bone, found in Siberia’s Denisova Cave, yielded DNA that didn’t match that of any known hominid. A few more fossils — three teeth and a limb fragment — plus genetic analyses indicated Denisovans were close relatives and occasional mating partners of Neandertals and Homo sapiens tens of thousands of years ago. But there was too little evidence to say what Denisovans looked like or how they behaved.

Discoveries reported in 2019 brought Denisovans into focus — but left plenty of room for interpretation. As fossils accumulate, investigators will grasp how Denisovan anatomy influenced the skeletal makeup of its mating partners in the Homo genus. Thanks to Denisovan discoveries, “we can now see that hybridization contributed to our own origins,” says paleoanthropologist John Hawks of the University of Wisconsin–Madison.

Ancient DNA evidence reported this year suggests that Denisovans fanned out into three genetically separate lines that mated with various human groups in Asia. That finding played into an emerging view of human evolution as a braided stream, with closely related species flowing into and out of genetic exchanges.

But testing that possibility requires finding more Denisovan fossils. The discovery of two pieces of a skull in the Siberian cave, reported this year, gave a glimpse of the anatomy that the ghost clan brought to the ancient hybridization scene (SN: 4/27/19, p. 15). The bone’s surprising thickness recalls H. erectus — a species dating back at least 1.8 million years. Yet a newly identified chunk of the girl’s finger bone looks like people’s digits today (SN: 9/28/19, p. 14).

These findings fit with the idea that Denisovans had a mix of their own skeletal traits plus characteristics like those of their breeding partners. That theme also emerged from a project that used the Denisovan girl’s DNA to reconstruct her skeleton and face (SN: 10/12/19 & 10/26/19, p. 24). The youngster’s portrait, which some researchers regard as too speculative, included a relatively flat, humanlike face but, like Neandertals, no distinct chin. Her broad nose had a look all its own.

This year also brought evidence that Denisovans traveled far beyond the Siberian cave. The population periodically inhabited that cave from nearly 300,000 to about 50,000 years ago, according to sediment studies (SN: 3/2/19, p. 11). But on the distant Tibetan Plateau, researchers identified a Denisovan lower jaw, dating to at least 160,000 years ago (SN: 6/8/19, p. 6). Denisovans’ arrival there fits previous evidence that Tibetans today inherited a Denisovan gene that aids high-altitude survival.

Denisovans may also have shared a sophisticated thinking ability with other Stone Age Homo populations. Engraved animal bones found in China, possibly etched by Denisovans, raised the likelihood that these hominids created objects with symbolic meanings (SN: 9/14/19, p. 8). Perhaps most intriguing is the report that at least three genetically distinct Denisovan populations split from the same Siberian population and mated with ancient humans elsewhere in Asia (SN 4/27/19, p. 15). People now living in parts of East Asia, Indonesia and Papua New Guinea have ancestors from those three Denisovan lines.

“The world was genetically complex 50,000 to 100,000 years ago,” says paleoanthropologist Bernard Wood of George Washington University in Washington, D.C. Wood suspects that three or four closely related Homo species, including Denisovans, interbred during that time.

Paleogeneticist E. Andrew Bennett of Paris Diderot University in France isn’t so sure about Denisovans’ status as a species. “The species concept is controversial and tricky,” he says. Since interbreeding occurred among Denisovans, Neandertals and H. sapiens, “it is better to talk about different populations, not different species.”

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Evidence of the three Denisovan lineages complicates the ghost clan’s identity, Hawks says. It’s unclear whether some Denisovan offshoots interbred enough to genetically blend in with H. sapiens groups or stayed largely to themselves, he says. “Are the Denisovans a species? Are they three species? Or are they all different populations of Homo sapiens?”

To decipher Denisovans’ place in human origins, archaeologist Eleanor Scerri of the Max Planck Institute for the Science of Human History in Jena, Germany, and colleagues advocate replacing the neatly branching traditional hominid evolutionary trees with a tangle of intertwined populations.

Fossil and genetic findings now support a scenario in which, between 500,000 and 300,000 years ago, H. sapiens groups across Africa — which varied in their skeletal makeup — genetically mixed and mingled until a package of physical traits typical of people today coalesced, the scientists argued September 23 in Nature Ecology & Evolution.

Populations in the Homo genus that periodically merged and separated probably inhabited Asia and Europe as well, producing Denisovans, Neandertals and perhaps other humanlike forms (SN: 5/11/19 & 5/25/19, p. 7), Scerri’s team suspects. As humans moved out of Africa, populations such as the Denisovans that had already adapted to local conditions contributed survival-enhancing genes to the newcomers, the team holds.

Even with those limited interactions, discoveries this year cement Denisovans’ status as coauthors of humankind’s long and winding evolutionary story.