A first-generation hybrid child of Neanderthal and Denisovan origin has been identified in the famous Denisova cave in Siberia, scientists reported in Nature on Wednesday.
This eureka moment in human evolution stemmed from a single fragment of bone about 90,000 years old, found in 2012 in the Denisova Cave, which is a large limestone cavern in Siberia’s Altai mountains.
The fragment, all of 2.5 centimeters long and dubbed “Denisova 11,” came from a teenage girl at least 13 years old, the researchers believe. To be exact, Denisova 11 was found in a layer dating to more than 50,000 years ago, but her Neanderthal mother lived about 90,000 years ago, say the authors. So, one is to understand, did she.
Yes: the tiny bit of bone turned out to belong to the teenage daughter of two different human species. The mind boggles.
Genetic analysis categorically proves the daughter's mixed antecedents. That is as opposed to looking at a modern skeleton with seemingly primitive characteristics, such as brow ridges, and arguing about whether it’s a human-Neanderthal hybrid or just a beetle-browed throwback. The early Homo sapiens found at Jebel Irhoud, Morocco, dated to 300,000 years ago, had beetled brows that could confuse any given Neanderthal.
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The discovery is the holy-grail moment evolution researchers have been hoping for since the discovery that at one time we were not alone in our humanity, and that we rubbed "shoulders" with other human species.
In Eurasia, our ancestors coexisted with at least three other hominins: Neanderthal, Denisovan, and the wee Homo floresiensis, nicknamed “hobbit,” which seems to have evolved in isolation on an Indonesian island, arising directly from Homo erectus – and with whom we did not mate. All other species or groups of humans have gone extinct, as far as we know.
“We knew from previous studies that Neanderthals and Denisovans must have occasionally had children together,” says co-author Viviane Slon, researcher at the Max Planck Institute for Evolutionary Anthropology and co-author of the article. “But I never thought we would be so lucky as to find an actual offspring of the two groups.”
Which raises the question of whether finding the hybrid girl’s remains was incredible luck or a sign that our ancestors weren’t that fussy in their mating habits.
Some call that ‘species’
Neanderthals and ancient modern humans split about 530,000 years ago, according to current genetic estimates. Neanderthal ancestors probably migrated out of Africa after that time and settled in Eurasia, Slon says.
By 430,000 years ago, early Neanderthals, or their direct ancestors, were already living in Spain.
The Denisovans arose shortly afterward in Eurasia, but are not thought to stem from Neanderthals. According to current thinking, Denisovans and Neanderthals are “sister-groups” that diverged from each other between 390,000 and 440,000 years ago, Slon told Haaretz.
Yet Caucasians have some Neanderthal genes, Asians have some Neanderthal and some Denisovan genes, and Melanesians have some Neanderthal and rather more Denisovan DNA. Africans have neither Neanderthal nor Denisovan DNA (though work is being done on possible “ghost DNA” of other, still unknown, hominins in certain African populations - and Andaman islanders.) So Homo sapiens clearly mixed with Neanderthal and Denisovan after leaving Africa.
Doesn’t the fact that the three hominin types – Denisovans, Neanderthals and us – could mate and have viable rug-rats actually make them one species with morphological and behavioral variations?
“This depends on the definition of ‘species’ that you choose to use; there are dozens, and some contradict each other entirely,” Slon says. “If you are thinking of what has been called the ‘biological species concept,’ that individuals are considered to be from the same species if they can mate and have fertile offspring, then Neanderthals, Denisovans and modern humans would all be considered the same species.”
But that would imply that polar bears and grizzlies are also the same, since they can mate and have viable cubs, she points out. So paleoanthropologists avoid the whole minefield and refer to the Neanderthal, Denisovan and sapiens as “groups.”
The bit of bone, by the way, was from one of the hybrid teenage girl’s long bones: but with no clear distinctive features, we can’t say if it was an arm or a leg.
Be that as it may, finding the actual “smoking bone” of a first-generation hybrid of two groups is still a huge archaeological coup. Talk about finding the missing link.
When Hairy met Sally
Scientists reached the conclusion that the teenage hybrid's mother was Neanderthal and her father Denisovan by extracting DNA from the ancient bones (easier said than done), sequencing it, and comparing the fragment’s genome with that of Neanderthals and Denisovans.
But the plot thickens. The Denisovan father did prove to be related to a Denisovan from another era whose remains were also found in the cave. Well and good. But the father also had traces of Neanderthal DNA, which had to have stemmed from interbreeding thousands of years before he lived. In other words, the Denisovan daddy had at least one Neanderthal ancestor.
“From this single genome, we are able to detect multiple instances of interactions between Neanderthals and Denisovans,” explains Benjamin Vernot from Max Planck, the third co-author of the study.
Complicating matters even more, the researchers deduced that the mother Neanderthal was genetically closer to Neanderthals who lived elsewhere in western Europe than to a Neanderthal who had dwelled earlier in the Denisova Cave. This implies that Neanderthals weren’t sedentary but roamed between western and eastern Eurasia, the scientists postulate.
Neanderthals and Denisovans evidently interbred more than once, though they may not have had the same stamping grounds. Moreover, hominin fossil remains are beyond rare, and the discovery of a first-generation Neanderthal-Denisovan child among the pitifully few remains found so far might indicate that mixing in the Late Pleistocene was common.
Which brings up another thought. We have some idea what Neanderthals looked like, thanks to reconstructions from skeletons. We have no idea what Denisovans looked like because of the paucity of remains, though we suspect they were large. All we have of Denisovans so far is four specimens: one finger and three teeth – plus this teenager’s long bone shard, all found in the Denisova Cave. For all we know, the two groups mixed because they couldn’t tell each other apart. Or care. Plus ça change, plus c'est la même chose?
Since the cave remains the only sure source of Denisovan remains, and since the highest concentration of their genes is in Melanesians, some think Denisovans once thronged Asia. Neanderthals lived throughout those parts of Europe not covered in ice, in Siberia and in the Levant. They seem to have spread east and west in temperate Europe. In any event, they clearly met in Siberia.
The earliest-known Denisovan in that Siberian cave lived at least 130,000 years ago. They are believed to have died out about 40,000 years ago.
Only one Neanderthal at the site has had its DNA sequenced. She is thought to have lived approximately 100,000 to 120,000 years ago, Slon says.
Denisova 11’s Neanderthal mother is estimated to have lived about 90,000 years ago. It is possible that the two species coexisted in Siberia, and maybe even in that very cave, for tens of thousands of years. Maybe one day scientists will figure out how these first cousins of humankind accomplished such peaceful coexistence.