Posted on Apr 13, 2019 in Evolution, Science
At some point in the past, far distant from this week’s release of the Event Horizon Telescope picture of a gargantuan 55-million-year-old black hole in an ancient elliptical galaxy, a distinct branch of human evolution, the Denisovans, disappeared, but not before interbreeding with modern humans. Today, people in places like East Asia and New Guinea still carry fragments of Denisovan DNA. Over the past decade, the Denisova Cave in a valley in the Altai Mountains of southern Siberia has yielded fossils harboring DNA dating back 200,000 years.
“Everyone said, these Denisovans, we have no idea how old they are,’” said Katerina Douka, an archaeologist at the Max Planck Institute for the Science of Human History in Germany, about the nagging fact that standard methods for dating ancient bits of bone and teeth have left scientists perplexed.
During the past six years, Douka and other experts have been creating a history of the Denisova Cave, reports Carl Zimmer in his New York Times “Matter” column. They have identified and dated 103 layers of sediment on the cave floor, as well as 50 items found in them, including bones, pieces of charcoal and tools. A timeline, Zimmer adds, “that shows that humans occupied the cave for perhaps as long as 300,000 years, and it raises some intriguing hints that Denisovans may have been capable of sophisticated thought, on par with modern humans.”
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In August 2018, researchers discovered a bone fragment from a girl whose mother was a Neanderthal and father was a Denisovan in a remote cave in Siberia. In the study, researchers estimate that this hybrid child lived between 79,100 and 118,100 years ago. Modern humans, scientists discovered, share a common ancestor with Denisovans and Neanderthals that lived roughly 600,000 years ago. Later — approximately 390,00 years ago — the Neanderthal and Denisovan lineages split.
The August finding in the cave in an isolated river valley in Siberia was “sensational” said Johannes Krause, who studies ancient DNA at the Max Planck Institute for the Science of Human History in Jena, Germany. “Now we have the love child of two different hominin groups, found where members of both groups have been found. It’s quite a lot of things happening in one cave through time.”
“Everyone said, ‘These Denisovans, we have no idea how old they are,’” said Katerina Douka, an archaeologist at the Max Planck Institute for the Science of Human History in Germany. Over the past six years, Dr. Douka and other experts have been creating a history of the cave, dating 103 layers of sediment on the cave floor, as well as 50 items found in them, including bones, pieces of charcoal and tools.
Douka and colleagues dated the Denisovan specimens using an approach called Bayesian modelling, reported Nature. Their favored version of this approach incorporated radiocarbon and optically stimulated luminescence ages, information about the excavated layers, and timing based on genetic data. It’s not until about 200,000 years ago that the oldest Denisovan DNA comes to light. The researchers estimated it to be between 217,000 and 185,000 years old. A Neanderthal DNA sample comes from a layer that formed between 205,000 and 172,000 years ago.
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The Max Planck scientists unveiled this chronology in a pair of papers published that shows that humans occupied the cave for perhaps as long as 300,000 years, raising some intriguing hints that Denisovans may have been capable of sophisticated thought, on par with modern humans.
The August research findings suggest that the woman may have been just a teenager when she died more than 50,000 years ago, too young to have left much of a mark on her world. But a piece of one of her bones, unearthed in in the cave in 2012, may make her famous. Enough ancient DNA lingered within the 2-centimeter fragment to reveal her startling ancestry: She was the direct offspring of two different species of ancient humans, Neanderthal and the mysterious Denisovan— extinct groups of hominins that separated from each other more than 390,000 years ago.
An analysis of the woman’s genome, continues Gretchen Vogel in Science, indicates her mother was Neanderthal and her father was Denisovan, the mysterious group of ancient humans discovered in the same Siberian cave in 2011. It is the most direct evidence yet that various ancient humans mated with each other and had offspring.
This bone fragments harbors the most direct evidence yet of ancient interspecies mating. Thomas Higham, University of Oxford.
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Based on other ancient genomes, researchers already had concluded that Denisovans, Neanderthals, and modern humans interbred in ice age Europe and Asia. The genes of both archaic human species are present in many people today. Other fossils found in the Siberian cave have shown that all three species lived there at different times.
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Viviane Slon, a paleogeneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who did the ancient DNA analysis, says when she saw the results, her first reaction was disbelief. Only after repeating the experiment several times were she and her Leipzig colleagues—Svante Pääbo, Fabrizio Mafessoni, and Benjamin Vernot—convinced.
That a direct offspring of the two ancient humans was found among the first few fossil genomes recovered from the cave suggests, Pääbo says, “that when these groups met, they actually mixed quite freely with each other.”
The Daily Galaxy, Andy Johnson via Nature, Science, and The New York Times
Image at top of page: Maxim Kozlikin and Tom Higham taking samples from the East Chamber at Denisova Cave (photo courtesy and copyright Sergey Zelinski, Russian Academy of Sciences).
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