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Modern humans cross-bred twice with Denisovans throughout history

Credits: Wikipedia

According to a recent study, modern humans coexisted and cross bred not only with Neanderthals, but also with another species of archaic humans, the mysterious Denisovans. The results of this study were published in the journal Cell.

Wide faces, tiny chins and prominent eyebrows: Neanderthals’ appearance is often the source of mockery. But mocking Neanderthals is a lot like mocking ourselves: Homo sapiens frequently had sexual relations with Homo neanderthalensis. Neanderthal genes now make up between 1 and 4% of the human genome. But that’s not all. The DNA of other archaic humans, the Denisovans, is also hidden within modern genomes.

A few years ago, two molars and a child’s pinkie bone, aged between 50,000 and 80,000 years old,  were found in the South of Siberia -more precisely in a cave in Altai. Thanks to well preserved DNA and to the contributions of genetic sciences, the researchers were able to identify a new and mysterious group of humans: Denisovan man. By developing a new method of analysing the genome, researchers in the University of Washington in Seattle (US) made the unexpected discovery of two distinct episodes of genetic mixing between Denisovans and modern man. This suggests a more diverse genetic history than we had previously suspected.

The researchers in this study examined over 5,500 genomes of modern humans from Europe, Asia and Oceania, searching for possible archaic DNA. After having found variations in the DNA, the scientists then compared these segments to Denisovan and Neanderthal sequences, from the samples taken in the Altai mountains.

Previous research had shown that if the Denisovans shared a common origin with the Neanderthals, they would have been almost as different from Neanderthals as Neanderthals are from modern humans. Previous works also showed that the Denisovans contributed to the DNA of several groups of modern humans. They bequeathed part of their DNA to around 5% of the genomes in the populations of Oceania, and some 0.2% to Asian and American Indian genomes. Scientists thus assume that this Denisovan DNA found in modern humans in Asia comes from cross breeding between the Denisovans and the Oceanians who had emigrated to Asia. This current study determines that there were two distinct episodes of interbreeding.

“I was surprised to note that there were two very different groups of Denisovans who contributed to the DNA of modern humans -this was not something I has expected to see”, says Sharon Browning, a geneticist in the University of Washington. The researchers suggest that ancestors of the Oceanians cross bred with a group of Denisovans in the South, while the Asian ancestors in the East mixed with a group who came from the North.

Thus there would have been at least three examples of modern humans cross breeding with populations of archaic humans -once with Neanderthals and twice with Denisovans. Scientists now plan to conduct further research on signs of cross breeding between modern humans and other archaic genetic lines in other populations across the world. This could be the case in Africa, but given that the climate is hotter, nobody has yet found archaic human fossils with enough DNA to allow for DNA sequencing.

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