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Ancient skull a new window on human migration, the encounters of Denisovan



Image of a large, illuminated groove with people working in it.
Enlarge / These excavations have identified Denisovan’s DNA in the sediment.

Dongju Zhang, Dongju Zhang, Lanzhou University

The Denisovans occupy a very strange place in human history. Like the Neanderthals, they are an early branch of the lineage that produced modern humans and then intermingled with modern humans. But we got to know Neanderthals about 1

50 years ago when we have any of their DNA sequences and have identified a set of anatomical features that define them. On the contrary, we did not know that Denisovans existed until their DNA suddenly appeared in a tiny little finger. And, to this day, we have not yet determined enough what’s left to actually say anything about what they look like.

However, over time, we have received more and more ancient DNA samples that provide a clearer picture of our interactions with this mysterious lineage. Now, two new reports describing ancient DNA provide some more details. One paper describes a modern human genome from Asia dating close to the time when species mating takes place. It provides additional evidence that there have been at least two cases of similar mating, and it helps clarify how primitive human populations moved across Asia. The second confirms that Denisovans lived along the Tibetan Plateau and may have adapted to the great altitude.

Mongolian skull

Back in 2006, mining in Mongolia’s Salkhit Valley brought out the tip of an apparently old skull. However, since it doesn’t have any exact characteristics, people argue over whether it could be Neanderthals. The man stands. However, preliminary DNA sequencing revealed that it belonged to modern humans, with a carbon chronology indicating its age was around 34,000 years old.

That is truly an important period in human history. At this time, there are distinct East Asian and Eastern Asian (or Siberian) populations, with the latter population somewhat related to Western Europe. Their history is very complicated. A 40,000 year old skeleton from near Beijing is clearly the closest to modern East Asians but is most closely related to a skeleton found in Belgium (! ??!?). A 45,000-year-old Siberian skeleton appears to be devoid of any modern relatives, while a 24,000-year-old individual from the same region has identified a population mixed with East Asians to create Native American ancestry. geography. But two other Siberian skeletons in the same period of time do not show that relationship and are generally Eurasian.

If you’re not confused after that, go back and read again.

With that mess, any other DNA from that era and region could come in handy. So the researchers did what became a standard procedure to process this old DNA. First, they looked for human DNA-matching sequences to pull out all of the human-like sequences. To eliminate pollution from modern humans, they then looked for signs of the most common damage that occurs as DNA ages. Anything that is obviously human and damaged is used to make the genome.

The end result is what you would expect, given the age of the skull’s crest. Most of the variations in DNA match modern human DNA, but there are some regions that match Neanderthals and Denisovans. Modern human parts are best suited for the Indigenous Eastern European and American populations, which confirms earlier results.

Breeding a lot

But it’s still almost as confusing as it once was. “The [newly described] Salkhit personally shares more and more alleles with Tianyuan [Beijing] Yana specimens ~ 31,000 years old from northeastern Siberia “, the researchers write,” but Tianyuan and Yana individuals share less alleles with each other than the Salkhit individual. “In general, the researchers conclude that, sometimes with the Western European and East Asian populations separate, there is some intercourse between Eastern Europeans and East Asians.

But of course, the newly described Siberian DNA has significant resemblance to skeletons from Belgium, suggesting that at least some of the Western European DNA is still being brought back into lineage.

Other ancient people

To the Neanderthals, the new Siberian skeleton is quite typical of modern Asian populations, with about 1.7% of its DNA coming from Neanderthals. It was harder to evaluate Denisovan’s content, but researchers discovered 18 large DNA fragments inherited from Denisovans. The size of these birds led the researchers to conclude that interbreeding took place about 10,000 years earlier. That is consistent with the complete absence of Denisovan DNA in the 45,000-year-old Siberian skeleton. And existing Denisovan DNA is more consistent with the amounts seen in later East Asian skeletons.

Interestingly, segments found in the new Salkhit genome do not overlap with those found in the modern human genome in Southeast Asia and the Pacific. The clear conclusion from this is that modern humans alternated with the Denisovans in at least two separate cases. That’s what has been shown by other results, but modern East Asians have DNA from both of these events. The Salkhit genome provides a clear cleavage between them.

Meanwhile, a separate article is about where the Denisovans live – specifically at the Karst Baishiya Cave on the edge of the Tibetan Plateau. At 3,000 meters (nearly 11,000 feet) above sea level, this was a very high environment that could have been a difficult place to house during the last ice age. However, part of the jawbone was found there. Although it does not yield any DNA, the protein fragments show that the jaw it came from belonged to a Denisovan.

DNA from dirt

Most ancient DNA samples were heavily contaminated by bacteria, with the DNA badly damaged and fragmented. As a result, researchers developed various processes to help them isolate human-like DNA and then recognize ancient DNA based on the damage pattern it accumulated. Gradually, it was realized that these same techniques could work even when pollution levels were higher and human sequences were even rarer: soil samples. So while we were unable to take the DNA out of the jawbone, one team decided that there might be some left in the environment it was born in.

So the team dug up the sediments on the cave floor, dating the different layers to make an estimate. Most layers have mammalian DNA which, based on damage, is quite old. So the researchers pulled out the human mitochondrial DNA and started sequencing it. Apparently Denisovan, with a bit of the ability to be a tiny chunk of modern human DNA.

Overall, there are indications that Denisovan was occupied more than 100,000 years ago until recently about 30,000 years ago. It’s a huge history of use, although we can’t tell whether it is continuous, seasonal or discrete. Researchers point out that 70,000 years is definitely the time to adapt to great altitudes. And that turned out to be consistent with another genetic finding: that some genetics adapted to the Tibetan altitude were inherited from Denisovans.

Science, 2020. DOI: 10.1126 / science.abc1166, 10.1126 / science.abb6320 (About DOIs).


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