The fourth Denisovan
The girl from Denisova Cave
Maxim Rousseau, Polit.roo
The authors of the new study confirmed that the child whose tooth was found in 1984 in the Denisova cave in Altai belonged to Denisovans, not Neanderthals. They also established that it was a girl and that she lived much earlier than other Denisovans known to science.
Recall that Denisovans are called representatives of a special species (or subspecies) of the genus Homo, related to both modern humans and Neanderthals. All these three species come from a common ancestor who lived about 700 thousand years ago. According to modern estimates, the separation of Neanderthals and Denisovans occurred at least 190 thousand years ago and possibly up to 470 thousand years ago. The name "Denisovsky man", or "Denisovets", is given after the Denisova cave, where the skeletal remains of this species were found. Outside of this cave, Denisovans, at least so far, have not been found.
In total, archaeologists managed to find four fragments, for which the Denisovans were later shown to belong. In 1984, a molar was found (Denisova 2). In 2000 and 2010 – two more molars (Denisova 4 and Denisova 8), and in 2008 – a fragment of the phalanx of the little finger (Denisova 3). The discovery of 1984 was made in layer 22.1 of the main gallery of the Denisova cave, the rest of the bones were found in layer number 11, which belongs to the period from 48 to 60 thousand years ago. In addition to these finds, other bone fragments were found in the Denisova Cave, which, as it turned out, belonged to Neanderthals. It was clear that Denisov's tooth 2 belonged to a child, since it is a milk tooth (lower right second molar), the other two Denisov teeth were permanent.
In 2010, the journal Nature published a study led by paleogeneticists Johannes Krause and Svante Paabo, who sequenced mitochondrial DNA from the Denisov phalanx 3. Due to the conditions of the Denisova Cave, where the average temperature is kept at around zero degrees, DNA in ancient bones has been well preserved. Scientists came to the conclusion that the owner of the phalanx belonged to a separate species of people. After this discovery, the name "Denisovsky man" appeared. In fact, scientists do not speak so categorically about the independent species status of Denisovans, assuming the possibility that Denisovans were a subspecies.
The study of nuclear DNA allowed us to determine that Denisov's phalanx 3 belonged to a woman, and Denisov's teeth 4 and 8 belonged to adult men. Scientists also learned that Denisovans interbred with ancient Homo sapiens and that modern inhabitants of Melanesia have up to 5% of the genome inherited from Denisovans. In 2014, scientists found that the Tibetans have a special variant of the gene that affects the synthesis of hemoglobin, which is useful for survival in cold climates, inherited from the Denisovan man, and in 2016 they determined that the Greenland Eskimos have variants of a number of genes associated with resistance to cold, also coincide with the variants of these genes in Denisovans. In 2013, when the genome of the Geibelberg man from the Spanish cave Sima de los Huesos was studied (about 400 thousand years ago), it turned out that in mitochondrial DNA he was close to both Neanderthals and Denisovans.
A new publication in the journal Science Advances (Slon et al., A fourth Denisovan individual) It is dedicated to the extraction of DNA from Denisov's tooth 2, the one that was discovered by the very first in 1984. The study was conducted by the same group from the Leipzig Institute of Evolutionary Anthropology of the Max Planck Society under the leadership of Svante Paabo, who had previously established the special status of Denisovans.
Scientists first did a CT scan of the tooth to preserve complete information about its structure for future researchers. The degree of tooth wear and the condition of its roots indicate that its owner was 10-12 years old.
Computed tomography of Denisov's tooth 2 (from an article in Science Advances).
The researchers then took a portion of the material (10 mg) to extract DNA. After sequencing, the DNA sequence was compared with previously obtained samples of Denisovans, as well as with the genomes of ten Neanderthals, five ancient Homo sapiens, five modern humans, chimpanzees and Heidelberg man from the cave of Sima de los Huesos. The first conclusion of the work was the confirmation that the owner of the tooth was a Denisovan. Scientists also determined gender by the absence of Y chromosomes in the nuclear genome. That is, the tooth belonged to a girl.
The second conclusion was made based on counting the number of mutations distinguishing mitochondrial DNA in four Denisov genomes. Scientists have suggested that the rate of DNA change in Denisovans was the same as in modern humans. And based on this assumption, they were able to date the bone fragments. It turned out that Denisov's tooth 2 was the oldest, it is 20.6 – 37.7 thousand years older than Denisov's tooth 8 and 54.2 – 99.4 thousand years older than Denisov's phalanx 3. Denisov 3 and Denisov 4 turned out to be almost contemporaries, the age difference between them is estimated at 3.7 – 6.9 thousand years. Although the numerical estimate of the difference depends on whether the rate of change in the mitochondrial DNA of Denisovans and modern humans is the same, in any case, it can be argued that Denisova 2 is older than Denisova 8 and significantly older than Denisova 3 and 4.
Based on the assessment of the antiquity of the layer in which the Denisov 8 bone was found and the age difference calculated from genetic data, scientists came to the conclusion that the girl (Denisova 2) lived at least 100 thousand years ago. This makes it one of the oldest hominins whose remains have been found in Central Asia. It also means that Denisovans lived in the cave for tens of thousands of years. Despite such a significant time range, scientists have noticed that the differences in the nuclear DNA of Denisovans are much smaller than the differences in the DNA of modern human populations. Perhaps this is due to the fact that Denisova Cave residents were a small isolated population, and the overall genetic diversity of Denisovans was still higher. But to confirm this, we need to find the remains of Denisovans who lived in other places and isolate their DNA.
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10.07.2017