Immediately after he was awarded the Nobel Prize for his work on the DNA of early human species, including Neanderthals, Svante Paabo published one of the largest genetic studies of this species in the journal Nature, writes The Economist.
The Neanderthals from the study lived 50,000 years ago in the Altai Mountains on the territory of modern Russia. The researched remains – 17 samples of bones and teeth belonging to 13 individuals – come from two caves located at a distance of about 100 km from each other. One of them, named Chagyrska, produced 11 individuals (three boys, three girls, three men, and two women), and the other, Okladnikova, produced two (a boy and a woman). Collectively, this work nearly doubles the number of described Neanderthal genomes. It also gives an insight into their social life.
It is extremely unlikely that all these persons were contemporaries. But researchers believe they have found both a trio and a pair of relatives. They did this by calculating a quantity called “DNA divergence.”
DNA divergence compares nuclear genomes by randomly selecting regions of their DNA and testing whether the two genomes match at each selected region. The more similar the DNA sequences are, the more closely related the two individuals are. Applying this approach to the Chagyr remains made it possible to identify the father, his daughter, and a close maternal relative who probably shared a grandmother with the father. Separately, this made it possible to match a young boy with an adult female relative, potentially a cousin, aunt, or grandmother.
The individuals in the Okladnikov Cave were not closely related either to each other or to anyone from the Chagyr Cave. However, the researchers found an intriguing connection. The mitochondrial DNA of the woman matched the mitochondrial DNA of the man from Chagyrska cave.
Mitochondrial DNA is transmitted unchanged from mother to offspring. It does not participate in sexual mixing, so it changes only under the influence of a random process of mutations. The absence of mutations that could distinguish the DNA of the individuals in question from each other suggests not only a common ancestor but also a relatively recent one.
Further analysis also showed that two mitochondrial DNA samples from Chagyrska were closer to the Okladnikov boy than to any of the other Chagyr representatives. And when the team looked at data on Y chromosomes, which are passed down unchanged from father to son, as well as their mitochondrial data, they were able to draw some preliminary conclusions about Neanderthal communities.
If members of a population mate more or less randomly with members of the opposite sex, the so-called coalescence time (how far in the past their last common ancestor lived) should be the same for mitochondrial (matrilineal) and Y-chromosomal (patrilineal) DNA. However, the researchers found that the average coalescence time for the Y chromosome is 500 years, while that for the mitochondrial genome is about 5,000 years.
To explain such a difference by an order of magnitude, scientists modeled different options. The best fit is that Altai Neanderthals lived in groups of about 20 individuals, with at least 60% of the females in the group having migrated there from elsewhere. The size of such groups is similar to that deduced for Paleolithic groups of Homo sapiens, which probably numbered about 25 members.