Fresh from his award a couple of weeks ago of the Nobel prize for his work on the DNA of early human species, including Neanderthals, Svante Paabo (or, more precisely, he and a group of his acolytes) has just published in Nature one of the largest genetic studies of its kind yet.
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These Neanderthals lived 50,000 years ago in the Altai mountains of Russia. The remains under study – 17 bone and tooth samples belonging to 13 individuals – came from two caves about 100km apart. One, called Chagyrskaya, brought 11 people (three boys, three girls, three men and two women). The other, Okladnikov, took two (a boy and a woman). Taken together, this work nearly doubles the number of Neanderthal genomes described. It also provides an interesting insight into Neanderthal social life.
It is highly unlikely that all these people were contemporaries. But the researchers believe that they have found both a trio and a pair of relatives. They did this by computing a so-called value DNA difference
DNA divergence compares nuclear genomes by selecting portions of them DNA randomly and checks if, for each selected segment, the two genomes match. The more likely the DNA Lines are, the closer, the two people can be considered to be related. Applying this method to Chagyrskaya’s remains revealed his father, his daughter and close maternal relatives who probably shared a grandmother with their father. Separately, he matched a young boy with an adult female relative, perhaps a cousin, aunt or grandmother.
The individuals in the Okladnikova cave were not closely related to each other or to anyone from Chagyrskaya. But the researchers found an interesting connection. Mitochondrial women DNA compared to one from Chagyrskaya.
Mitochondrial DNA passed down intact from mother to offspring. It is not involved in sexual mixing, so it only changes with the random process of mutation. The lack of mutations that could have made a difference to the DNA of the individuals from each other suggesting not only a common ancestor, but a relatively recent one.
Further analysis also showed that two of the mitochondrial DNA samples from Chagyrskaya were closer to the Okladnikov boy than to any of the other Chagyrskayans. And when the team looked at data on it Y-chromosomes, which pass from father to son, as well as their mitochondrial data, they could draw uncertain conclusions about Neanderthal communities.
If members of the population mix more or less randomly with those of the opposite sex, the so-called convergence interval – how long in the past the most common ancestor was modern living – the same for mitochondrial (matrilineal) and Y– chromosome (patrilineal) DNA. The researchers, however, found that the average convergence time for the Y-chromosome 500 years, and that for the mitochondrial genome was about 5,000 years.
To explain this order of magnitude difference, they modeled different possibilities. The one that best fits the data was that the Neanderthals of the Altai lived in groups of about 20 people, with at least 60% of the women in a group having migrated there from other places. The size of these groups is similar to those produced for Palaeolithic bands of Homo sapienswhich probably had about 25 members.
When dealing with the ancestors and cousins of mankind it is easy and shameful to over-interpret the scant data available – and indeed practitioners of the subject have been guilty to do this in the past. These findings should therefore be treated with caution. But if nothing else, this study shows that the methods that awarded Dr. Paabo have increased the data collection available for such speculation in a unique way. ■
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