Fresh from being awarded the Nobel Prize a few weeks ago for his work on the DNA of early human species, including Neanderthals, Svante Paabo (or, more accurately, he and a group of his cronies) has just published Nature one of the largest genetic studies of its kind to date.
These Neanderthals lived in the Altai Mountains in Russia 50,000 years ago. The remains examined – 17 bone and tooth samples from 13 people – came from two caves about 100 km apart. One named Chagyrskaya produced 11 individuals (three boys, three girls, three males and two females). The other, Okladnikov, delivered two (a boy and a woman). Taken together, this work almost doubles the number of Neanderthal genomes described so far. It also gives a tantalizing glimpse into Neanderthal social life.
It is extremely unlikely that all of these individuals were contemporaries. But researchers believe they’ve found both a trio and a pair of relatives. They did this by calculating a value called DNA divergence.
DNA divergence compares nuclear genomes by randomly selecting sections of their DNA and checking that the two genomes match for each selected section. The more similar the DNA sequences are, the closer it can be assumed that the two individuals are related. Applying this approach to the remains of Chagyrskaya revealed a father, his daughter, and a close maternal relative who likely shared a grandmother with the father. Separately, a young boy was matched with an adult female relative, possibly a cousin, aunt, or grandmother.
The individuals in Okladnikov Cave were not closely related to each other or to anyone from Chagyrskaya. But the researchers found an intriguing connection. The woman’s mitochondrial DNA matched that of a man from Chagyrskaya.
Mitochondrial DNA is passed intact from mother to offspring. It is not involved in sexual intermingling, so it changes only through the random process of mutation. The lack of mutations that could have differentiated the DNA of the individuals in question suggests not only a common ancestor, but a relatively recent one.
Further analysis also showed that two of Chagyrskaya’s mitochondrial DNA samples were closer to the Okladnikov boy than any other Chagyrskayans. And when the team looked at data on Y chromosomes passed intact from father to son, as well as their mitochondrial data, they were able to draw some tentative conclusions about Neanderthal communities.
If members of a population mate more or less randomly with those of the opposite sex, the so-called coalescence period — how far in the past their most recent common ancestor lived — should be the same for mitochondrial (matrilineal) and Y-chromosome (patrilineal) DNA. However, the researchers found that the average coalescence time for the Y chromosome was 500 years, while that for the mitochondrial genome was around 5,000 years.
To explain this order of magnitude difference, they modeled various possibilities. The one that best fitted the data was that the Altai Neanderthals lived in groups of about 20 individuals, with at least 60% of the females in a group having migrated from elsewhere. The size of such groups is similar to that inferred for Paleolithic bands of Homo sapiens, which probably had around 25 members.
When it comes to mankind’s ancestors and cousins, it’s easy and tempting to overinterpret the scarce data available — and practitioners of the subject have indeed been guilty of it in the past. These conclusions should therefore be taken with caution. But last but not least, this study shows that the methods that Dr. Paabo brought in its prize, greatly expanded the data pool for such speculation.