Saturday, May 08, 2010

why the Neanderthal genome matters

Amidst all the ado about the UK general election, few people around here will have noticed that a draft genome of our Neanderthal (or Neandertal, as they spell it in the US and in Germany) relatives was published in Science magazine on Friday, 7.5. What press coverage there was focussed on the finding that all modern non-Africans analysed in comparison have a very small number of “typical Neanderthal” genetic traits, suggesting that the small group of early humans that emigrated from Africa and then went on to populate the rest of the world must have interbred with an even smaller number of Neanderthals.

Personally, I find that specific result neither surprising nor terribly important. What I find much more exciting about this project is, firstly, the phenomenal difficulty that had to be overcome (which is why the project is discussed in the “crazy” section of my recent book, The birds, the bees, and the platypuses), and secondly, the usefulness of Neanderthals as an external reference point for genetic and biomedical studies of our own species.

To the first point, as I have both worked with DNA and dug for Neanderthal remains, I was awestruck a few years ago, when Svante Pääbo and colleagues set out to sequence a complete genome, based on a 50-microgram DNA sample from a single rotten bone (if memory serves, 50 micrograms of DNA may well be invisible to the naked eye). Apparently, they have a little more material now (seeing they report a composite genome derived from three individuals), but still. If you consider that only 10 years ago the draft sequence of Homo sapiens, a species of which we have more than 6 billion specimens walking around, required a monumental effort and astronomical cost to sequence even in a crude draft version, the success of the Neanderthal sequencing project just a decade later is just mindboggling. Especially because the risk of contamination with modern human DNA must be severely limiting the sequencing effort. Every bone fragment that any Homo sapiens has touched with their bare hands is probably a lost cause already.

Regarding the second point, we are only beginning to get an impression of the diversity of individual human genomes, but as we are accumulating more and more personal genomes and begin to draw medical conclusions from them, it is important to have an “ancient” reference for comparison, to be able to deduce which of the genetic variants is the original one and which is the mutation. To some extent, the chimpanzee genome can serve as a stop-gap in this role, but Neanderthal being so much closer to us, it is a much more useful reference point. The new research puts the parting of ways at 270,000 to 440,000 years, while chimps and humans parted company some five million years ago.

Finally, in the analysis of the few genetic variants that we non-Africans seem to have inherited from Neanderthals, there are already some interesting clues for medical research. Most intriguingly, one of the genes has been linked to autism. A few years ago, there was a short story published in Nature, exploring the possibility that we have our “nerdy” genes (an overdose of which may cause autism) from Neanderthals, which implies that Neanderthal society failed because of a lack of social instincts. This story could now turn out to be amazingly prescient.


References:

R. E. Green et al., Science 2010, 328, 710.
H.A.Burbano et al., Science 2010, 328, 723.

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