Editorial: A giant leap in sequencing Neanderthal genome

Published in The Hindu on February 25, 2009

The successful completion of a rough draft of the Neanderthal genome — this is the first time that a genome of an extinct organism has been sequenced — marks a giant leap in evolution science. The announcement coincided with the 200th birth anniversary of Charles Darwin. The genome sequence of the Neanderthals, man’s closest relatives who became extinct about 30,000 years ago, would help in getting a better insight into human evolution and throw more light on what makes humans unique. Svante Paabo and his colleagues from the Max Plank Institute for Evolutionary Anthropology in Leipzig, Germany, deserve credit for their extraordinary feat of sequencing the genome from DNA extracted from 38,000-year-old limb bones of two female Neanderthals found in a cave in Croatia. Sequencing genomes of extinct organisms is a challenging task. The genetic material starts degrading the moment an organism dies. It also becomes contaminated with microbes that invade the decaying material and with modern human DNA while the fossils are handled. That more than 96 per cent of the DNA extracted belonged to microbes left Paabo with only a small quantity of Neanderthal material to sequence about 60 per cent of the genome. Repeated sequencing is mandatory to confirm the accuracy of the outcome. This is all the more compelling in this case because the full genome has not been sequenced. Since getting hold of DNA material that is less contaminated is extremely difficult, one has to go for improving the four-year-old technique of sequencing ancient DNA to study even the degraded and contaminated samples.

The decoding of small stretches of DNA from mitochondria in 1997 revealed for the first time that the Neanderthal was not an immediate ancestor of humans but a separate species altogether. In fact, humans split from the Neanderthals some 600,000 years ago but co-existed in Europe and parts of Asia till the latter became extinct. Since the two co-existed, it becomes important to find out whether interbreeding — if it had existed at all — contributed substantially to the modern human genome. The 1997 study of mitochondrial DNA from Neanderthals did not find any evidence to support interbreeding. Scientists are now looking at specific genes to further ascertain this. If the two had indeed interbred, the genome of modern Europeans should be similar more to that of Neanderthals than to that of modern Africans. Preliminary results from the Croatian samples show that the genome of Europeans is not similar to that of Neanderthals. In all probability, the two closest relatives shared a common ancestor and evolved in separate ways.