Thursday, November 16, 2006

New DNA sequencing of Neanderthals sheds light on their relations with humans

Latest DNA sequencing of bone material from a Neanderthal man has given clear indications that the Neanderthals are distant relatives of humans and that they were more than 99.5 per cent genetically identical to the modern man.

Two international teams working on DNA samples recovered from the leg bone of the Neanderthal who died 38,000 years ago also found that Neanderthals and the ancestors of modern human beings rarely interbred. No matter how much interbreeding may have taken place in the nearly half-a-million years that the two were separate species, it left little or no mark on the genetic code of either one, the researchers say.

Edward Rubin, a researcher at the U.S. energy department's Lawrence Berkeley National Laboratory and a senior author of a study appearing in the journal Science, says there is no evidence of mixing 40,000, 30,000 years ago in Europe. "We do not exclude it, but see no evidence," he says.

Rubin's team had collaborated with a team from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, led by Svante Paabo, a group that first extracted ancient mitochondrial DNA from a Neanderthal sample some 10 years ago. The two teams used different methods to isolate and sequence part of the Neanderthal's DNA.

Paabo's team had reported its work in 1997, saying Neanderthals did not mix with modern humans. Even Rubin's genetic analysis of the Neanderthals suggest that there was little sexual contact between the two, at least according to the genes recovered from the bone, which the researchers found belonged to a male, who lived in Croatia.

Both Neanderthals and modern humans had descended from Homo erectus, which left Africa and spread around the world about 1.5 million years ago. While Neanderthals lived in Europe and the Middle East until about 30,000 years ago, the ancestors of modern humans, known as Cro-Magnon, had migrated out of African about 10,000 years earlier.

By comparing the genes of the Neanderthals with some of those from the complete human gene map, the researchers have calculated that Neanderthals differed from humans by about three million base pairs of genes, out of a total code of more than three billion pairs. The genetic code of chimpanzees differs from humans' by about 50 million base pairs.

The researchers believe between 500,000 and 700,000 years ago the two lineages had split but they continued to interbreed, yet drifting apart genetically. About 370,000 years ago, the mixing stopped and the family tree split, with one branch becoming the Neanderthals and the other humans. This conclusion has been possible after assessing some 1.1 million DNA letters of the Neanderthal genome.

Paabo and Rubin are now working to deliver a rough draft of a Neanderthal genome in about two years.

All the while, researchers have been wondering what actually led to the extinction of the Neanderthals and whether the two human species interbred during the millennia when they belonged to the same environment and habitat.

Genetic experts are thrilled at the development. They say a three-way comparison, now possible, between the human, chimpanzee and Neanderthal genomes will help scientists understand how the modern humans became unique.


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