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Neanderthal DNA secrets unlocked

A genetic breakthrough could help clear up some long-standing mysteries surrounding our closest evolutionary relatives: the Neanderthals. Scientists have reconstructed a chunk of DNA from the genome of a Neanderthal man who lived 38,000 years ago. The genetic information they extracted from a thigh bone has allowed them to identify more than a million building blocks of Neanderthal DNA so far. “The sequence data will serve as a DNA time machine,” said co-author Edward Rubin, from the Joint Genome Institute in Walnut Creek, California, US. Having a Neanderthal genome will also throw light on our own evolution.

Studying the Neanderthal genome will shed light on the genetic changes that made our species what it is, after the evolutionary lineages of Neanderthals and modern humans diverged from one another. It could also reveal what colour hair, eyes and skin Neanderthals had, whether they were capable of modern speech, shed light on aspects of their brain function and determine whether they contributed to the modern human gene pool. Researchers have already sequenced mitochondrial DNA (mtDNA) from 12 Neanderthals. This is DNA from the cell’s powerhouses, and which is passed down from mother to child.

While mtDNA has confirmed that Neanderthals were indeed different from us, the information gleaned from it is limited. The double-stranded DNA molecule is held together by chemical components called bases. Adenine (A) bonds with thymine (T); cytosine(C) bonds with guanine (G). These “letters” form the “code of life”; there are 3.2 billion base-pairs in the Neanderthal genome. Written in the DNA are genes, which cells use as starting templates to make proteins; these sophisticated molecules build and maintain the body. To answer more detailed questions about our evolutionary cousins, scientists had to extract DNA that came from the cell’s nucleus. This nuclear DNA encodes most of an organism’s genetic blueprint. Researchers used cutting-edge DNA sequencing techniques to retrieve genetic material from the Neanderthal femur found in the Vindija Cave, Croatia.

Writing in Nature journal, Professor Svante Paabo and colleagues describe how they recovered more than one million base-pairs – the building blocks of DNA – by directly reading the genetic sequence. In another paper published in Science magazine, Professor Rubin’s team used a different approach called metagenomics, in which the fragments of Neanderthal genetic material were incorporated into bacteria that were then copied themselves, generating a living “library” of DNA sequences. This method resulted in the recovery of 65,250 base-pairs of Neanderthal DNA. While direct sequencing allows scientists to recover more genetic material, it is a random process. The metagenomic approach should allow scientists to call up specific genetic sequences of interest from the DNA library in a targeted manner.

Professor Paabo told BBC science correspondent Pallab Ghosh that he planned to look at the form of the gene FOXP2 in Neanderthals; this gene is implicated in the development of language skills and has undergone evolution in modern humans since our divergence from chimpanzees. “We have two little snippets of genes involved in skin and hair colour, but they don’t give any hint of a special variant that would be of interest,” Paabo told BBC News. Our closest evolutionary relatives are still something of a mystery. The two teams basically agree, within their margins of error, that the evolutionary lineages of Neanderthals and modern humans split somewhere around 500,000 years ago. This fits with previous estimates from mtDNA and archaeological data. Professor Paabo, from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his team also show that Neanderthals came from a very small ancestral population of about 3,000 individuals.

At their peak, Neanderthals dominated a wide range – stretching from Britain and Iberia in the west, to Israel in the south and Uzbekistan in the east. This stocky, muscular human species was our closest evolutionary relative. Modern humans entered Europe about 40,000 years ago; and within 10,000 years, the Neanderthals had largely disappeared from the continent. By 24,000 years ago, the last survivors had vanished from their refuge in the Iberian Peninsula. The question of whether modern humans and Neanderthals mated when they encountered each other 40,000 years ago is highly controversial. One US scientist recently suggested modern humans might have acquired a variant of the brain gene microcephalin through interbreeding with Neanderthals.

Edward Rubin’s team found no evidence for a Neanderthal contribution to the modern gene pool, but Professor Paabo’s analysis hints at a possible contribution in the other direction – from modern humans into Neanderthals. The researchers say more extensive sequencing is needed to address this possibility. Professor Chris Stringer, from London’s Natural History Museum, said the results “confirm the distinctiveness of the Neanderthals, and support previous estimates of the divergence time. “Research will now extend to complete the whole genome of a Neanderthal and to examine Neanderthal variation through time and space to compare with ours.” The researchers aim to produce a rough draft of the full Neanderthal genome sequence over the next two years.

BBC News
December 5, 2006

Original web page at BBC News