CHICAGO--The details are in the DNA: Neandertals probably did not contribute to the gene pool of modern humans.

With the first rough draft of the Neandertal genome in hand, "what's very clear from the data we have is that if you look at [genetic] contributions from Neandertals into modern humans today, that contribution is, if any, very small," said evolutionary geneticist Svante Pääbo in his plenary speech to the 2009 AAAS Annual Meeting.

The rough draft, first announced at a news conference on 12 February in Chicago,  is a technological feat in ancient DNA studies. The Neandertal fossil fragments yielding the draft's genetic material are tens of thousands of years old, surviving in severely degraded tissue and surrounded by bacteria that may provide their own confounding genetic signals.

As with most ancient remains, "what you inevitably find is DNA degraded into short little pieces," said Pääbo, a researcher at the Max Planck Institute for Evolutionary Anthropology. Faced with this fragmentation, his team of researchers undertook a three-year effort to squeeze the maximum amount of useful Neandertal DNA s out of the fossils obtained from four different sites across Europe.

Several upgrades in DNA sequencing technology were a big help, but the researchers also found new ways to guard against contamination of the Neandertal DNA sample by modern human DNA.

Fossils were excavated by workers wearing sterile clothing and masks, samples were handled under clean room conditions, and known Neandertal gene sequences were tagged with a special genetic marker, among other  "safety" measures, said Pääbo.

After sequencing 63 percent of the Neandertal genome, the researchers are ready to tackle one of the most persistent mysteries in human evolution. Were Neandertals kissing cousins of modern humans? Put more directly: Did they mate, and did Neandertal genes make their way into the modern human genome?

One way to approach the problem is to compare the Neandertal genome with genomes from modern European, African, and Asian modern humans. If Neandertals mated with modern humans in Europe, as some paleoanthropologists have suggested, the Neandertal DNA should be more closely similar to the genome of today's European population than to the African or Asian genome.

But instead, Pääbo and his colleagues found that the Neandertal genome was equally different from the Asian, African, and European genetic branches of the human family tree.

The researchers can now compare individual genes between modern humans and Neandertals, looking for DNA  variations or fingerprints of natural selection processes that have occurred since we diverged genetically from our closest evolutionary relatives.