DNA Reveals Long, Separate History of Neanderthals and Humans

The first-ever analysis of Neanderthal nuclear DNA from a 38,000 year-old Croatian fossil suggests Neanderthals and modern humans split into separate species about 370,000 years ago. The finding is reported in the 16 November issue of the journal Science.

Neanderthals and modern humans can trace their ancestry back to a hominid population that lived about 706,000 years ago, said James Noonan of the U.S. Department of Energy Joint Genome Institute and colleagues.

Based on these dates, the two groups were on their way to becoming separate species long before anatomically modern humans appeared on the scene and dispersed from Africa to the rest of the world about 200,000 years ago, according to the researchers.

During a teleconference with reporters on Tuesday 14 November, Science lead author Edward Rubin said the Neanderthal and modern human genome appear to be 99.5% identical, and that the 3 million base pair differences between the two species were "sort of a drop in the bucket" in the context of the human genome's 3 billion base pairs.

The Neanderthal sequence data, which will be freely available to other researchers on the Internet, is "a DNA time-machine that will tell us about biology and aspects about Neanderthals that we could never get from their bones and the limited number of associated artifacts," Rubin said.

The genetic evidence suggests that Neanderthals did not contribute in a significant way to the modern human gene pool, the Science researchers said. They found no trace of Neanderthal-specific gene variations in the European gene pool, where mixing between modern humans and Neanderthals is thought to be most likely.

The study is the first time that ancient DNA from the cell nucleus, rather than cell mitochondria, has been sequenced in Neanderthals. Nuclear DNA offers a better glimpse of the entire Neanderthal genome, since mitochondrial DNA is only inherited maternally.

In fact, the analysis uncovered hominid genetic sequences from the Y chromosome, indicating that the fossil leg bone came from a Neanderthal male, Rubin said.

Noonan and colleagues used bacteria to build up a "library" of all the ancient DNA fragments extracted from the Croatian fossil. The researchers directly sequenced these tiny fragments and compared their sequences to those in the human and chimpanzee genomes. Differences among the sequences can be used to estimate the date that the species split from each other.

Svante Paabo, a co-author of the Science paper and lead author on an accompanying paper in the journal Nature, said his research team will use DNA from the Croatian fossil to complete a rough first draft of the entire Neanderthal genome in two years.

This rough draft could help researchers pinpoint the exact differences in biology and behavior that separated modern humans and Neanderthals. But Rubin said geneticists would first have to know more about how genes affect biology and behavior in modern humans.

"For the Neanderthal, we're never going to bring it back to life, but we're going to be able to compare it to what we know about the human genome...and the more we know about the human genome, the more we will know about Neanderthals," Rubin said.

Becky Ham

15 November 2006