"However, at the same time, our data shows no indication of any genetic overlap or exchange [interbreeding] between humans and Neanderthals," he cautioned. "So, if the question is, 'Did any Neanderthal genes find their way into modern humans?' our data so far would suggest that didn't happen."
Jeffrey Laitman, a professor and director of the Center for Anatomy and Functional Morphology at the Mount Sinai School of Medicine in New York City, said the finding could help solve an anthropological puzzle.
"We've been going around in circles since the discovery of Neanderthals about 150 years ago, trying to figure out, 'are they us or are they not us,' " said Laitman, who was not involved in the research.
"Are they a sub-group of humans or a separate species? That's the crux of the issue. This genome work speaks very strongly that they are a distinct species and, equally importantly, that there is no evidence that there was any mixture -- no interbreeding -- between the two species," he said.
Rubin announced the findings at a press conference held Tuesday.
His team began its work by developing a permanent "nuclear DNA library" containing their Neanderthal DNA specimen extracts.
The nuclear DNA extracts they worked with are found in the nucleus of a cell and are considered to be the most reliable sequencing source for examining evolutionary issues.
The new work goes a step beyond previous efforts to map Neanderthal DNA. All prior Neanderthal sequencing work had been based on examinations of mitochondrial DNA -- bits of genetic material that are inherited exclusively from the mother and are viewed as less useful.
Rubin's group overcame technological limitations and have for the first time been able to sequence more reliable nuclear DNA. They established a Neanderthal gene library and were able to efficiently isolate and study specific DNA sequences from their archives.
After confirming their sample as being 98 percent Neanderthal DNA, with only two percent modern human contamination, the researchers identified mutations that might account for comparative differences between human and Neanderthal DNA sequences.
Based on this work, they now calculate that the most recent common ancestor of humans and Neanderthals lived approximately 706,000 years ago.
This date, they noted, was not the point in time when the two species actually split apart but rather reflects the onset of mutative differences within the common ancestor population that would eventually lead to such a split.
The split itself, they determined, occurred an estimated 339,000 years later. From this point onward, two genome pools existed that were 99.5 percent identical, but 0.5 percent distinct.
Long after their split, humans and Neanderthals are known to have cohabitated in Europe up until about 30,000 years ago, when the Neanderthals disappeared.
While some anthropologists have argued that Neanderthals and modern humans interbred, Rubin's team found no evidence to support that theory. However, he noted that more research is needed to bring an end to the controversy.
Nonetheless, the researchers hailed their sequencing process as a move away from "inference and speculation" when it comes to developing a better understanding of the relationship between humans and Neanderthals.
"Clearly, we're at the dawn of Neanderthal genomics," Rubin remarked at the press conference. His team's goal is to sequence the full Neanderthal genome within two years, he said.
Rubin added that the advent of DNA analysis could change anthropology itself, moving the science beyond an analysis of bones and artifacts to the genetic and biological makeup of groups like the Neanderthals.
Laitman was equally enthusiastic.
"This is a major piece of work that is really going to make a major impact in the field of genetics," he said. "And I think the power of genetic analyses such as this one is that it really answers a number of long-standing questions, such as whether or not our closest ancestral relatives, Neanderthals, are seated at the same family table. And this work indicates that they are not."
"It's fascinating to find that it's that half-percent difference that makes us what we are and makes them what they were," Laitman observed. "It looks like a small difference. But, in fact, it's enormous."
More information
For more on genome sequencing, visit the Human Genome Project.
