Neanderthal DNA reveals human divergence

Chip off Neanderthal bone opens a door to our past

2 research teams extract genetic information from a 38,000-year-old fossil, a break that may help clear up mysteries about human evolution.

By Peter Gorner
Tribune science reporter

November 16, 2006

Opening up ancient DNA to modern science, scientists say they have extracted detailed genetic information from a 38,000-year-old piece of Neanderthal bone that may help explain the evolutionary relationship between the ancestors of today's humans.

The researchers suggest Neanderthals probably shared a common individual ancestor with Homo sapiens around 706,000 years ago. The two populations split about 370,000 years ago and set about evolving in separate ways, according to dual reports published this week in the journals Science and Nature.

The DNA showed no evidence of extensive interbreeding between Neanderthals and humans, though the issue is far from settled.

Whether Neanderthals were wiped out by more advanced early humans or interbred with them--or both--is one of the most contentious questions in anthropology.

With the new genetic information, scientists hope to be able to compare Neanderthal genes to those of humans and chimpanzees to help determine what it means to be human--as well as solve such enigmas as whether Neanderthals had a language and the color of their skin and hair.

"We're at the dawn of Neanderthal genomics. Think about the state of Egyptology until we cracked the code of hieroglyphics--this is that important," said Edward Rubin of Lawrence Berkeley National Laboratory in Berkeley, Calif., leader of one of two international teams working on the project.



`DNA time machine'

"Here's a DNA time machine that will tell us things about Neanderthals that we could never get from their bones and artifacts."

Svante Paabo, head of the other team and a paleogeneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, announced plans to produce a rough draft of the entire Neanderthal genome in two years.

"This is wonderful. Who would have thought 15 years ago that we would be talking about the genetic structure of an extinct form of human?" said paleontologist Fred Smith, chairman of anthropology at Loyola University and a leading authority on Neanderthals, who was not connected to the research. "The work these teams are doing is absolutely fantastic."

Neanderthals were short and stocky, the stereotypical beetle-browed caveman. They lived in Europe and western Asia, coexisting with modern humans until they disappeared from the Iberian peninsula about 28,000 years ago.

"We care about Neanderthals because we know a lot about them," said Milford Wolpoff of the University of Michigan, an authority on Neanderthal paleontology. "We know a lot about them because they had the courtesy to bury their dead, and we figured out how to find them."



DNA deteriorates

But because DNA deteriorates over time, it was difficult to find a fossil that yielded enough surviving genetic material to be analyzed.

The fragment used in these studies was a piece of leg bone about 2 1/2 inches long--a fossil that had been stored away with minimal human handling, reducing the risk of contamination by modern DNA.

The researchers said their experiments indicated that the material they were able to extract was, in fact, Neanderthal DNA.

But Smith, who did considerable work on the Neanderthals in Croatia and has been in the cave where the fragment was found, expressed some concern about the specimen. While the bone is likely to be from a Neanderthal, he said, it is not proved.



Questions about the bone

"If I look at the specimen as an anatomist, I cannot say for sure that it is a Neanderthal," Smith said. "It's just too small a fragment. What we need are other specimens; things we're absolutely sure are Neanderthal."

Still, he praised the teams' findings. "It's outstanding work," he said. "But in science, it's important to understand the limitations to some pioneering work."

Rubin's team reported recovering 65,250 of the Neanderthal's estimated 3 billion base pairs using the sequencing technologies employed in the Human Genome Project as well as a new method to clone and complete fragmented DNA.

Using similar techniques but preparing the DNA differently, Paabo and his colleagues sequenced 225,000 fragments of DNA that finally yielded a million base pairs.



No evidence of interbreeding

As for interbreeding between the species, Rubin's team found no evidence for it, but Paabo reported he couldn't rule out a modest amount of gene flow.

To experts who have argued for admixture--the minority view--the latter finding is justification.

"There's a whole bunch of human genes in the Neanderthal genome that are more recent than 200,000 years. How did they get there?" said Wolpoff. "If divergence was early ... the only way genes could get in there that are younger than 200,000 years is through interbreeding."

Other experts said the breakthroughs announced Wednesday eventually should help clear up mysteries about human evolution.

"Humans went through several key stages of evolution during the last 400,000 years," said Jonathan Pritchard, a professor of human genetics at the University of Chicago who led the team that analyzed the sequencing data.

"If we can compare human and Neanderthal genomes, then we can possibly identify what the key genetic changes were during that final stage of human evolution."