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Neandertal DNA Partially Mapped, Studies Show

Kimberly Johnson
for National Geographic News
November 15, 2006
 
Modern humans' closest relatives, the Neandertals, broke off from the family tree about 500,000 years ago, according to one of two new studies that analyzed DNA from the extinct species Homo neandertalis.

Nuclear DNA from a 38,000-year-old Neandertal (often spelled Neanderthal) fossil leg bone from Croatia was sequenced and compared to DNA from modern humans and chimpanzees.

The findings, published today in the journal Nature, also suggest that the entire Neandertal population was derived from a relatively small ancestral group of 3,000 individuals.

The second study, released simultaneously by the journal Science, analyzed DNA from the same ancient Croatian bone, revealing for the first time that modern humans and Neandertals share 99.5 percent of their genetic makeup.

But their analysis didn't find evidence that modern human and Neandertal DNA mixed, seeming to counter recent conclusions that Neandertals interbred with humans to the point of total absorption, leading to their extinction.

Overall, the dual projects signal the dawn of Neandertal genomics, says Edward Rubin, director of the U.S. Department of Energy Joint Genome Institute and co-author of the Science study.

"In many ways it will change some aspects of anthropology," Rubin said. "We're never going to bring [Neandertals] back to life, but we will be able to compare [the species' genetics] to the human genome."

Neandertal Library

The Nature study team used an advanced new machine to perform high-throughput nuclear DNA sequencing.

Using this method, the team, led by Richard Green of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, was able to isolate about one million DNA base pairs, creating the beginnings of a Neandertal genome "library."

Base pairs are the "letters" of the genetic alphabet that are combined in DNA as genes to code for everything from hair color to body shape (get an overview of human genetics).

"Neanderthals are the hominid group most closely related to currently living humans, so a Neanderthal nuclear genome sequence would be an invaluable resource for annotating the human genome," the Nature study authors wrote.

Using a different type of DNA analysis, the Science team has revealed that there are about three million base pair distinctions between modern humans and Neandertals, study co-author Rubin says.

The genetic differences between humans and Neandertals is "a drop in the bucket" compared to the estimated 30 million to 50 million base pair differences between humans and chimpanzees, he said.

Learning to Read

Based on the latest findings, scientists say they are on their way to fully sequencing the Neandertal genome, possibly as soon as two years from now.

Some experts in the field, however, remain skeptical.

Putting together a complete Neandertal genetic library could take decades, according to archaeologist John Shea, who teaches at the State University of New York in Stony Brook.

And "accumulating a library is a first step," said Shea, who was not involved in the research released today.

"We need to know how to read the books, i.e., to know what differences in particular genes mean for growth and behavior."

But scientists are a long way from having a sufficient number of samples to conclusively test ancestry, he says.

DNA breaks down during fossilization, which also complicates analysis. What's more, Neandertal bones are typically soft and easily contaminated by fixatives or excessive handling once they are excavated.

"There are vastly more contaminated specimens" than pure ones, Shea said. "If you find that a Neandertal fossil has modern human DNA, is it mixed lineage or contamination?"

Anthropologist Erik Trinkaus, who was involved with last month's interbreeding study, says that today's DNA reports are consistent with findings based on non-DNA fossil analysis.

"All of this only says that the DNA analysis is probably close to correct, if not new in its conclusions," said the scientist, who is based at Washington University in St. Louis, Missouri.

"In other words, their work so far is solely a test of whether they can do it, what we call a pilot study." Going further may be a challenge.

"They have one individual and are unlikely to get very many more, given the poor organic preservation of most Pleistocene [1.8 million years ago to 11,500 years ago] fossil humans," Trinkaus said.

"Since evolution consists of changing patterns of variation, they cannot address evolutionary questions."

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