Genetic Study Revises Human Family Tree

Genetic Study Revises Human Family Tree

By NICHOLAS WADE

A hauntingly brief but significant message extracted from the bones of a Neanderthal who lived at least 30,000 years ago has cast new light both on the origin of humans and Neanderthals and on the long disputed relationship between the two.

The message consists of a short strip of the genetic material DNA that has been retrieved and deciphered despite the age of the specimen. It indicates that Neanderthals did not interbreed with the modern humans who started to supplant them from their ancient homes about 50,000 years ago.

The message also suggests, said the biologists who analyzed it, that the Neanderthal lineage is four times older than the human lineage, meaning that Neanderthals split off much earlier from the hominid line than did humans.

The finding, made by a team of scientists led by Dr. Svante Paabo of the University of Munich in Germany, marks the first time that decodable DNA has been extracted from Neanderthal remains and is the oldest hominid DNA so far retrieved. The DNA was extracted from the original specimen of Neanderthals, found in the Neander valley near Dusseldorf, Germany, in 1856 and now in the Rheinisches Landesmuseum in Bonn.

"This is obviously a fantastic achievement," said Dr. Chris Stringer, an expert on Neanderthals at the Museum of Natural History in London.

Many anthropologists had tried to extract DNA from Neanderthal bones without success. "Clearly, it's a coup," Dr. Maryellen Ruvolo, an anthropologist at Harvard University, said of the Munich team.

The Neanderthals were large, thick-boned individuals with heavy brows and a brain case as large as that of modern humans but stacked behind the face instead of on top of it. They lived in Europe and western Asia from 300,000 years ago, dying out about 270,000 years later.

For the latter part of that period they clearly coexisted with modern humans but the relationship between the two groups, whether fraternal or genocidal, has been debated ever since the first Neanderthal was discovered. Early humans and Neanderthals may have interbred, as some scientists contend, with modern Europeans being descended from both; or the two hominid lines may have remained distinct, with humans displacing and probably slaughtering their rivals.

The new finding, reported in Friday's issue of the journal Cell, comes down firmly on the side of Neanderthals having been a distinct species that contributed nothing to the modern human gene pool. Describing it as "an incredible breakthrough in studies of human evolution," Stringer said the results showed Neanderthals "diverged away from our line quite early on, and this reinforces the idea that they are a separate species from modern humans."

Dr. Ian Tattersall, a paleontologist at the American Museum of Natural History in Manhattan, said the finding "fits well into my view of the fossil record," although there was still a "very tenacious notion of Neanderthals having given rise to Homo sapiens or interbred with them."

The new work is also being praised by scientists who study ancient DNA, a lively new field, which has included reports of DNA millions of years old being retrieved from dinosaur bones, fossil magnolia leaves and insects entombed in amber.

Although these reports have appeared in leading scientific journals like Science and Nature, other scientists have been unable to reproduce them. In at least in one case the supposed fossil DNA was contaminated by contemporary human DNA.

But a leading critic of these claims of ancient DNA extraction, Tomas Lindahl of the Imperial Cancer Research Fund in England, has given the new work his seal of approval, calling it a "landmark discovery" and "arguably the greatest achievement so far in the field of ancient DNA research."

The Munich team took great pains to verify that it had a genuine sample of Neanderthal DNA. Working in sterile conditions, team members isolated it in two different laboratories and distinguished it from the human DNA, which contaminated the bones. Their work is "compelling and convincing," Lindahl wrote in a commentary in Friday's Cell.

The DNA recovered from the Neanderthal is known as mitochondrial DNA, a type especially useful for monitoring human evolution. Mitochondria are tiny, bacteria-like organelles within the cell and possess their own DNA. They exist in eggs, but not in sperm, and so are passed down through the female line. Unlike the main human genes on the chromosomes, which get shuffled each generation, the only change to mitochondrial DNA is the accidental change caused by copying errors, radiation or other mishaps.

Once a change, or mutation, becomes established in mitochondrial DNA, it gets passed on to all of that woman's descendants. Tracking mutations is a powerful way of constructing family trees. The branch points on such a family tree can also be dated with some plausibility if at least one of them can be matched to known event in the fossil record, like the parting of the human and chimpanzee lines.

The Munich team focused on a particularly variable region of mitochondrial DNA and reconstructed the Neanderthal version of it, 378 units in length. Comparing it with modern human DNA from five continents, they found it differed almost equally from all of them, signaling no special relationship with contemporary Europeans, as would have been expected if Neanderthals and modern humans interbred.

In addition, the family tree of Neanderthal mutations, when compared with those of the chimpanzee and human, yielded a distinctive pattern of variations. In the authors' interpretation, Neanderthals branched off the hominid line first, followed by humans much later.

According to the fossil and archeological record, humans and Neanderthals diverged at least 300,000 years ago. The mitochondrial DNA evidence agrees well with this date, the authors say, since individual genes would be expected to diverge before the divergence of populations.

From fossil evidence the human and chimpanzee lines are thought to have diverged some 4 million to 5 million years ago, a date that helps anchor the tree drawn from the new genetic data. The split between Neanderthal and human mitochondrial DNA, which marks the start of the split between the human and Neanderthal lineages, would have occurred between 550,000 and 690,000 years ago, the authors say, while the individual from whom all modern human mitochondrial DNA is descended, would have lived 120,000 to 150,000 years ago.

Acknowledging the uncertainty in these dates, the authors say they show at least that Neanderthal lineage is four times as old as the human lineage, as measured by mitochondrial DNA.

The Munich team's report ranges over the three treacherous fields of paleoanthropology, ancient DNA and the genetics of human evolution. Paabo has criticized many claims of ancient DNA and sought to lay out his methods with care. That was part of the reason for choosing to publish his work in Cell, a journal that specializes in rigorous molecular biology, rather than more widely read journals like Science and Nature.

Dr. Mark Stoneking of Pennsylvania State University, an expert on human evolution and a member of the Munich team, said Cell offered more space to describe the team's methods.

The interpretation of the Neanderthal mitochondrial data may be open to debate on the dating.

"Deriving these dates involves making a lot of supposition about the neutrality of the mitochondrial genome and the speed of accretion of new changes," Tattersall said.

But Ruvolo said the Munich team's methods seemed sound and its interpretation of the data was likely to be accepted.

"There could be minor quibbles over the dates but the overall properties of the tree won't change," she said, referring to the new family tree.

Other Places of Interest on the Web

The journal Cell http://www.cell.com/