Gene test probes Neandertal origins - Brief Article

Science News, July 8, 2000 by B. Bower

A technique for discerning the similarity of DNA specimens supports earlier genetic evidence that Neandertals were a dead-end species, a new study finds.

Neandertal DNA exhibits substantial genetic differences from the DNA of both ancient Homo sapiens and modern humans, reports a team led by Lutz Bachmann of the Field Museum in Chicago.

"These data support the hypothesis that Neandertals were not ancestors of anatomically modern [humans]," the scientists assert in the June AMERICAN JOURNAL OF HUMAN GENETICS.

Bachmann's team examined DNA from Neandertal fossils from between 110,000 and 50,000 years ago and a H. sapiens fossil from about 35,000 years ago.

The group relied on a standard method for comparing genomes. For each of the fossils, the researchers measured the extent to which the nuclear DNA chemically bonded to nuclear DNA samples from the fossils, modern humans, and chimpanzees. Strong bonds between DNA samples signify a close evolutionary relationship.

Compared with the two Neandertal samples, DNA from the early H. sapiens fossil exhibited a markedly different pattern of binding strengths to the set of samples, the researchers say.

Chimpanzee DNA, however, elicited roughly equal amounts of binding from the Neandertal, early H. sapiens, and modern-human DNA samples, they add.

Differences in the binding propensities of DNA from Neandertal and H. sapiens may reflect contrasts in the length of repeated nucleotide sequences between the species, the investigators theorize. The variations probably evolved rapidly, they argue.

Anthropologist Erik Trinkaus of Washington University in St. Louis views the new genetic findings as inconclusive regarding Neandertals' evolutionary status. This measure, he says, provides a rougher estimate of genetic similarity than studies that compare specific mitochondrial DNA nucleotide sequences of Neandertals and modern humans (SN: 2/6/99, p. 88).

"[Bachmann's group] used a crude measure of DNA differences," Trinkaus says. "How big of a genetic difference is needed to discriminate between two species?" he asks. "Answers to that question are very subjective."

Trinkaus argues that fossil evidence shows signs of considerable interbreeding between Neandertals and modern humans (SN: 5/8/99, p. 295). Genetic remnants of that interbreeding may have diminished enough over time to escape the notice of current DNA probes, he asserts.