Munich

Skull school: researchers in Italy have sparked a row over DNA studies of human evolution. Credit: G. BERTORELLE

When Giorgio Bertorelle published his latest findings on ancient mitochondrial DNA earlier this month, he claimed that his work added yet more weight to the idea that modern humans evolved independently of Neanderthals. But his paper may have a different significance — experts say that the techniques involved, which are also used by other groups, are dogged by problems so fundamental that they can never be used to draw conclusions about the evolution of modern humans.

Bertorelle, based at the University of Ferrara in Italy, claimed to have extracted mitochondrial DNA, which evolves independently of chromosomal DNA, from the bones of Cro-Magnons, Homo sapiens from around 25,000 years ago. The extracted sequences are strikingly similar to modern human DNA, and are very different from those of Neanderthal specimens (D. Caramelli et al. Proc. Natl Acad. Sci. USA 100, 6593–6597; 2003).

But experts question whether Bertorelle, and others who are attempting similar experiments, can rule out contamination with material from modern humans. “Cro-Magnon DNA is so similar to modern human DNA that there is no way to say whether what has been seen is real,” says Svante Pääbo, a director at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

Alan Cooper, an evolutionary anthropologist at the University of Oxford, UK, published the accepted standards of contamination avoidance in 2000 (A. Cooper and H. N. Poinar Science 289, 1139; 2000). He has deliberately contaminated teeth and bones with viral DNA by soaking them in liquid containing the virus. Cooper says that not even extreme decontamination methods, such as application of acid or bleach, can eradicate such DNA completely.

Bertorelle agrees that it is impossible to rule out contamination, but says he used all of the standard precautions in his study. “We repeated the procedure on a bone from a horse taken from the same layer of the site as the human bones came from,” he says. This bone yielded only horse DNA, he notes, whereas it would also have yielded human DNA if contamination had been a problem.

But such controls cannot definitively rule out contamination of the Cro-Magnon sample. Researchers such as Pääbo and Cooper see no way to resolve the issue, and are now increasingly questioning the value of studies such as Bertorelle's. “We know that contamination is almost impossible to avoid,” laments Cooper.