It's official: Neanderthals and humans didn't date

New Scientist vol 178 issue 2395 - 17 May 2003, page 14

WE ARE not partially descended from Neanderthals, a study of ancient DNA has confirmed. This settles the controversy over whether ancient humans bred with Neanderthals. If they did, it was so rare that Neanderthals failed to pass on any genetic legacy to modern people.

The results are "extremely important", says archaeologist Paul Pettitt at Britain's University of Bristol. Cro-Magnon man, the earliest known modern human to reach Europe, invaded the eastern edge of the Neanderthals' range in Western Asia around 45,000 years ago. Within 10,000 to 15,000 years Cro-Magnons had spread to Western Europe and Neanderthals were extinct. But an argument has raged over whether Neanderthal genes survived in our DNA.

The case for interbreeding was bolstered by the discovery in 1998 of a boy's skeleton in Portugal that apparently displayed both Cro-Magnon and Neanderthal features. The child lived around 24,500 years ago, long after the Neanderthals disappeared, suggesting that traits, and hence DNA, from the extinct hominids survived their elimination.

But genetic evidence began to cast doubt on the interbreeding hypothesis. Comparisons of mitochondrial DNA from Neanderthal specimens and living people suggested a definite separation. Proponents of interbreeding countered that you would expect differences after 30,000 years of evolution. The real comparison, they said, would be between modern people, Neanderthals and Cro-Magnons.

This is exactly what Giorgio Bertorelle at the University of Ferrara in Italy and his team have done. The researchers compared sequences from sixty modern Europeans, twenty non-Europeans, four Neanderthals and three Cro-Magnons - one from Australia and two from Italy. The Australian sample is around 40,000 years old (Proceedings of the National Academy of Sciences, vol 100, p 6593).

The mitochondrial DNA sequences of the Neanderthals differed from those of the modern people by between 23 and 28 base pairs per 360 tested, clearly separating them from living populations. But the Cro-Magnons' sequences were indistinguisable from modern humans'. "[The Cro-Magnons] had sequences that living individuals still have," says Bertorelle, "they are nothing to do with the Neanderthal sequences."

At least one proponent of the interbreeding idea is not convinced. Milford Wolpoff of the University of Michigan, Ann Arbor, says that the most recent Neanderthal specimen is the most similar to the human sequences. If the two lineages were evolving independently then this sequence should be the most different, he says. But Bertorelle counters that Wolpoff is arguing over one or two mutations. That is far too small a difference to draw such a conclusion from four samples.

Pettitt, on the other hand, believes the evidence is so strong it even casts doubt on the idea that the two hominids regularly came into contact. Every colonisation in history has ended with the conquerors and conquered interbreeding, he points out. If there was no interbreeding there may not have been much contact, he speculates.