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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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Did your ancestor date a Neanderthal?

And if so where and when?

My latest Mind and Matter column discusses the debate about how non-Africans got their 1-4% Neanderthal DNA:

So did we or didn't we? Last week saw the publication of two new papers with diametrically opposed conclusions about whether non-African people have Neanderthal-human hybrids among their ancestors-a result of at least some interspecies dalliance in the distant past.

That non-Africans share 1% to 4% of their genomes with Neanderthals is not in doubt, thanks to the pioneering work of paleo-geneticists led by the Max Planck Institute's Svante Paabo. At issue is how to interpret that fact. Dr. Paabo originally recognized that there are two possible explanations, hybridization (which got all the press) or "population substructure."

The second explanation goes like this. Around 350,000 years ago, when the ancestors of Neanderthals first left Africa and began to populate Eurasia, the African population from which they came would not have been genetically homogeneous. The early Neanderthals would therefore have been more closely related to some of their African cousins than to others.

That would have continued to be true for long afterward, because the genes of northern Africans would not have mixed fully with those of southern Africans. It might still have been true when the ancestors of modern Eurasians left Africa, about 65,000 years ago. In other words, northern Africans might still have had genetic similarities to Neanderthals, compared with southern Africans, left over from more than 350,000 years ago-and they might have brought that genetic similarity with them to Europe.

It is this argument that Dr. Andrea Manica and Dr. Anders Eriksson have tested with mathematical modeling and found plausible. The model uses over 100 populations in Africa to represent a diversity of geographical races. "Hopefully," says Dr. Manica, "Everyone will become more cautious before invoking hybridization, and start taking into account that ancient populations differed from each other probably as much as modern populations do."

Dr. Paabo, however, along with Dr. David Reich of Harvard University and others, has now come to the opposite conclusion. Measuring the length of genetic sequences that remain intact in the DNA shared by modern people and Neanderthals, these researchers infer that "the last gene flow from Neanderthals into Europeans likely occurred 37,000-86,000 years ago, and most likely 47,000-65,000 years ago." That is much too late for the substructure theory: Neanderthals were a separate species long before then. But because this time frame matches the period of modern people's exodus from Africa, it strongly supports the notion of recent interbreeding.

Moreover, as Drs. Paabo and Reich point out, there is intriguing archeological evidence for where and when the interbreeding might have happened. Around 100,000 years ago, modern humans occupied caves in what is now Israel. By 70,000 years ago, in cooler times, the Neanderthals were in the area. By 50,000 years ago, the moderns had come to the Middle East to stay. These long epochs of to-and-fro migration must have given opportunities for sexual overlap.

The population-substructure argument also struggles to explain an even larger genetic overlap between yet another extinct human form, the "Denisovans," and people from Southeast Asia and Australasia, presumably originating from an eastern exodus out of Africa. With the addition of this case, the substructure argument requires two different, distinct human groups with different samples of pre-modern-human genetic material (Neanderthal and Denisovan) to have migrated at different times-and to have ended up in the same parts of the world as ancient populations whose genes they happen to share.

It is probably more plausible (and newsworthy) to imagine that when modern people spread around the Indian Ocean, they too encountered a distantly related human species and dallied with them under the palms.