Dec. 4, 2007 -- Did modern humans interbreed with Neanderthals and, if so, did the mating result in a half-human, half-Neanderthal hybrid ? The answer is possibly yes to the interbreeding but no to the hybrid, according to the authors of a new study that is already making waves among leading anthropologists.
At the center of the study, published in the Journal of Human Evolution, and the current debate, is a 29,000-year-old Romanian skull that is one of the oldest fossils in Europe with modern human features. But those features aren't quite a perfect match with us, which has led some experts to suspect it was a cross between a Neanderthal and a modern human.
That's not so, according to Katerina Harvati, and her colleagues Philipp Gunz and Dan Grigorescu.
"It differs from living people only in subtle ways, and always well within the range of modern human variation," Harvati, who led the recent study, told Discovery News.
"It has, for instance, slightly heavier eyebrows than the average person, and is generally somewhat more robust than average," she added, explaining that modern humans have gradually evolved to become more slight and slender than Upper Paleolithic people were.
Harvati is a senior researcher in the Department of Human Evolution at the Max Planck Institute for Evolutionary Anthropology and is an adjunct associate professor of anthropology at City University of New York Graduate School.
She and her team took detailed 3-D measurements of the Romanian skull, called Cioclovina calvaria, and compared these with a similar head shape analysis of Neanderthals, modern humans and fossils of other hominids found in Europe, Africa and countries bordering the eastern Mediterranean Sea.
The researchers also studied animal hybrids and developed an unprecedented list of proposed criteria for evaluating whether or not a fossil specimen is a hybrid.
The criteria include the following: greater or much smaller size than the parental species, on average; evidence for developmental instability; possible occurrence of rare attributes, such as having extra teeth or bone joints; and possessing an intermediate shape.
"Cioclovina did not meet any of these criteria -- a strong refutation of the hypothesis that it represents a hybrid," Harvati said.
The scientists support the Single Origin model of human evolution. This holds that modern humans evolved between 200,000 and 100,000 years ago in a single location -- mostly likely Africa -- with subsequent migration displacing archaic hominid populations, including Neanderthals, around the world.
The researchers, however, do not rule out that interbreeding may have taken place.
"Modern humans and Neanderthals are very closely related species, so it is possible that, like living closely related species of primates today, they could have interbred to a limited degree," she said, adding that if it occurred, "it was probably a rare event and the result was not significant in evolutionary terms."
Ian Tattersall, curator in the Division of Anthropology of the American Museum of Natural History in New York City, told Discovery News that he is "thoroughly in agreement" with the new study, and that "the strenuous search for a Neanderthal/modern human hybrid has yet to turn up any evidence of such a thing."
Eric Delson, chairman of the Department of Anthropology at Lehman College, City University New York, also supports the conclusions.
"The results that Dr. Harvati and colleagues obtained on the Cioclovina cranium agree well with the widespread opinion that Neanderthal-modern hybridization was rare at best in Europe," he said.
Delson added that, when combined with recent genetic studies that have found "indications of low to nonexistent" levels of Neanderthal genetic imprinting on modern humans, the new findings lead "us to reject widespread hybridization and thus a major influence of Neanderthals on later human populations in Europe."
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