"But what we found," said Zollikofer, a paleoanthropologist, "was [that children] had the same differences as adults" of their respective populations.
In juveniles, environmental factors such as physical activity by the individuals couldn't explain the occurrence of such differences. Therefore, he concluded, "the differences were deeply genetic."
The two Swiss researchers studied 16 Neandertal fossils from across the ancient group's geographical range. One specimen came from Gibraltar, off the coast of Spain, and others from as far east as Uzbekistan, in central Asia. Several more came from Israel, but political difficulties left one well-preserved specimen in Iraq off limits.
The scientists spent eight years traveling to study the fossils, which also required packing them carefully in plaster casts and shipping them back to Switzerland for additional examination.
They used a computer-modeling technique to compare these Neandertal fossils with those of modern humans. The latter group of fossils represented people of many ethnicities and included some ancient individuals who could have passed unnoticed among people today.
Eleven of the Neandertal specimens examined were "immature," meaning that not all of the individuals' adult teeth had grown in. That enabled Ponce de León and Zollikofer to reconstruct how different parts of the faces of Neandertals and modern humans changed as both groups grew older.
New Look at Juveniles
About half of the fossils of Neandertals and ancient humans that anthropologists have found are of juveniles. "It's a demographic fact that 50 percent of [those ancient] populations died before reaching maturity," said Zollikofer.
In past scientific studies, the remains of children were usually excluded from the sample because they were assumed not to reflect the developmental patterns that could reveal differences between the adults of two species. Ponce de León and Zollikofer, however, have found that even young members of a species exhibit full-blown growth patterns parallel to those of adults in the groups.
The new research contradicts the claims made by discoverers of a hotly debated fossil find in Portugal's Lapedo Valley two years ago. The so-called Lagar Velho fossil, of a young child who died 24,500 years ago, appears to be Neandertal, but has distinct characteristics of a modern human.
Its discoverers, who included João Zilhão, director-general of the Portuguese Institute of Archaeology, saw it as evidence that humans and Neandertals interbred and produced hybrid offspring. In a paper he wrote about the fossil last year, Zilhão observed: "Neandertals were just peopleperhaps a little funny-looking, but people nonetheless."
But most anthropologists still disagree with that conclusion, and Zollikofer's evidence bolsters the argument that humans and Neandertals did not interbreed, and probably were not biologically capable of doing so.
Nevertheless, the Lagar Velho specimen is "amazingly interesting," said Zollikofer. He and Ponce de León have joined a large effortled by Zilhão, another Portuguese anthropologist, and Erik Trinkhaus of Washington Universityto publish a book describing the Lagar Velho find and exploring its implications for our understanding of human evolution.
Zollikofer plans to study the Lagar Velho fossil using the same computer-aided technique he applied to the Neandertal specimens. Determining whether the head and facial growth patterns of the child are more characteristic of Neandertals or of modern humans may help answer uncertainties about its alleged mixed parentage.
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