Neanderthals and modern humans not only coexisted for thousands of years long ago, as anthropologists have established, but now their little secret is out: they also cohabited.
At least that is the interpretation being made by paleontologists who have examined the 24,500-year-old skeleton of a young boy discovered recently in a shallow grave in Portugal. Bred in the boy's bones seemed to be a genetic heritage part Neanderthal, part early modern Homo sapiens. He was a hybrid, they concluded, and the first strong physical evidence of interbreeding between the groups in Europe.
"This skeleton demonstrates that early modern humans and Neanderthals are not all that different," said Dr. Erik Trinkaus, a paleoanthropologist at Washington University in St. Louis. "They intermixed, interbred and produced offspring."
Although some scientists disputed the interpretation, other scientists who study human origins said in interviews last week that the findings were intriguing, probably correct and certain to provoke debate and challenges to conventional thinking about the place of Neanderthals in human evolution.
Neanderthals and modern humans presumably were more alike than different, not a separate species or even subspecies, but two groups who viewed each other as appropriate mates.
Recent DNA research had appeared to show that the two people were unrelated and had not interbred. Neanderthals lived in Europe and western Asia from 300,000 years ago until the last of them disappeared on the Iberian peninsula about 28,000 years ago. In the prevailing theory today, modern humans arose in Africa less than 200,000 years ago and appeared in great numbers in Europe, starting about 40,000 years ago.
The new discovery could, at long last, resolve the question of what happened to the Neanderthals, the stereotypical stocky, heavy-browed "cave men." They may have merged with modern humans, called Cro-Magnons, who appear to have arrived in Europe with a superior tool culture. In that case, some Neanderthal genes survive in most Europeans and people of European descent.
The skeleton of the boy, buried with strings of marine shells and painted with red ocher, was uncovered in December by Portuguese archeologists led by Dr. Joo Zilhao, director of the Institute of Archeology in Lisbon. The discovery was made in the Lapedo Valley near Leiria, 90 miles north of Lisbon.
Realizing the potential significance, Dr. Zilhao called in Dr. Trinkaus, an authority on Neanderthal paleontology, who went to Lisbon and examined the bones in January.
The boy, who was about 4 years old at death, had the prominent chin and other facial characteristics of a fully modern human. But his stocky body and short legs were those of a Neanderthal. Dr. Trinkaus compared the limb proportions with Neanderthal skeletons, including some children. He said he was then sure of the skeleton's implications.
"It's a complex mosaic, which is what you get when you have a hybrid," Dr. Trinkaus said. "This is the first definite evidence of admixture between Neanderthals and European early modern humans."
The age of the skeleton, determined by radiocarbon dating, showed that full Neanderthals had apparently been extinct for at least 4,000 years before the boy was born. "This is no love child," Dr. Trinkaus said, meaning that this was not evidence of a rare mating but a descendant of generations of Neanderthal-Cro-Magnon hybrids.
Dr. Trinkaus and Dr. Zilhao have completed a more detailed scientific report to be published soon in the Proceedings of the National Academy of Sciences. DNA tests on the skeleton have not yet been done.
Other Neanderthal specialists reacted favorably to the discovery. Dr. Fred H. Smith of Northern Illinois University in De Kalb called it "very convincing and absolutely right."
Dr. Smith noted that he had come upon other skeletal material in central Europe that raised the possibility of interbreeding between the groups. Though most scholars in the field will probably accept the possibility of interbreeding, he said, a significant number will probably not.
The more ardent exponents of the out-of-Africa hypothesis of modern human origins may be holdouts. They have argued that early modern humans all emerged from Africa and wiped out the Neanderthal population in Europe. Whether the relationship was fraternal or genocidal has been much debated. But many have argued that the two groups were distinct, with humans displacing and probably slaughtering their rivals.
Dr. Chris Stringer, an expert on Neanderthals at the Museum of Natural History in London, who is a leader of the out-of-Africa forces, said that he was willing to consider the Portuguese findings with an open mind. He told The Associated Press that the current evidence was not sufficient to convince him of Dr. Trinkhaus's hybrid interpretation.
An alternative theory, known as regional continuity, holds that the earliest human ancestors arose in Africa and spread around the world more than a million years ago. Modern humans then emerged in different regions through separate evolution and interbreeding. A leading advocate of this theory is Dr. Milford Wolpoff, a paleontologist at the University of Michigan in Ann Arbor.
"This find should be devastating to the out-of-Africa people," Dr. Wolpoff said. "It shows their theory doesn't work, at least in Europe. And it shows that fundamentally, Neanderthals are the same species we are and they contributed their genes to European ancestry."
By now, scientists said, only a small fraction of Neanderthal genes have survived, the European gene pool having been further mixed through migrations during the spread of agriculture and invasions from the east.
But Dr. Wolpoff cautioned that it would take more than one skeleton to tell the effects of interbreeding apart from ordinary evolutionary changes, the result of genes modifying in response to environmental stresses.
Dr. Alan Mann, a specialist in human evolution at the University of Pennsylvania, called the Portuguese hybrid skeleton "some of the most important data we ever got about Neanderthals in human evolution," but said he was not sure that interbreeding had been established.
Dr. Trinkaus said the discovery "refutes strict replacement models of modern human origins" and also seemed to undermine interpretations of recent DNA research. Two years ago, Dr. Svante Paabo of the University of Munich in Germany, reported that a study of the genetic material DNA from Neanderthal remains and living humans indicated that Neanderthals did not interbreed with the modern humans.
At the time, scientists said the DNA results reinforced the idea that Neanderthals were a separate species from modern humans. If the new findings are correct, though, the two groups were probably more like different races of the same species.
"The problem with the DNA research was the interpretation," Dr. Trinkaus said. "It's demonstrably wrong. All that they showed is that Neanderthal biology is outside the range of living humans, not modern Homo sapiens back then."
Dr. Alan Templeton, an evolutionary geneticist at Washington University, said that some hybridization occurs without the effects showing up, for example, in mitochondrial DNA, which is passed only through the mother. "But if you look deep enough in evolutionary time, you find a lot of interbreeding," Dr. Templeton said. "That is what humanity is all about: we interbreed a lot."