Researchers have long debated what happened when the indigenous Neanderthals of Europe met "modern humans" arriving from Africa starting some 40,000 years ago. The end result was the disappearance of the Neanderthals, but what happened during the roughly 10,000 years that the two human species shared a land?

A new review of the fossil record from that period has come up with a provocative conclusion: The two groups saw each other as kindred spirits and, when conditions were right, they mated.

How often this happened will never be known, but paleoanthropologist Erik Trinkaus says it probably occurred more often than is generally imagined.

In his latest work, published last week in the journal Proceedings of the National Academy of Sciences, Trinkaus, of Washington University in St. Louis, analyzed prehistoric fossil remains from various parts of Europe. He concluded that a significant number have attributes associated with both Neanderthals and the modern humans who replaced them.

"Given the data we now have, it would be highly improbable to argue there is no Neanderthal contribution to the early European population that came out of Africa," Trinkaus said. "I believe there was continuous breeding between the two for some period of time.

"Both groups would seem to us dirty and smelly but, cleaned up, we would understand both to be human. There's good reason to think that they did as well."

The conclusion, one of the strongest to date in this debate, remains controversial, and it has potentially broad implications. It suggests, for instance, that humans today should still have some Neanderthal genes. It also means that the unanswered question of why the Neanderthals died out is even more puzzling -- because under this scenario they were quite capable of living successfully alongside the more modern newcomers.

But Trinkaus says the fossil record is the best information available, and it increasingly points to an "admixture" theory -- that Neanderthals who had lived in Europe for about 400,000 years shared the land and, to some extent, their genes with the migrants from the south who began arriving 40,000 years ago.

As with all theories regarding the Neanderthals, there are problems with the one Trinkaus and others are advancing. So far, analysis of modern humans' DNA has turned up no identifiable Neanderthal genetic material. Instead, it points to a common East African male ancestor from about 100,000 years ago and a common East African female from 170,000 years ago. Because the sampling remains limited, evolutionary geneticists generally do not say their findings settle the matter -- although an ongoing mapping of the Neanderthal genome by European researchers may change the equation.

Chris Stringer, a top paleontology researcher at the Natural History Museum in London, said the genetic evidence has kept him "on the fence" regarding Trinkaus's theory of more widespread interbreeding. He said Neanderthals and modern humans from Africa would be considered distinct "homo" species, making interbreeding less likely but not impossible. Under stressed conditions, he said, zebras and horses will mate, as will lions and tigers, so related humans might have done the same.

But one genetic trait of modern Europeans makes him doubt there was any major Neanderthal input -- the fact that most humans today are genetically ill-adapted to cold weather. Only some native Indian populations, as well as people in the north of Eurasia and aborigines in Australia (who experience deep cold at night), have good genetic defenses to cold. Since Neanderthals lived in Europe for hundreds of thousands of years, through ice ages and frigid conditions, they would have become genetically suited to such conditions, Stringer said. The fact that Europeans are not, he added, suggests that any Neanderthal contribution to their makeup is limited.

Although Neanderthals live in the public imagination as hulking and slow-witted "Alley Oops," Trinkaus and others say there is no reason to believe they were any less intelligent than the newly arrived "modern humans." Neanderthals were stockier and had larger brows, sharper teeth and more jutting jaws, but their brain capacity appears to have been no different than that of the newcomers.

One geneticist, Bruce T. Lahn of the University of Chicago, has even proposed that Neanderthals may have provided genetic material that helped in human brain development. Lahn has been studying such genes, in particular a gene called microcephalin. A mutation of that gene can cause microcephaly, which leaves a child with a very small head and serious neurological problems.

Lahn's studies indicated that a new and more powerful version of the gene arose in modern humans about 40,000 years ago. In a paper last year, he concluded that the two microcephalin genes are so different that they must have diverged about 1 million years ago, around the time of the split between those that would become Neanderthals and the homo sapiens destined to become modern humans.

Lahn's explanation of the information: The newer and better version of the gene evolved in a separate species -- most likely Neanderthals -- and then entered the gene pool of modern humans through interbreeding around the time that modern humans reached Europe.

If Neanderthals were in some ways better suited for life in a sometimes very cold Europe, and if they contained brain capacity that may have been quite similar to that of modern humans, why did they die out?

Trinkaus says that while there is no evidence that they were any less intelligent or capable than the newcomers, they seem to have had a less evolved social structure and less ability to develop new technologies. As the number of migrants from the south increased, he said, Neanderthals were to some extent absorbed into the arriving population and to some extent were outcompeted for resources.

By the time the Neanderthals were dying about 30,000 years ago, the fossil record suggests that about 10 to 20 percent of the genetic material in European humans was from Neanderthals, he says.

Some, perhaps most, of that genetic material was selected out of the human genome in ensuing generations, but Trinkaus says there probably remains some Neanderthal in many of us.