Scientists say they've found the remains of a prehistoric female whose mother was a Neanderthal and whose father belonged to another extinct group of human relatives known as Denisovans.

The 90,000-year-old bone fragment found in southern Siberia marks the first time a direct offspring of these two groups has been discovered, according to a study published Wednesday in the journal Nature.

Both groups disappeared by about 40,000 years ago. Neanderthals lived in Europe and Asia, while fossils of Denisovans are known only from the cave where the fragment was found.

Past genetic studies have shown interbreeding between the two groups, as well as with our own species, which left a trace in the DNA of today's people. But the new study is the first to identify a first-generation child with Neanderthal and Denisovan parents.

"It's fascinating to find direct evidence of this mixing going on," said Svante Paabo, one of the study's lead authors.

Paabo said he was surprised by the discovery, given how relatively few remains of our evolutionary relatives have been found around the world.

The cave near Mongolia where the bone was found contains some remains attributed to Neanderthals as well as Denisovans. But finding an actual offspring of the two groups — which are more different from each other than any two present-day human groups — seemed like a rare stroke of luck, Paabo said.

"The fact that we stumbled across this makes you wonder if the mixing wasn't quite frequent," said Paabo, a geneticist at Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

The finding doesn't reveal how often such mating occurred and where, said Ron Pinhasi, a physical anthropologist at the University of Vienna who wasn't involved in the study.

"Had it happened frequently, we would not have such divergence between the Denisovans and Neanderthal genomes," he said.

The newly discovered DNA could be interpreted in different ways, said Anders Eriksson, evolutionary population geneticist at King's College London who wasn't involved in the study.

"I think they convincingly showed that genetically this individual falls halfway between the Neanderthal and Denisovan fossils found in the same cave," he said. "But I'm less convinced that it is necessarily a first-generation offspring of a union between Neanderthal and Denisovan."

The fossil could instead have come from a population with roughly an equal mix of Neanderthal and Denisovan ancestry, he said. It will take analysis of more fossils to find out, he said.

The researchers said the small bone fragment likely came from the arm or leg of a female who was at least 13 years old at the time of death. Comparison with other ancient DNA showed that the genes she inherited from her mother were more closely related to Neanderthals who lived later in Europe than to other Neanderthal remains found in the cave, suggesting a wave of westward migration.

Together with previously discovered remains of a Homo sapiens that had a Neanderthal ancestor four to six generations earlier, Paabo said the latest find supports the theory that the now-extinct ancient lineages may have been absorbed through interbreeding with modern humans, rather than wiped out through warfare as is widely believed.