DNA extracted from the fossilised toe of a 50,000 year-old Siberian Neanderthal woman has revealed that she was the child of two closely related parents.
The findings, published in the scientific journal Nature, also indicate incest was not uncommon among Neanderthals, modern humans' nearest extinct relatives.
Discovered by Russian archaeologists in a Siberian cave in 2010, the toe bone allowed scientists to extract, sequence and analyse the woman's genome.
After running the information through a series of simulated inbreeding scenarios, the researchers established her parents were either half-siblings who shared the same mother or double first cousins. The genetic clues also raised the possibility of her parents coming from different generations of the same extended family.
One of the researchers, University of California at Berkley population geneticist Montgomery Slatkin, said this could include an aunt and nephew, an uncle and niece, a grandfather and granddaughter or a grandmother and grandson.
Deputy director of the University of Adelaide's Australian centre for ancient DNA Jeremy Austin said given the odds of randomly finding the only Neanderthal bone with evidence of incest was slim, this toe bone likely told a common story of inbreeding.
He said the woman lived close to the period when Neanderthals went extinct and the shrinking population would have offered limited partnering opportunities.
"On that basis, it's not particularly surprising," Professor Austin said. "You can imagine they would have been living in very small and isolated family groups."
Led by researchers from Germany's Max Planck Institute of Evolutionary Anthropology, the international team analysed a high quality genome sequence of the woman. The genome sequencing also suggested low levels of genetic diversity, which is an effect of having a small population which wasn't very mobile.
The genetic information extracted from the toe also revealed frequent interbreeding between Neanderthals and the lesser known cousins of modern man, the Denisovans.
To intrigue things further there was also suggestions of another group of as yet unnamed human relatives. Kay Prufer from the Max Planck Institute of Evolutionary Anthropology said it was possible this was Homo erectus, but that it could only be confirmed by further studies.
For Professor Austin, this was more interesting than the incest finding.
"This new DNA sequence from this toe bone suggests that Denisovans and Neanderthals were also interbreeding," he said.
Previously scientists had been able to establish breeding had occurred between Neanderthals and modern humans and between Denisovans and modern humans.