The remains of one of the earliest modern humans to inhabit eastern Asia have been unearthed in a cave in China. The find could shed light on how our ancestors colonised the East, a movement that is only poorly understood by anthropologists.
Researchers found 34 bone fragments belonging to a single individual at the Tianyuan Cave, near Beijing.
Details of the discovery appear in Proceedings of the National Academy of Sciences journal.
Radiocarbon dates, obtained directly from the bones, show the person lived between 42,000 and 39,000 years ago.
"For this time period, which is critical for understanding the spread of modern humans around the world, we have two well-dated human fossils from eastern Asia," said co-author Professor Erik Trinkaus, from Washington University in St Louis, US.
"We have remains from the Niah Cave from Sarawak on Borneo, and now this specimen from China. As you go west, the next specimens are from Lebanon. There's nothing in between."
According to the "Out of Africa" theory, modern humans (Homo sapiens) evolved in East Africa and then spread out across the globe about 70,000 years ago, replacing earlier, or archaic, human populations, such as the Neanderthals, with very little, if any, interbreeding.
The Tianyuan remains display diagnostic features of modern H. sapiens. But co-author Erik Trinkaus and his colleagues argue, controversially, that the bones also display features characteristic of earlier human species, such as relatively large front teeth.
The most likely explanation, they argue, is interbreeding between early modern humans emerging from Africa and the archaic populations they encountered in Europe and Asia.
"The pattern we see across the Old World is basically a modern human in terms of its newly emerged characteristics, but also a minority of traits that are absent or lost in the earliest modern humans in East Africa," Professor Trinkaus told the BBC News website.
"The question is where did they get them from? Either they re-evolved them, which is not very likely, or, to some degree, they interbred with archaic groups.
"Sex happens. I find this neither disturbing nor surprising."
He added that evidence from the animal world suggested two closely related species, which have been separate for less than two million years, could interbreed successfully when given the opportunity to mate.
One example from the UK is the Scottish wildcat, which is being absorbed into domestic cat populations through interbreeding.
The domestic cat and the wildcat are distinct species separated by hundreds of thousands, if not millions, of years, and have very different body sizes. Despite this, pairings produce fertile, viable offspring.
Signs of disease
The view of interbreeding between Homo sapiens and archaic humans is controversial. Other palaeoanthropologists say that some of these features are simply retained from ancient African ancestors.
And most genetic evidence gathered from present-day humans does not appear to support significant interbreeding between modern humans from Africa and archaics.
The researchers' analysis of the bones has revealed several interesting details about the Tianyuan individual's lifestyle.
The person's age at death was estimated by how much the teeth had worn down. This put the individual in their late 40s or 50s.
But the lack of a pelvis among the remains means that it is not possible to say with any certainty what sex the human was.
The Tianyuan specimen shows several signs of disease. The individual had lost a number of teeth before death, not unusual considering their age.
The researchers also identified several lesions, or growths, on the leg bones, which appear to have been caused by a condition affecting the muscle attachments around both knees.
Whatever condition these were caused by, however, it does not appear to have disabled the person, because the remainder of the leg bones suggest they kept active.
The single toe bone which was unearthed seems to suggest the individual wore shoes, pushing back the earliest known evidence for footwear by about 10,000 years.
An earlier study by Professor Trinkaus shows that human small toes became weaker during the stage of prehistory known as the Upper Palaeolithic, and that this can probably be attributed to the adoption of sturdy shoes.
The invention of rugged shoes reduced humans' reliance on strong, flexile toes to grip and balance.