"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.

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.

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.

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 Tianyuan specimen shows several signs of disease. The individual had lost a number of their 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 the person 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.

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