From New Scientist:

Stone Age toe could redraw human family tree

10 August 2011 by Colin Barras

ON THE western fringes of Siberia, the Stone Age Denisova cave has surrendered precious treasure: a toe bone that could shed light on early humans’ promiscuous relations with their hominin cousins.

New Scientist has learned that the bone is now in the care of Svante Pääbo at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who revealed the first genetic evidence of interbreeding between ancient humans and other hominins (New Scientist, 30 July, p 34).

There are tantalising hints that the find strengthens the case for a third major group of hominins circulating in Eurasia at the same time as early humans and the Neanderthals. It might possibly even prove all three groups were interbreeding (see diagram).

The Denisova cave had already yielded a fossil tooth and finger bone, in 2000 and 2008. Last year, Pääbo’s DNA analysis suggested both belonged to a previously unknown group of hominins, the Denisovans. The new bone, an extremely rare find, looks likely to belong to the same group.

It is a very exciting discovery, says Isabelle De Groote at London’s Natural History Museum. “Hominin material from southern Siberia is rare and usually extremely fragmentary.”

The primitive morphology of the 30,000 to 50,000-year-old Denisovan finger bone and tooth indicates that Denisovans separated from the Neanderthals roughly 300,000 years ago. At the time of the analysis, Pääbo speculated that they came to occupy large parts of east Asia at a time when Europe and western Asia were dominated by Neanderthals. By 40,000 years ago, Homo sapiens was also moving around much of the region. But the Denisovans remain known only from the finger and tooth fossils - not enough information to formally assign them to their own species.

That may change with analysis of the newly discovered toe bone.