An ancient femur found in a Chinese cave is unlike any bone formerly discovered, suggesting it belonged to a previously unknown human species that lived alongside modern man just 14,000 years ago.

The distinctive shape of the bone indicates that the species would have walked differently from humans today, according to the New Scientist. And based on the size of the bone, the scientists behind the analysis report in their paper, published in PLOS One in December 2015, that an adult would have weighed 50kg, which is far smaller than other humans who lived at the time.

“When you put all the evidence together the femur comes out quite clearly resembling the early members of Homo,” Darren Curnoe, from the University of New South Wales in Sydney, Australia, who led the research team, told the New Scientist.

But while the earliest homo species lived around 2 million years ago and more recent humans, such as Neanderthals, became extinct some 40,000 years ago, researchers believe the newly discovered species would have lived far more recently, and alongside modern humans.

The 14,000-year-old bone fragment, which was found in the Muladong Cave in southwestern China in 1989 but was not studied for 25 years, has been painted with red clay, which is indicative of burial rituals. It seems to have been broken in a way that allows access to the bone marrow, and shows evidence of being butchered and being burned in a fire alongside other meat, according to the New Scientist.

Curnoe believes that homo sapiens mated with this newfound species, and possibly also ate them and used their bones as tools.

The theory is supported by bones discovered in Longlin cave in Guangxi Province in 1979, which Curnoe examined in 2012. These fossils were dated to 11,500 years ago and are thought to show a combination of homo sapiens traits and those of an archaic human.

Our understanding of mankind’s evolutionary roots is far from clear-cut, and this finding could indicate a new branch in human development. “If true, this would be rather spectacular and it would make the finds of truly global importance,” Michael Petraglia, co-director of the Centre for Asian Archeology at the University of Oxford, who wasn’t involved in the work, told the New Scientist.

But other paleoanthropologists believe the bone’s distinctive features come from variations within the species, rather than a distinct species. Chris Stringer, head of research into human origins at the Natural History Museum in London, told the Guardian that he is “cautious” about the discovery. “It is an isolated bone. It is not even half a femur,” he said.

Curnoe is hoping to extract DNA from the bones, to build evidence about the fossil’s evolutionary origins. For now, he told the Guardian, the discovery raises many more questions to be answered.

“The riddle of the Red Deer Cave people gets even more challenging now: Just who were these mysterious stone age people? Why did they survive so late? And why only in tropical southwest China?,” said Curnoe.

But the discovery wouldn’t be the first time scientists discovered the complexities of humans’ evolutionary roots. Earlier this year, researchers discovered 15 skeletons belonging to a new species of ancient human in South Africa. The homo sapiens evolutionary tree is starting to get crowded.