After years of studying a mysterious thigh bone from a cave in China, scientists said Thursday they believe it represents an ancient species of human that persisted much longer than previously thought.

The 14,000-year-old bone was uncovered in 1989 in Maludong, known as the Red Deer Cave. The trove of fossils it was initially found with went unstudied until 2012.

The partial femur, though relatively young in age, looks like the bones of far older species like Homo habilis and early Homo erectus that lived more than 1.5 million years ago, said the study in PLOS ONE.

"Its young age suggests the possibility that primitive-looking humans could have survived until very late in our evolution, but we need to careful as it is just one bone," said co-author professor Ji Xueping from the Yunnan Institute of Cultural Relics and Archaeology in China.

Until now, scientists believed that the only pre-modern humans who survived in what is now Europe and Asia were Neanderthals and Denisovans, both of which disappeared some 40,000 years ago.

They are believed to have vanished soon after modern humans entered the region, but the new bone suggests another primitive human could have survived far longer, perhaps until the end of the last Ice Age.

"The new find hints at the possibility a pre-modern species may have overlapped in time with modern humans on mainland East Asia, but the case needs to be built up slowly with more bone discoveries," said co-author Darren Curnoe from the University of New South Wales.

The thigh bone is small, with a thin shaft much like the bones of Homo habilis, which lived 1.5-2.8 million years ago.

The individual would have weighed only about 110 pounds (50 kilograms), which was quite small for pre-modern and Ice Age human standards.

- Mysterious species? -

When researchers first announced their discovery of bones in the Red Deer Cave in 2012, describing them as containing an unusual mix of modern and primitive features such as a jutting jaw, broad nose and prominent brow ridges, they were greeted with skepticism by some scientists.

The findings stoked debate over whether or not they represented a new evolutionary line.

"The new discovery once again points towards at least some of the bones from Maludong representing a mysterious pre-modern species," the research team said in a statement.

However, more work needs to be done before scientists can declare a new species of human has been found.

For now, researchers believe there may have been more different kinds of human living until very recently in southwest China than previously thought.

"The unique environment and climate of southwest China resulting from the uplift of the Tibetan Plateau may have provided a refuge for human diversity, perhaps with pre-modern groups surviving very late," Ji said.

"The riddle of the Red Deer Cave people gets even more challenging now," added Curnoe.

"Just who were these mysterious Stone Age people? Why did they survive so late? And why only in tropical southwest China?"