A 14,000-year-old thigh bone may upend human history.

Unearthed in southwest China, this femur resembles those of an ancient species of humans thought to be long extinct by the Late Pleistocene, scientists say. The scientists compare the leg bone to ancient and modern human femurs in a paper published Thursday in the journal PLOS ONE, arguing that this specimen represents a population of ancient humans that lived surprisingly recently.

If they're right, this could dramatically change the way we see human history.

Today, our species, Homo sapiens, are the only humans to walk the Earth. But it hasn't always been that way.

At times, ancient human species, like Neanderthals, Denisovans, H. erectus, and H. habilis, overlapped. Some even intermingled with our own species, as Denisovan genes show up in some modern humans living today.

Scientists thought that the last time there was more than one species of human on Earth was tens of thousands of years ago. One of our closest cousins, Neanderthals, for example, are thought to have died out about 40,000 years ago.

"Until now, it was thought that archaic humans on mainland Asia had survived no later than around 100,000 years ago," study author Darren Curnoe tells the Monitor in an email. "So, to find a human bone that resembles very ancient humans that is only around 14,000 years old is a real surprise."

"Now, it is only one bone, so we need to be a bit careful," Dr. Curnoe says. But if it does represent these ancient humans, "there must also have been overlap in time between archaic and modern humans for tens of thousands of years in Southwest China."

David Begun, a paleoanthropologist at the University of Toronto who is not affiliated with the study, tells the Monitor in an interview, "I'm not convinced."

"To me, it's just a Late Pleistocene, Early Holocene population that just looks a little bit different, that really doesn't have anything especially archaic about it," Dr. Begun says. "I certainly don't buy the argument that it is some kind of holdover from an Early Pleistocene, early Homo lineage, pre-Neanderthal or something like that. I'm not convinced by the evidence at all."

So what was Curnoe and his colleagues' evidence in the first place?

The scientists analyzed the femur by measuring and comparing physical features on the bone with both ancient and modern specimens.

Discovered among other fossils in Maludong, also known as Red Deer Cave, the femur "is very small; the shaft is narrow, with the outer layer of the shaft (or cortex) very thin; the walls of the shaft are reinforced (or buttressed) in areas of high strain; the femur neck is long; and the place of muscle attachment for the primary flexor muscle of the hip (the lesser trochanter) is very large and faces strongly backwards," Curnoe says.

By looking at measurements and traits of the bone, he says, "we found a clear association between the femur and the bones of the earliest members of the human genus Homo."

But Begun says the leg bone is too fragmentary to say all that. "It lacks most of what you would want to have in a femur to really say something about it," he says. "You'd want to have the head of the femur, the hip joint itself, and that's not here. It only preserves about a third of the length of the femur."

The specimen also shows a lot of damage, Begun says. "Because of how fragmentary the specimen is and how damaged it is, I'm not convinced that the measurements really tell us much."

This isn't the first specimen from Maludong the team has described and named as a member of an ancient human species. In 2012, they published a paper on skulls found at the same site, suggesting the same thing – that these fossils represent a surprising population of ancient humans.

To survive so recently, this group of people would have likely been an isolated population.

The region where the bones were found is unique, Curnoe explains. Tectonic uplift created the Qinghai-Tibetan Plateau and the area is also quite tropical.

So, Curnoe says, "The Maludong femur might therefore represent a relic, tropically adapted, archaic population that survived relatively late in this biogeographically complex, highly diverse and largely isolated region."

The Maludong specimen isn't the first that scientists have claimed is more recent evidence of ancient humans. Homo florensiensis, nicknamed "Hobbit" for its short stature, was found to have lived on the Island of Flores in Indonesia as late as 17,000 years ago.

"Honestly, it's not the same kind of situation as we have in Flores," Begun says. "It's just not the same thing because the archaic signal, the primitive signal is just not very clearly developed."

"I could be wrong," Begun admits. "But frankly, I'm not convinced."

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"Without the more diagnostic parts of the bone, like the head of the femur and a complete neck and more of the shaft," he says, "it's just very very difficult to say anything about a specimen like that."

But Curnoe is unfazed by such a reaction. "Our work is bound to receive a mixed reaction because for some of our colleagues the idea that archaic humans could have survived until the end of the Ice Age in East Asia will be difficult to accept," he says. "There is simply no convincing some, regardless of what we might have found."