A newly discovered leg bone fossil suggests one of our ancient ancestors survived longer than previously believed, and may have even existed alongside modern humans into the Ice Age.
Found in Red Deer Cave, southwest China, the partial femur is similar to those of some early human species commonly thought to have disappeared during the Late Pleistocene.
Researchers argue, however, that a comparison between the discovered fossil and other ancient femurs, as well as those of modern humans, could challenge current notions of human evolution.
Professor Ji Xueping from Yunnan Institute of Cultural Relics and Archaeology says their discovery could represent a population of some ancient humans who survived much more recently than had been thought possible. Yet it isn't safe to arrive at a conclusion just yet, as the discovery only includes one bone, he adds.
Although dated to just 14,000 years ago, the bone fragment shows similarities to those from species such as Homo habilis or Homo erectus, who first walked on the Earth more than 1.5 million years ago.
Darren Curnoe, study co-author and associate professor at the University of New South Wales, says it has long been assumed that the last time more than just one species of human trod the Earth around 100,000 years ago. Hence, it is surprising to find a 14,000-year-old pre-modern human bone that is comparable to ancient, primitive human bones, he adds.
"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," he explains.
The partial femur from China shows a number of characteristics that link it to the most ancient members of the human evolutionary chain, the researchers explain; it is small, with a narrow shaft, and the outer layer of the shaft is very thin.
Measurements and traits of the bone suggest a "clear association between the femur and the bones of the earliest members of the human genus Homo," Curnoe argues.
Not all experts in the field agree, however. University of Toronto paleoanthropoligist David Begun, who was not involved in the study, says he is 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," he 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."
The researchers believe their discovery represents a mysterious pre-modern human species, or a possibility that the unique environment in southwest China may have resulted in the diversity of pre-modern species. However, they also believe more work needs to be done as the Red Deer Cave fossils still have many stories to uncover.
The study was published in the journal PLOS One.