As firelight flickered on the back of the cave, a group of people ate deer, porcupine and otter. Then one of the group solemnly took a large bone off the fire, broke it in half, sucked the bone marrow out, then carefully painted the broken bone with red clay and buried it in the cave. This ritual was observed because this bone belonged to another human species. One they shared not only the forest with, but also their beds.
This is the remarkable – though so far tentative – picture emerging from controversial discoveries from two caves in south-west China. If true, some think it could overturn our understanding of what it means to be human.
Among the discoveries appears to be a primitive human species which most closely resembles the earliest human species, Homo habilis and Homo erectus.
But while these lived about 2 million years ago, this new species lived just 14,000 years ago, says UNSW’s Darren Curnoe, who led the team behind the discoveries. This would make it the most recent human species to have gone extinct.
“If true, this would be rather spectacular and it would make the finds of truly global importance,” says Michael Petraglia at the University of Oxford, who wasn’t involved in the discoveries. The work is excellent, he says, but is likely to leave many in the field unconvinced.
One of the most exciting pieces of evidence in the story is a hominin femur found in Maludong cave in south-west China, alongside other human and animal bones. It shows evidence of having been burned in a fire that was used for cooking other meat, and has marks consistent with it being butchered for consumption.
It has also been broken in a way that is often used to access the bone marrow.
It’s hard to know if the bone was actually cannibalised by the H. sapiens whose remains have also been found in the area.
Unusually, it had been painted with a red clay called ochre, something often associated with burial rituals. While many other bones were eaten in the cave, only the ones from human species were painted.
It’s hard to know if the bone was actually cannibalised by the H. sapiens whose remains have also been found in the area, Curnoe says, but all the evidence points towards that conclusion.
“We don’t know it was cannibalism,” he says. “We’ve got cut marks that would be consistent with butchering.”
But things got interesting when the team tried to identify the bone. “Our work shows clearly that the femur resembles archaic humans,” Curnoe says. Yet the sediment the bone was found in dated to just 14,000 years ago.
The shaft of the bone is very narrow and it has a thin outer layer, yet the walls are reinforced in areas of high strain. There is also a notch where muscle would have joined the bone which is much larger than in anatomically modern humans, and it faces more towards the back of the bone.
“These features suggest it walked differently,” says Curnoe. And judging by the size of the bone, Curnoe estimates the adult human would have weighed about 50 kilograms – much smaller than other known Ice Age humans.
“When you put all the evidence together, the femur comes out quite clearly resembling the early members of Homo,” says Curnoe.
If confirmed, says Petraglia, this would change our understanding of human evolution.
Besides Homo floresiensis, also known as “the Hobbit”, which was confined to an Indonesian island up to around 18,000 years ago, the most recent archaic humans were thought to be the Denisovans and Neanderthals, which became extinct quickly after H. sapiens came through their lands some 40,000 years ago.
“This turns that on its head,” says Curnoe. “Its young age shows that remarkably primitive-looking humans must have shared the landscape with very modern-looking people at a time when China’s earliest farming cultures were beginning to flourish.”
But some in the field have doubts that such a young bone can be from something so archaic. “It is not an archaic human,” says Erik Trinkaus at Washington University in St Louis. Trinkaus thinks the differences in the bone are a result of natural variation within a population, not a new species.
Henry McHenry at the University of California, Davis, is more ambivalent. He says the femur looks very odd, but that it does seem to have similarities to very archaic humans.
The second cave
Further supporting evidence might come from Longlin cave, a few hundred kilometres north, where another stash of human bones, including an almost complete skull, were found – some as early as 1979. Curnoe and Ji Xueping at the Yunnan Institute of Cultural Relics and Archaeology in China reanalysed these bones and dug up more, describing them in 2012.
Curnoe and his colleagues analysed skull and facial bones and argue they belong to a hybrid of our own species and something more archaic – quite likely the creature that once walked on the now-painted femur. They have preliminarily dated that hybrid to just 10,500 years ago.
One of the less complete bones found at Maludong cave had been cut and had holes dug near the top of it, suggesting it was used as a vessel for carrying and drinking liquid.
What all this hints at, Curnoe and colleagues say, is that H. sapiens was mating with an archaic human species, possibly eating them, and using the hybrid offspring bones as tools. But to back up these controversial claims, we will need DNA from these bones, says Petraglia.
“Ultimately, what we’d like is DNA evidence,” agrees Curnoe, “But so far we’ve had no luck.”
The burning of many of the bones and the tropical climate have degraded the DNA. He says technology has improved, though, and they will continue trying.
DNA could answer some of the thorniest questions raised by these findings. Is the archaic human they found the mysterious Denisovan, so far only known from a finger and a tooth found in a Siberian cave? Or is it a new species, suggesting a rich array of human species existed in Asia at this time? And do we carry any of their genes?
Curnoe says the discoveries point to a profound shift in our understanding of what it means to be human. Adding to the now well-established interbreeding that occurred with our cousins the Neanderthals and Denisovans, we can no longer consider ourselves a single lineage that emerged from Africa.
“We had particular notions of our evolution: that we found ourselves evolving in isolation in Africa and quickly replaced all the other species that were around because we thought we were superior to them. And it happened very quickly, without question, and without biological interaction,” Curnoe says. “But the hybridisation story has turned that on its head.”
Michael Slezak is the Guardian Australia’s environment reporter and previously New Scientist magazine’s correspondent in Australasia. He will edit next year’s edition of The Best Australian Science Writing (published by UNSW Press/NewSouth). This article was first published in New Scientist in December 2015, under the original title ‘New species of human may have shared our caves – and beds’.