Scientists have discovered the fossilized remains of an unusual human-like creature that lived long ago. Exactly how long ago is still a mystery — and that's not the only mystery surrounding this newfound species.
The bones have a strange mix of primitive and modern features, and were found in an even stranger place — an almost inaccessible chamber deep inside a South African cave called Rising Star.
"It is perhaps one of the best-known caves in all of South Africa," says Lee Berger, who studies human evolution at the University of the Witwatersrand, Johannesburg.
In 2013, some local cavers found some fossils inside Rising Star cave. Berger had asked them to be on the lookout, so they brought him photos.
"And there I saw something I perhaps thought I'd never see in my life," recalls Berger. "That is, clearly primitive hominin remains lying on the floor of a cave."
A jaw and a skull were just sitting there in the dirt — usually such bones are encased in rock.
Berger was excited, but he knew he personally could never reach this fossil site. To get into the cave chamber, you have to climb a steep, jagged rockfall called Dragon's Back, then wiggle through a small opening that leads to a long, narrow crack.
The crack is only about 7 1/2 inches wide, and goes down more than 30 feet. Squeezing through it is the only way to reach the chamber of bones at the bottom.
Since he couldn't go, Berger sent in his tall, skinny 16-year-old son. "When he came out after 45 minutes, he stuck his head out. And to tell you how bad I am, I didn't say: 'Are you OK?' I said: 'And?' And he says, 'Daddy, it's wonderful.' "
Berger got funding from the National Geographic Society to excavate the site. And he advertised for research assistants on Facebook — for skinny scientists who weren't claustrophobic. Six women took the job.
They worked in the chamber almost like spacewalkers, communicating with researchers outside via cameras and about 2 miles of fiber optic cable. The team in the chamber used paintbrushes and toothpicks to gently unearth fossil bones — there were more than 1,550 of them, an incredible treasure trove. The researchers describe their find Thursday in a journal called eLife.
"Often I was wondering, 'How on Earth are we going to get that fossil out?' because the density of bones in that chamber was so great, it was like a puzzle to get each fossil out," says Becca Peixotto, one of the scientist-cavers and a doctoral student in anthropology at American University.
The bones come from at least 15 individuals, says John Hawks, a paleoanthropologist from the University of Wisconsin, Madison who was on the team that studied the bones.
"We have every age group represented" among the fossils, he says. "We have newborns; we have children of almost every age; we have adults and old adults."
He says these creatures were short — less than 5 feet tall — and thin. They have a particular combination of features that has never been seen before. "It's a new species to science," says Hawks. Researchers have named it Homo naledi, because "naledi" means "star" in a local South African language.
"They have a very small brain — they are not human-like at all in their brain," Hawks says. "It's around a third the size of a human brain today."
But the creatures had feet like us, and walked in a very human-like way. Their hands were also like ours, but their fingers were more curved.
The researchers also tackled this question: How did these human-like creatures get into such a crazy spot? It looks as though the cave chamber has always been hard to reach.
There are no animal bones there, except for a handful of bits from birds and mice. There's no evidence that a carnivore dragged the human-like creatures in, or that they somehow got washed in. And there's no evidence of a mass death, such as a cave accident.
Berger believes someone had to have put the bodies there.
"Homo naledi was deliberately disposing of its dead in a repeated, ritualized fashion in this deep underground chamber," he says.
That's quite a claim — that kind of ritual has been thought to be unique to modern humans or our very close relatives.
And really, the whole discovery — from the bones to their bizarre location — has perplexed experts on human evolution.
"To be honest, I would really distrust anyone who thinks they understand what the significance of these finds is," says Bernard Wood, a paleoanthropologist at George Washington University.
Usually scientists can tell how old fossilized bones are, but in this case the geology of the cave gives no clues. The bones could be less than 100,000 years old or several million years old.
"These folks do not have an age, yet they have some remarkable fossils, and the context of them is also remarkable," says Wood. "It's not only remarkable, it's also rather weird. But nonetheless, the fossils are important. So the community is, I think, struggling to work out what it all means."
He notes that only a small section of the cave chamber has been excavated, and it looks like many more bones are down there.
"There is the potential for thousands of specimens in that cave," says Wood. "Intellectually, it's a real puzzle. And I think it's going to take scientists quite a time to get their heads around what the real significance of these discoveries is."