Deep inside the Rising Star cave system, 30 miles northwest of Johannesburg, South Africa, there's a hidden chamber that's almost impossible to reach.
To get inside, cavers and scientists crammed themselves into a crack that was just over 7 inches wide. Then they had to descend 50 feet down a pitch-black chute, mostly the same size as its entrance.
"It's insane ... some of the most difficult conditions ever for paleontologists to work in," says Dr. William Harcourt-Smith, a researcher with the paleontology division of the American Museum of Natural History and a professor of paleoanthropology at Lehman College.
But at the bottom of that narrow chute, scientists found a treasure trove: thousands of bones belonging to a previously unknown species closely related to humans, a small hominin (an extinct early relative of humans). This predecessor to modern humans walked upright and yet had a tiny brain, and it's already totally transforming the way that scientists understand how our species evolved.
The new human ancestor is called Homo naledi, named after the Dinaledi chamber of the cave. Dinaledi, meaning "many stars," is itself named after the Rising Star cave system where it was discovered.
This chamber has been explored only over the past two years, and the remarkable findings within are being revealed for the first time on Thursday.
One of the most remarkable things about this discovery is that researchers think the bones were found where they were because these human ancestors buried bodies in this chamber intentionally, depositing them there over a period of time. This behavior is unheard of for hominin creatures that appear as primitive as these.
This discovery was announced Thursday by the University of the Witwatersrand, the National Geographic Society, and the South African National Research Foundation, with findings described in two papers published in the journal eLife and featured as the cover story in the October issue of National Geographic Magazine.
The discovery and 'underground astronauts'
Two years ago, South African cavers Rick Hunter and Steve Tucker - now National Geographic explorers - knocked at the door of National Geographic explorer-in-residence Lee Berger of the University of the Witwatersrand.
They had been exploring the Rising Star cave, having received some training on looking for fossils of early human ancestors, and they had gone off map, into a previously unknown crevice that led to a deep chamber. There were bones there, and as Berger described it on a press call, "they realized this may be something interesting."
They brought photos to show some of what they had seen. Berger, who earlier in his career discovered another new species of early human ancestor, Australopithecus sediba, realized that this could be an unprecedented finding.
The cave was incredibly hard to access, so Berger and other researchers put out a call on social media. They were looking for a team that had the physical capability and stature to enter the Dinaledi chamber - as well as the academic training required to identify and excavate precious fossils that could tell the story of the hominins that preceded us on this planet.
They found a team of what they call "underground astronauts," six female scientists and cavers with degrees in archaeology and paleoanthropology.
They first entered the cave on November 10, 2013.
Becca Peixotto, a member of the team whom Berger describes as "one of the six people willing to quite literally risk their lives on a daily basis," said on the press call that "it was like a puzzle to get each fossil out."
They had to carefully remove each individual bone and then figure out how to get it up the 7-1/2-inch-wide, 5o-foot-deep chute.
One of the first skull fragments was particularly challenging. It would help reveal the size of the head and the brain of this early human cousin, but getting this fragment up and out without breaking it was daunting.
Three team members passed it to one another to get the skull up the chute before carrying it down the section of the cave known as the Dragon's Back and then going on a "Superman crawl" through another tiny shaft that would bring them out to the light of day.
It was "just a quiet relay of 'take this,' - 'OK I've got it,'" Peixotto says. But there was a huge cheer as they emerged into the South African sun.
What the bones reveal
The team has excavated almost 1,600 fossils from the cave, but thousands more remain inside and are still being examined. At least 15 different individuals are represented in the remains that have been studied so far, identified by their teeth.
This is the largest collection of hominid fossils ever discovered in Africa, and it changes what we thought about how the related groups of species to which we belong evolved.
Previously, we thought it likely that the gradual evolution of Homo sapiens came about with species gaining modern characteristics like our feet alongside our larger brains. But these new naledi creatures - which are much better represented than our collections of almost all other hominins - show that the evolution of these human ancestors wasn't so straightforward.
Their feet were incredibly similar to those of modern humans, says Harcourt-Smith, who led the study of the newly discovered creature's feet. Homo naledi stood about 5 feet tall, and yet they had a skull whose volume was only about one-third of ours, a tiny brain in comparison with that of the modern human. Despite their ability to walk upright, with stiff feet and toes that couldn't grasp things as easily as more primitive animals, they had shoulders and hands indicating they would have been quite comfortable climbing through trees and, perhaps, through caves.
This confounds the idea of how humans evolved, Harcourt-Smith tells Tech Insider, making the tree of evolution "more bushy." What we think about as the journey from early primates to modern humans is more confusing and takes more turns than previously believed.
More confusingly, we have no idea how old the fossils are. From their physical structure, their morphology, Berger says it looks as if Homo naledi evolved at the early stages of all creatures that fall under the Homo genus, perhaps up to 2.8 million years ago.
But we have no idea whether these particular specimens come from then or from a much more relatively recent time period. By email, Berger tells Tech Insider that for now it's "impossible to answer" whether they could have come from even the relatively recent past.
Despite how primitive Homo naledi appears to be, it's even theoretically possible that they could be young, dating back to sometime in the past 30,000 to 100,000 years, Harcourt-Smith writes in a follow-up email. "If it were young, then we would have a truly fascinating story of a remnant population hanging on for a long time," he says.
There's no saying whether they are a missing link in our own evolutionary chain or just a related cousin who diverged somewhere along the way.
Researchers haven't been able to use carbon dating or DNA testing on the bones yet because doing so would require destroying some, and they didn't want to do that before describing the findings.
But the particularly shocking thing is that there isn't other material in this particular chamber that can be used to date these fossils. That's what leads researchers to believe that these bodies were buried - intentionally placed - in this deep, isolated tomb, 100 meters from the cave's entrance.
Burying the dead
Behavior like the intentional repetitive placement of dead bodies (burial) is something Berger described on the press call as something we didn't think any hominin species "even contemplated before 300,000 years ago."
The team has considered other possible explanations.
Geological features show that the bodies arrived in the cave over a period of time, meaning this wasn't a one-off event or catastrophe of some sort. Teeth show that the remains come from individuals of many different ages, from young children to teenagers to elderly adults. There aren't signs of violence, falls, or cannibalism. And there are almost no remains from any other creature, indicating that this was a place that had to be sought out deliberately - not a place that some kind of creature dragged its prey.
Yet the idea that a hominin with such a primitive brain would have carried its dead to the same spot over time defies everything we've thought about our early ancestors. "It's a game changer, it's new, it's not something you'd ever expect from something with such a small brain," Harcourt-Smith says.
And yet they've ruled out all of their other hypotheses for how those bodies could have arrived in this underground tomb.
Now, Harcourt-Smith says, "it's up to someone to come up with a better explanation."