Scientists say they've discovered a new member of the human family tree, revealed by a huge trove of bones in a barely accessible, pitch-dark chamber of a cave in South Africa.

The creature shows a surprising mix of human-like and more primitive characteristics — some experts called it "bizarre" and "weird."

And the discovery presents some key mysteries: How old are the bones? And how did they get into that chamber, reachable only by a complicated pathway that includes squeezing through passages as narrow as about 17.8 centimetres?

The bones were found by a spelunker, about 48 kilometres northwest of Johannesburg. The site has yielded some 1,550 fossils since its discovery in 2013. Those, in turn represent at least 15 individuals, ranging in age from babies to elderly individuals.

Human Ancestor Homo naledi

This March 2015 photo provided by National Geographic from their October 2015 issue shows a reconstruction of Homo naledi's face by paleoartist John Gurche at his studio in Trumansburg, N.Y. (Mark Thiessen/National Geographic via AP)

Researchers named the creature Homo naledi (nah-LEH-dee). That reflects the "Homo" evolutionary group, which includes modern people and our closest extinct relatives, and the word for "star" in a local language. The find was made in the Rising Star cave system.

The creature, which evidently walked upright, represents a mix of traits. For example, the hands and feet look like Homo, but the shoulders and the small brain recall Homo's more ape-like ancestors, the researchers said.

'This stuff is like a Sherlock Holmes mystery.' - Bernard Wood, George Washington University

Lee Berger, a professor at the University of the Witwatersrand in Johannesburg who led the work, said naledi's anatomy suggests that it arose at or near the root of the Homo group, which would make the species some 2.5 million to 2.8 million years old. The discovered bones themselves may be younger, said Berger, an American.

At a news conference Thursday in the Cradle of Humankind, a site near the town of Magaliesburg where the discovery was made, bones were arranged in the shape of skeleton in a glass-covered wooden case. Fragments of small skulls, an almost complete jawbone with teeth, and pieces of limbs, fingers and other bones were arrayed around the partial skeleton.

Berger handed a skull reconstruction to Deputy President Cyril Ramaphosa, who kissed it, as did other VIPs. Berger beamed throughout the unveiling.

Berger said almost every bone in the body was represented multiple times among the fossils found, making the new species "practically the best-known fossil member of our lineage."

The researchers also described the discovery in the journal eLife. They said they were unable to determine an age for the fossils because of unusual characteristics of the site, but that they are still trying.

'No way we can judge'

Berger said researchers are not claiming that neledi was a direct ancestor of modern-day people, and experts unconnected to the project said they believed it was not.

Rick Potts, director of the human origins program at the Smithsonian Institution's Natural History Museum, who was not involved in the discovery, said that without an age, "there's no way we can judge the evolutionary significance of this find."

If the bones are about as old as the Homo group, that would argue that naledi is "a snapshot of ... the evolutionary experimentation that was going on right around the origin" of Homo, he said. If they are significantly younger, it either shows the naledi retained the primitive body characteristics much longer than any other known creature, or that it re-evolved them, he said.

Eric Delson of Lehman College in New York, who also wasn't involved with the work, said his guess is that naledi fits within a known group of early Homo creatures from around two million year ago.

Slender scientists

Besides the age of the bones, another mystery is how they got into the difficult-to-reach area of the cave.

The chamber where the bones were found was 90 metres from the cave entrance, accessed via a passage as narrow as 18 centimetres wide. It was excavated by a team of six women who were both experienced as scientists and cavers and slender enough to squeeze though. They included Marina Elliott, a PhD student at Simon Fraser University, one of several Canadians involved in the study.

The researchers said they suspect the naledi may have repeatedly deposited their dead in the room. That would be a startling discovery, as humans are the only animals known to do that.

 Alternatively, the cave may have been a death trap for individuals that found their own way in.

homo-naledi-skull-jaw

Researchers announced Thursday in the journal eLife and at a news conference in the Cradle of Humankind, a site near the village of Magaliesburg, that they have found a new member of the human family tree. (eLIFE)

"This stuff is like a Sherlock Holmes mystery," declared Bernard Wood of George Washington University in Washington, D.C., who was not involved in the study.

Visitors to the cave must have created artificial light, as with a torch, Wood said. The people who did cave drawings in Europe had such technology, but nobody has suspected that mental ability in creatures with such a small brain as naledi, he said.

Potts said a deliberate disposal of dead bodies is a feasible explanation, but he added it's not clear who did the disposing. Maybe it was some human relative other than naledi, he said.

Not everybody agreed that the discovery revealed a new species. Tim White of the University of California, Berkeley, called that claim questionable. "From what is presented here, (the fossils) belong to a primitive Homo erectus, a species named in the 1800s," he said in an email.

At the news conference in South Africa, Berger disputed that.

"Could this be the body of Homo erectus? Absolutely not. It could not be erectus," Berger said.

The research was supported by Wits University, the National Geographic Society and South African DST/NRF. Ongoing exploration and conservation of the Rising Star site is supported by the Lyda Hill Foundation. 

South Africa Human Ancestor

South Africa Deputy President Cyril Ramaphosa kisses a reconstruction of Homo naledi's face during the announcement made at Maropeng Cradle of Humankind in Magaliesburg, South Africa, Thursday, Sept. 10, 2015. (Themba Hadebe/Associated Press)

With files from CBC News