On September 10, scientists announced the truly remarkable discovery of thousands of bones belonging to a previously unknown species of human ancestor.
They gave the new species the name Homo naledi, after the Dinaledi chamber of the Rising Star cave where the bones were found.
The finding is already starting to change our understanding of how our own species evolved. At the same time, it's prompted some scientists who weren't involved in the research to raise questions about what it really means.
The single cache of "more than all other hominid bones discovered in Southern Africa" is something that's helping us solve the story of our biological family in the genus Homo, Lee Berger, the lead researcher on the Homo naledi expedition said on a press call.
There are many stunning components to this finding, which is featured on National Geographic magazine's October cover. This newly discovered species is unlike any we've ever seen, with feet that are fairly close to those of modern humans, yet a brain the size of an austrolopith — a type of ancient human relative that preceded the Homo genus.
Perhaps most incredibly, Berger and his team concluded that after ruling out other possibilities, the most likely way that these skeletal fossils ended up in the chamber that they did — down a 50 foot chute that was almost impossible to find, about 300 feet into a dark cave — was that they were placed there intentionally.
Still, as with any new discovery, scientists are questioning certain aspects of the story. Some ask how to best classify the bones, how we know we've truly found a new species, and how it's possible to argue that creature with a brain the size of a gorilla's was intentionally disposing of its dead.
Questioning scientific findings is completely normal and doesn't take away from the remarkable aspects of the discovery. Science is a process of skepticism, chipping away at facts and growing more and more certain along the way. Here's what some researchers still want to know — and how the many scientists who worked on this discovery have answered those questions so far.
How can we say this is definitely a new species?
Several anthropologists argue that without knowing the age of the bones — the researchers haven't dated them yet — we can't be sure that these specimens aren't from an already-known species, like Homo erectus.
"If they are as old as two million years, then they might be early South African versions of Homo erectus, a species already known from that region," anthropologist William Jungers of Stony Brook School of Medicine tells The Guardian.
In that same Guardian story, anthropologist Christoph Zollikofer of the University of Zurich argues the same. "The few 'unique' features that potentially define the new species need further scrutiny, as they may represent individual variation, or variation at the population level," he says.
The idea that perhaps specimens don't fully represent a new species has been controversial in anthropology before, notably with Homo floresiensis, a small hominid also known as "the Hobbit." There was lots of argument over whether it was a separate species or just a deformed specimen of another known one — though most recent research has come down in favor of "the Hobbit" being a unique species.
But in the case of the new discovery, the researchers involved say that they were able to find bones from 15 different individuals, and they believe there are more still to be found. The characteristics that make Homo naledi unique are found in all the skeletons they've looked at, meaning they aren't some sort of deformity or pathological difference.
"We had so much material to work with," John Hawks, an anthropologist at the University of Wisconsin and a senior author on a paper describing the new species, said on the press call.
The modern feet, austrolopith-like skull, curved fingers, ape-like shoulders, and Homo-like brow make for a unique creature, unlike any we've ever seen, William Harcourt-Smith, a researcher who led some of the work on the new specimen, tells Tech Insider.
Is it really so closely related to us?
The Homo part of Homo naledi indicates that it's part of the same genus — a wider classification than species — as modern humans. But even that part is up for debate.
"If I'd been writing those papers I wouldn't have assigned this to the genus Homo," Ian Tattersall, curator emeritus of the American Museum of Natural History tells Tech Insider.
In some ways this new species seems so primitive that it could be considered an austrolopith, he says. The feet and shape of the skull (though not its size) were more like primitive Homo specimens, but many other features could have come from a creature that predates our more direct relatives.
Despite these similarities with more primitive ancestors, the researchers involved in the new discovery say that there are enough characteristics that are associated with other species classified as Homo that they think that their new finding could share a common ancestor with Homo sapiens (us) and Homo erectus.
And as Tattersall says, "there are no absolute rules involved here."
Where exactly a species fits into a classification system is a judgment call for scientists that they'll come to agreement on over time, he says. "I think this is all going to shake out."
Did Homo naledi really bury its dead?
The most controversial argument the discoverers of Homo naledi made was that it looked like the bodies they found had been intentionally placed deep inside this cave. In other words, this creature was repeatedly disposing of their dead in the same place, a behavior that's shocking for a creature with a brain one-third the size of our own.
"Intentional disposal of rotting corpses by fellow pinheads makes a nice headline, but seems like a stretch to me," Jungers tells The Guardian. Another anthropologist told them that these "claims are clearly for the media."
The researchers involved in the work know that anything like "burial" sounds crazy. As far as we knew until now, members of our family tree didn't even contemplate intentional body disposal until around 300,000 years ago — and though we have no idea how old these fossils are, they look like they could be far older.
But they say that they've ruled out all of their other hypotheses for how those bodies could have got to this underground tomb.
The bodies look like they arrived there over time and come from individuals of many different ages. There aren't signs of violence 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.
And just because the bodies were deliberately deposited in a cave, researchers aren't suggesting any kind of ritual or religion; the intent could have been purely practical, even some kind of basic safety measure.
If they weren't placed there intentionally, Harcourt-Smith says, "it's up to someone to come up with a better explanation."
Tattersall agrees, saying that it is, "by elimination, the only rational possibility that they have." But he says there's much more to learn.
"This really is a work in progress that has gone from absolutely nothing to where we're at today with a lot of logistical and hard work and analysis, all in two years," he says. "For a vast amount of material, that's a very short time to digest. So look upon this as a progress report, and the beginning of a really exciting journey."
