In October 2013 scientists working in South Africa announced that they had discovered a trove of fossil human remains in the pitch-black depths of an underground cave system. They began a rapid recovery effort that yielded some 1,550 specimens of bones and teeth—just a fraction of the material at the site, yet already the largest assemblage of human fossils ever found in all of Africa. Now the team has published its eagerly anticipated analyses of the remains, and the conclusions are startling. The researchers suggest that the fossils represent a previously unknown species in our genus, Homo, one that had a peculiar mix of physical traits and engaged in surprisingly sophisticated behavior for its brain size. But the age of the fossils has yet to be determined, leaving other scientists unsure of what to make of them.

Cavers collected the bones from a difficult-to-reach chamber 30 meters below ground within the Rising Star cave system in South Africa’s Cradle of Humankind region, which is famous for its human fossils. In their paper describing the new remains, published today in the journal eLife, paleoanthropologist Lee Berger of the University of the Witwatersrand in Johannesburg and his colleagues report that the remains include multiples of nearly every element of the skeleton and represent at least 15 individuals. For a field in which even an isolated bone can constitute a major discovery, this find is an absolute windfall.

The fossils exhibit a combination of primitive features that bring to mind our ancient australopithecine predecessors (including Lucy and her ilk) and features that are associated with Homo. For instance, the pelvis has a flared shape like that seen in Australopithecus, whereas the leg and foot resemble those of Homo sapiens. Likewise, the skull combines a small braincase with a cranium that is otherwise built like that of early Homo. The teeth, meanwhile, are small like those of modern humans, yet the third molar is larger than the other molars—a pattern associated with australopithecines. And the upper limb pairs an Australopithecus-like shoulder and fingers with a Homo-like wrist and palm. “All that combined leaves us with a really, really strange creature,” Berger remarks.

Standing about 1.5 meters tall, with a small brain, clever hands and a body built for upright-walking as well as climbing, this creature possessed a unique mosaic of traits that Berger and his co-authors think reveals a new species of human. Given the many Homo-like traits evident in the bones—particularly in those regions that contact the environment (namely, feet, hands and teeth)—the team put the creature in the genus Homo, rather than Australopithecus, calling it H. naledi.

But exactly where H. naledi belongs in the human family tree, apart from somewhere on the Homo branch, is unclear. The confusion arises in large part from the fact that thus far the researchers have been unable to determine the age of the bones. The bones could be several million years old or tens of thousands of years old, though the team seems to favor the idea that H. naledi stems from a point close to the origin of Homo. (The oldest known fossil of Homo is a lower jaw bone from Ethiopia that dates to 2.8 million years ago).

However old the Homo naledi bones turn out to be, they will dramatically impact how scientists interpret human evolution, Berger says. If the remains are quite old, then certain physical and behavioral traits associated with later Homo emerged surprisingly early, and possibly in species that did not give rise to H. sapiens. If the remains are intermediate in age, some of their more australopithecine-like traits might be the result of reversals, in which a more primitive trait re-evolves, possibly because it becomes adaptive again in some way. If the remains are young, then scientists will have to come to terms with the fact that a small-brained human species with tool-wielding hands managed to persist alongside larger-brained human species—possibly including H. sapiens—for an amazingly long time. In that case, says team member John Hawks of the University of Wisconsin, perhaps H. naledi was among the archaic human species that interbred with H. sapiens and thereby contributed DNA to the modern human gene pool, like Neandertals did. (The team plans to try to obtain DNA from the H. naledi bones, though the warm, damp conditions in the cave system are not ideal for preservation of ancient DNA.)

The mix-and-match anatomy of the H. naledi bones is not the only puzzling aspect of this discovery, however. At other fossil sites in the Cradle of Humankind, fossils are encased in sediment and animal bones are found mixed in with the human remains. The bones of humans and animals alike accumulate in the caves there through catastrophic events such as falling down a hole in the ground into an underground cave and getting trapped, or becoming dinner for the large carnivores that denned in the caves. But the Rising Star bones are not encased in sediment, nor do any remains of any vertebrate animals, apart from a few rodents and birds, accompany them. Given the absence of any evidence to indicate that Homo naledi fell or washed into the underground chamber or was transported there by a predator, the discovery team suggests that this small-brained human deliberately disposed of its dead. Furthermore, the location of the H. naledi bones in a chamber that appears to have always been lightless and difficult to access suggests that the humans went to great lengths to deliver the bodies there, and possibly needed an artificial light source (perhaps a simple torch) to do so. The behavior is important because it implies that H. naledi had, as Hawks puts is, “a shared cultural knowledge of mortality.” Paul Dirks of James Cook University in Australia and his colleagues published their analysis of the geology of the site and how the bones got there in a second paper published today in eLife.

The team’s claims have met with skepticism. “I find [the discovery] fabulous but confusing,” says Susan Antón of New York University, who studies the evolution of Homo.  She notes that the remains highlight an ongoing debate among paleoanthropologists about what constitutes the genus in the first place. Early Homo fossils tend to be scrappy at best, which makes it hard to figure out which traits first distinguished our genus from Australopithecus. H. naledi has multiple body parts preserved, but “we don’t have any idea how old this stuff is or whether it’s relevant to the origin of Homo,” Antón comments.

Bernard Wood of George Washington University agrees with the authors that the remains represent a new species, but he does not think that they will force experts to revise the overarching story of human evolution. Instead he suspects that bones represent a relic population that might have evolved its odd traits in relative isolation in South Africa, which he describes as a cul de sac at the bottom of the African continent. Wood points to another small-brained species of Homo, H. floresiensis from the island of Flores in Indonesia, as another example of such a relic population.

The suggestion that small-brained H. naledi was systematically disposing of its dead has likewise raised eyebrows. “It would be quite radical,” says Alison Brooks of George Washington University. “There are people who think Neandertals didn’t bury their dead,” she observes. (Neandertals are our closest relatives; they had brains as large as our own and engaged in a host of sophisticated behaviors. Whether or not they buried their dead is a matter of some debate. ) “I don’t want to rule it out entirely that they’re right, but I just think it is so far out there that they really need a higher standard of proof.” Brooks adds that the team would have a better case if it could show that the remains all date to the same time period.

But to other observers, the team’s preferred explanation for how all those bodies of H. naledi ended up together without any animal remains in the mix rings true. Travis Pickering of the University of Wisconsin, who has worked at fossil sites in the Cradle of Humankind for the past 20 years, calls the circumstances of  H. naledi fossils unique, and says that intentional disposal of the remains by other humans is the most sensible explanation for the phenomenon. But “whether that means Homo naledi was a rather cultural advanced species with well-developed mortuary practices or simply an atavistic one that had the sense not to cohabit with rotting corpses is currently unanswerable,” he remarks. Pickering adds that it is impossible to say whether the H. naledi individuals were lured or pushed into the cave to be murdered, or whether they were placed there, once dead, as part of a ritual. In fact, another, contemporaneous human species might have disposed of H. naledi’s bones in that spot.  

The discovery team focuses on the possibility that H. naledi disposed of its own dead, however--a behavior that is not necessarily unexpected, in Pickering's view. Observations of modern chimpanzees demonstrate that apes with modestly sized brains distinguish death from life. Surely, he says, a human species with a brain larger than a chimp’s would be able to make the same type of connection. “What is important about the new observations out of Rising Star is that they might be confirming this very point,” Pickering offers.

For his part, Hawks notes that the behavioral insights from Rising Star hint at an interesting parallel to the anatomical story. “We have all these things we think of as human. From an anatomy point of view walking upright is human, a large brain is human, tool-making hands are human. But all of these things happened at different times in different ancestors. The package we think of as human did not appear simultaneously," he observes. “I don’t know why we would think behavior is any different—a package evolved and different parts appeared at different times.”