As we very recently noted, the origin of our own genus is a mystery. Some of the features that help define Homo were found in a species, Australopithecus sediba, that belongs to an entirely different genus. Most of the earliest remains that appear to be Homo are fragmentary and tell us little about the individual they came from.

All of that makes a recent find from South Africa all the more exceptional. A cave in the Cradle of Humankind World Heritage Site has yielded a nearly complete skeleton and parts of 14 others. The remains define a new species, Homo naledi, that had a mix of features from Homo and Australopithecus, as well as a few of its own. The biggest mystery: when did the species live? Currently, we don't have even the slightest hint.

The World Heritage Site is in the same area that yielded the A. sediba fossils. People exploring a cave found what appeared to be remains in an area that's extremely difficult to reach. Lee Berger of the University of the Witwatersrand helped organize an expedition, backed by National Geographic (which has provided all the images here). A call went out for paleontologists with caving experience who could fit through an 18cm wide passage, resulting in a crew of six women who helped excavate part of the site.

In an unusual practice for the field, the work was broadcast online and described on social media. It was clear at the time that the team were excavating hominin bones, but the full significance of their find wasn't entirely clear. Now, a collection of papers describe some of the new species' features.

Meet the new family member

A typical adult of Homo naledi stood about 1.5m (4.9 feet) tall and weighed only 45kg (about 100 lbs). Its skull was very small, with a cranial volume similar to that of the larger Australopiths; every species of Homo except the hobbits (H. floresiensis) had a larger brain. Its shoulders are also ape-like, which is commonly interpreted as indicating it retained the ability to climb trees; the rib cage is constructed like that of Australopiths.

Yet many features align it more closely with Homo. The feet and ankles are largely similar to those of modern humans, although its hips looked Australopith-like. Meanwhile, some of the leg bones have features unique to the species. Overall, the authors conclude that it was able to stride efficiently.

The skull, despite its small size, has many features similar to those of our own genus. The fingers are long and curved, yet appear to have the ability to perform fine-scale manipulations, potentially allowing H. naledi to use tools. Its jaw looks like those of other early members of our genus, but the teeth were small and similar in size to those of humans.

In the end, the authors conclude that many of the features—like the jaw and teeth, or foot and ankle—that form integrated systems are similar to other members of our own genus. Thus, they call the species Homo. "Naledi" means star in a local language and references the Rising Star cave system in which it was found.

The bones that were recovered so far (more await further expeditions) come from an astonishing 15 individuals—the authors call it "the most comprehensive representation of skeletal elements across the lifespan, and from multiple individuals, in the hominin fossil record." That reveals a striking feature of H. naledi: it wasn't that diverse. Human populations generally vary in body mass by about 15 percent; H. naledi varies only by nine percent. Many of the features unique to the species were found in every single individual excavated.

Bewildering diversity

But if this population of H. naledi wasn't diverse, it appears that our early ancestors were. The new species contains a number of systems that appear to be typical of Homo, but mixes them in with traits that are typical of earlier species. The same is true for other early species from our genus. The key thing is that the Homo-like systems were different in different species. Or, as the authors put it, "such features appeared in different combinations in different fossil samples."

That's interesting (and challenging) from the perspective of figuring out how a complete "modern" set of traits got assembled in a single species. But it also means that any fragmentary samples aren't especially informative, because a modern-looking foot won't necessarily be linked to equivalent femurs and hips. Again, quoting the authors, "In light of this evidence from complete skeletal samples, we must abandon the expectation that any small fragment of the anatomy can provide singular insight about the evolutionary relationships of fossil hominins."

The other problem here is that there's no way of dating the H. naledi skeletons (more on that in a bit). The small skull size would suggest it appeared early in our lineage, as do many features that are shared with species like H. habilis and H. rudolfensis. But it's also technically possible that the species was a hold out that retained earlier features even as our own lineage was acquiring more modern ones.

Looking for a date

For many skeletons, this can be solved by dating. Typically, fossils are buried in nice sedimentary layers. A combination of dating nearby layers and tracking features like volcanic deposits and magnetic pole reversals can provide a reasonably precise date. But these skeletons were simply buried in the detritus that accumulated on a remote cave floor.

Another way of providing dates for fossils is to look at the other species that are found with them and figure out when those went extinct. This again works well if there's a good sedimentary deposit, since those encase bones indiscriminately. Here, there were essentially no other animals, except a few skeletons from birds and small rodents. Thus, the only thing buried with H. naledi were animals that had gotten lost in the cave.

So, at the moment, there's no way of telling when H. naledi lived, and thus infer its potential relationships with other hominin species.

Signs of culture?

Those of you paying close attention will notice something weird about the information that's been presented. The cave in which these skeletons were found was remote enough that almost no other animals ever reached it. Yet there were at least 15 individuals of this same species found at the site.

The clear implication is that these skeletons were unlikely to have gotten there by accident. Instead, they were likely to be placed in the cave. And, since H. naledi belongs to our own genus, the most likely explanation is that other members of this population were the ones doing the placing.

The cave wasn't necessarily always this remote or difficult to access, but it's clear that there must have been some effort involved in getting individuals to the location. It's tempting to over-interpret this as a sort of burial, but it's dangerous to try to infer the mental processes in a species where we have no understanding of the mental capacity.

So, like the age and evolutionary relationships of H. naledi, the process that put so many of them in a single cave will remain a mystery for the moment. But as there's still plenty of material to excavate there, you can bet some very thin paleontologists will make their way to the site before too long. And, with time, they might find ways to tell us a bit more about this species.

Open Access at eLife, 2015. DOI: 10.7554/eLife.09560 (About DOIs).