They were slender and tall for their kind, on average standing about 5 feet tall and weighing just under 100 pounds. Their brains were tiny—about the size of an orange. Yet these creatures did something remarkable: They took great care with their dead, placing their bodies in a deep, dark cave chamber that was only accessible through a narrow crack just 7 inches wide.

Who were they? Our newest human relatives: Homo naledi.

An international team of researchers announced today they have made a truly stunning discovery: an entirely new species of hominid, or ancient human relative. More than 1500 bones from 15 individuals who share a similar morphology—one that's unique among hominids—have been unearthed in South Africa, making this cache of hominid bones the largest ever discovered in Africa of a single species. There's likely many more awaiting discovery. The scientists' research was published online in two papers in the science journal eLife.  

“It’s one of the most extraordinary discoveries made in the history of the studies of human evolution,” Lehman College paleoanthropologist William Harcourt-Smith, a co-author on the paper, said today via Skype during a press event at the American Museum of Natural History, where he is a resident research assistant. 

Dinaledi skeletal specimens. The "skeleton" is actually a composite of elements that represent multiple individuals. Image credit: Berger et al. in eLife.

The bones of H. naledi  were first spotted in October 2013 in the Rising Star cave system in South Africa’s Cradle of Humankind World Heritage Site, home to 40 percent of the world’s human ancestor fossils, by paleoanthropologist Lee Berger, a research professor at the University of the Witwatersrand and a National Geographic explorer-in-residence. (Berger previously discovered the early hominid species Australopithecus sediba in the region.) Naledi means “star” in Sesotho, a local South African language.

What they found was tantalizing, but largely out of reach, "because it was found deep within the cave system,” Harcourt-Smith said. Few researchers could fit through the 7-inch entrance to the chamber, known as Dinaledi, to explore it further. 

Berger put out a worldwide call on social media for help from experienced—and small-bodied—cavers and scientists. The majority of the Rising Star Expeditions work, done in November 2013 and March 2014, was undertaken by a crack team of “underground astronauts”: a half-dozen female scientists and cavers who had both the experience to handle such an extreme location and the small, slim body type to gain access to the space.

What they brought to the surface is extraordinary, as scientists began to learn in May 2014, when more than 50 experienced and early-career researchers came together in Johannesburg to study and analyze the treasure trove of fossils. The bones have yet to be dated.      

WHY H. NALEDI IS SO UNUSUAL

There are several reasons why this find is simply amazing. For one, only in one other place in the world—the Atapuerca cave site in Spain—have so many ancient hominid remains been recovered in one location. The bones also represent nearly every element of the H. naledi skeleton, multiple times. And all ages have been found: infants, children, adult men and women, the elderly. Considering that we've identified many ancient relatives from a painfully limited number of fossils, having so many bones from so many individuals across the lifespan is remarkable.

The individuals are morphologically homogeneous (meaning they all look alike) but they look like nothing else in the human fossil record, the researchers say. They’re a fascinating mix of primitive, human-like, and utterly unique.  

For instance, their tiny brains are similar in size to the more-ancient genus Australopithecus—Lucy being the most famous example—but are housed in a skull with a jaw and teeth that are closer to early examples found in Homo, our own genus. Their shoulders are suited for climbing, which would have been handy for spending time in trees. But their feet and ankles are quite modern, and well adapted for walking. Their hands, especially their wrists and fingers, are mostly Homo-like and could have conceivably been used to make tools (though none have been discovered), and yet their fingers are distinctively curved—another helpful feature for gripping tree branches. Their range of body mass is similar to small-bodied modern human populations.

Finally, the fact that they appear to have deliberately disposed of their dead is astounding, and completely unprecedented among ancient hominids. While we have some evidence for Neanderthal burials, we humans are generally thought to be the only ones to bury our dead.

DISPOSING OF THE DEAD

The idea that H. naledi deliberately placed their dead in the cave chamber was so implausible to the researchers that they explored virtually every other explanation first. But the bones show no sign of mass death, either accidental or intentional, or marks from carnivores or scavengers. Nor is there any indication that water or some other natural process deposited the remains there. Moreover, out of the 1550 fossil elements recovered in the cave, which has never been open directly to the surface, only about a dozen are not hominin—and these few pieces are isolated mouse and bird remains.

In short, the only visitors to this cave appear to have been H. naledi bringing their dead here. 

“It’s a fascinating example of what we used to think was a rather advanced human behavior, this tendency to dispose of the dead, in a small-brained, more primitive member of our genus,” Harcourt-Smith said. “So there’s an extraordinary behavioral story alongside the fact that we have a new species.”

Among early hominids, this behavior is “very, very unusual,” paleoanthropologist Ian Tattersall, curator emeritus at AMNH, told mental_floss at the press event. (Tattersall was not involved in the study.) “We see it in only one other place: Atapuerca.”

Paleoanthropologist Dean Falk, one of the world’s leading researchers on the evolution of the human brain, told mental_floss in an email that it’s not unexpected that H. naledi might be capable of advanced behaviors. She points to Homo floresiensis, the 3-foot-tall “Hobbit” that lived in on the Indonesian island of Flores 95,000 to 17,000 years ago; its brain was small but had advanced features, and it was an avid stone tool maker.

“The Hobbit showed us that small hominin brains can be organized in advanced ways, so we shouldn't preclude higher cognitive abilities in H.naledi based merely on the apelike size of the brain,” she noted.

SHOULD IT BE IN THE GENUS HOMO?

Whether the species should be placed in the genus Homo will likely be robustly debated, Tattersall said: “Definitely they have a new species down there, there’s no question about it. Whether it belongs in the genus Homo is going to be a discussion point.”

He suspects there’s more than one species in the Dinaledi chamber. “I wouldn’t be surprised if there were more than one thing in there,” he told mental_floss. “They illustrated three skulls. One is really broken up, so I won’t say much about it. And the other two look very different from each other. One that does look like probably a regular Australopithecus skull, and the other has a bit of a forehead and a brow ridge.

“There’s a lot more stuff that has yet to be recovered. It will be really interesting to see what kind of variety of morphologies emerges,” he said. “In terms of the morphological variety and whether there might be more than one hominid down there, that would be very exciting; it could mean that at least some of them were not being thrown down by there by members of their own species. That would be a really cool and really complicated thing going on there.”

In a commitment to open access research, the researchers have made all the fossils available online in full-color, high-resolution, 3D scans at MorphoSource for scientists to use for research, teaching, and display; those with a 3D printer can print out H. naledi bones.

“This is a wonderful example of open access science,” Harcourt-Smith said. “So often in the human evolution world, it can be rather hard to get access to certain fossils. I love that other people will be able to add to the debate. And indeed, there will be debate.”