Puzzling skeleton helps reveal how ancient human relative grew up

More than 200,000 years ago in what is now South Africa, a child, standing about three feet tall, somehow died before reaching adulthood. The young human relative’s body came to rest inside a deep, dark cave, alongside the remains of at least 14 others of its kind.

The remains went unnoticed and undisturbed until 2013, when spelunkers in South Africa’s Rising Star cave system came across hundreds of bone fragments and teeth from a newfound cousin to modern humans, a relative now called Homo naledi. Now, researchers poring over the cave’s fossils have pieced together the child’s partial skeleton—the first such juvenile H. naledi skeleton ever assembled.

Recovered from the cave system’s Dinaledi chamber, the individual—named DH7 (Dinaledi Hominin 7)—is thought to have died between eight and 15 years old. The newly described bones, unveiled today in the journal PLOS ONE, consist of a lower right jawbone and 16 fragments from the rest of the body. Such complete skeletons from young hominins other than modern humans and Neanderthals are exceedingly rare because the smaller, softer bones are less likely to fossilize.

The skeleton could help researchers figure out how H. naledi grew up—and whether it matured more like modern humans, or more like our earlier ancestors.

“What was super exciting about this was that we were able to have proven associations with a single individual,” says study co-author Lee Berger, a paleoanthropologist at South Africa’s University of the Witswatersrand and National Geographic Society explorer-at-large. “We can look at dental development against body development and try to infer ... the way that Homo naledi developed.”

Finding juveniles is particularly significant, says University of Tuebingen paleoanthropologist Katerina Harvati-Papatheodorou, who was not involved with the new study. “Because fossil hominids did not necessarily follow the same growth patterns as modern humans, such information can also tell us how different or similar their growth would have been to our own or to other extinct species,” she says in an email.

A cave full of Homo naledi

Most early human fossils are found as scattered, individual bones, not as sets that clearly belong to the same individual. Researchers have found only a handful of associated juvenile skeletons from the ancient hominins Australopithecus afarensis, Australopithecus sediba, and Homo erectus.

But the new Homo naledi skeleton stands out. The other early hominin species lived more than a million years ago, while H. naledi is much closer to our species in time. DH7 and the other fossils found with the skeleton were deposited between 226,000 and 335,000 years ago, just as signs of modern humans appear in Africa. But even though these two species may have coexisted, H. naledi had several physical features—such as its hips and shoulders—that more closely resembled ancient hominin cousins.

The find marks the latest discovery from the Dinaledi chamber, which Berger’s Rising Star Expedition has excavated with the support of the National Geographic Society since 2013. Just getting into the chamber is no easy feat. Team members have to squeeze their equipment—and themselves—through a passage less than eight inches wide.

Enduring the tight space proved worthwhile. When study co-author Marina Elliott led excavations in the chamber in 2013 and 2014, she and her colleagues uncovered a treasure trove of bones. Scattered on the surface of the cave floor and buried within less than four carry-on suitcases’ worth of excavated dirt, the team found some 1,550 bones and teeth belonging to at least 15 Homo naledi individuals, ranging from infants to adults. To date, the chamber has yielded more than 1,800 fossils.

Elliott carefully documented where each bone fragment was found, so once the remains were removed from the cave, the team could work out which bones belonged to which individual—a challenge akin to solving 15 different puzzles when some pieces are lost and the rest are jumbled together.

By looking at the bones’ maturity and location within the cave, researchers began to piece together DH7—and right away, they knew the fossil was special. Some of DH7’s bones were articulated as they would have been in life, a sign that the bones had been buried surrounded by the original soft tissue. The remains were so well protected, DH7’s left shinbone still had both ends—a profoundly rare find.

In young hominins, including modern humans, the knobby ends of limb bones don’t properly fuse to the bones’ shafts until the individual is done growing. As a result, the ends of fossil juveniles’ limb bones are often scattered or missing altogether.

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Part Ape, Part Human

“This is an incredible find,” says lead study author Debra Bolter, a paleoanthropologist at Modesto Junior College in Modesto, California. “Most of the time, our stuff isn’t inside of a cave system and isn’t protected from all the other elements, like wind and rain and stampeding of African wildlife.”

Hominin youth

How old was DH7? For now, researchers aren’t sure. If H. naledi matured as fast as earlier hominins, such as H. erectus or A. sediba, then DH7 died between the ages of 8 and 11 years old. If H. naledi matured more slowly like modern humans and Neanderthals, however, DH7 would have died between 11 and 15 years old.

Researchers don’t yet know enough to distinguish between the two scenarios. H. naledi’s anatomy is a mosaic of features that resemble both earlier and later hominins. Its curved hand bones look a lot like far older and faster-developing species, but some of its other features, such as its feet and ankles, look strikingly like those of slower-growing modern humans.

What’s more, different parts of the H. naledi body may mature at different paces. A 2017 paper in Science showed that Neanderthal children matured in nearly the same way as modern humans—but their spines grew in a different pattern. “It was a surprise—we didn’t know that!” says Antonio Rosas González, the lead author of the 2017 study and a paleoanthropologist at Spain’s National Museum of Natural Sciences, who was not involved in the new study. “In the case [of H. naledi], we don’t know, and that would be a second step of the research.”

H. naledi’s anatomical mishmash becomes all the more puzzling once researchers consider its brain. Despite H. naledi’s many humanlike features, its brain is only about two-fifths the size of ours, in line with far older and more primitive human ancestors.

DH7's left tibia, or shinbone, was found with its unfused ends, which suggests that the bone was entombed with its surrounding soft tissue.

Photograph by Marina Elliott and Evolutionary Studies Institute, University of the Witwatersrand

“My first reaction would be that, since H. naledi is characterized by a much smaller brain size than Homo sapiens, which is closer to H. erectus or A. sediba, as well as a relatively small body size, that its maturation patterns would also be more similar to those species despite some similarities noted with H. sapiens,” Harvati-Papatheodorou says.

However, the brain organization of H. naledi appears to have been more complex than earlier hominins’, with more development in areas possibly linked to tool production. Other signs of H. naledi’s smarts have emerged from the Dinaledi chamber. Berger’s team has interpreted the fossil cache as a sign that H. naledi may have deliberately disposed of their dead, an idea that ignited intense debate when the team unveiled H. naledi in 2015.

Normally, small brain sizes correspond to faster development times. But with its humanlike features and complex brain, H. naledi may have been the exception to the trend. “You’re beginning to see that maybe Homo naledi is breaking all the rules,” Berger says.

Measuring the rate of H. naledi’s development requires figuring out how old DH7 was when the individual died. Luckily, the research team has exactly the material they’d need to figure out DH7’s exact age: some of its teeth.

As teeth develop, they accrete enamel by the day, leaving faint lines in the structure that resemble tree rings. By counting up these lines in some of DH7’s molars, researchers could potentially pin down, to the day, the age of DH7. But doing so comes with costs. Either researchers cut into and destroy part of DH7’s teeth, or they subject the teeth to powerful x-rays, which might destroy any preserved proteins that could reveal more about H. naledi’s relationship to other hominins, including us.

“We just have to be careful that what we do to get one piece doesn’t destroy an attempt to get any other kind of data,” Bolter says.