A gay naledi A child’s skeleton could shed some light on the evolutionary origins of our own species’ long childhoods. Humans take much longer to grow up than other great apes, which may be related to our larger brains and more complex cognitive skills. Anthropologists are still trying to understand exactly when and how that reality came about. To do this, they worked with juvenile skeletons from just a handful of species besides ours: Neanderthals, homo erectus† Australopithecus afarensis† A. sediba† But now they can also watch gay naledi†
In 2013 and 2014, paleoanthropologists unearthed the partial skeleton of a gay naledi child dating from 335,000 to 226,000 years ago. The skeleton, now called DH7, has most of a left leg with the bones still articulated — even some of the tarsals, the small bones that make up the ankle. The bones also include a right thigh bone (femur) and hip bone (ischium), a right arm, and part of a right lower jaw and a pair of teeth. The shafts of the long bones were not fully fused with their ends, or epiphyses, indicating that the young hominins were still growing when they died.
It’s hard to say exactly how old the DH7 was, though, because we don’t know how fast gay naledi children matured. If the child’s development was quite rapid, as in earlier hominids, then DH7 was probably between eight and 11 years old. But if gay naledi children developed more slowly, like Neanderthals and modern humans, the bones could look the same when they were 11 to 15 years old.
Interestingly enough, DH7 was at about the same developmental stage (if not the same age in years) as a 1.6 million year old homo erectus child from Lake Turkana, Kenya, and a 1.98 million year old A. sediba child from Malapa cave, South Africa. It’s impossible not to wonder what these three adolescents might have in common, and how they might be different if they weren’t separated by hundreds of millennia of time and evolution.
Putting an old puzzle together
Once anthropologist Debra Bolter and her colleagues found the articulated left leg, they searched the Dinaledi Chamber collection for other bones at about the same stage of development that may have belonged to the same individual. The bones they selected had to have been found nearby, looked like they were about the same length and age as DH7 at death, and no duplicate pieces that had already been found. It was a bit like rummaging through a box of puzzle pieces looking for things that fit into the same part of an image. They found the right femur, a right ischium, a right upper arm bone (humerus), part of a right lower jaw bone (mandible), and some fragments of hand bones (metacarpals).
And maybe even more of DH7 can be found. Bolter and her colleagues aren’t sure yet how deep the sediment at the base of the Dinaledi Chamber is or how many fossil bones it can still hold. “Therefore, we cannot predict what other remains of this person may yet be recovered,” they wrote.
The wisdom teeth of DH7 had not yet grown in; in modern humans, that happens for most people in their late teens or early 20s. Meanwhile, the epiphyses on the ischium and the ends of the femur and shinbone (tibia) were well-developed, but not yet fused — in other words, DH7 was not a very young child, but was definitely growing. If anthropologists looked at a skeleton of our species, they would have said that the child was between 11 and 15 years old.
That, however, is the challenge. We know how long it takes a human child to reach that growth stage. We even know more or less the same for H. erectus† anthropologists counted enamel layers in teeth of the aforementioned 1.6 million-year-old skeleton, which turned out to be between eight and nine years old at death. But we don’t know how long that kind of growth lasted gay naledi children.
Teeth tell (part of) the story
The best clue we have is DH7’s teeth. In our species, the adult premolars all grow in before the second molars appear. But in older hominids like A. sediba and H. erectus, that is not the case; their sequence of adult tooth growth is more like modern great apes. Based on half of the right jaw of DH7, gay nalediThe adult teeth came in a more human-like pattern.
Once again, gay nalediThe tooth roots still look more ape-like. Meanwhile, the pattern of enamel deposition on the surface of his teeth is, in the words of Bolter and her colleagues, “unlike any other humanoid.” That lines up well with everything else we know gay naledi so far. It had the appearance of A. sediba (143.5 cm for adults) and a brain size somewhere between A. sediba and H. erectus (but already organized like ours).
We still don’t know how these three species—A. sediba† H. erectusand gay naledi– are related to each other or to us.
So what does it all mean? Some anthropologists have suggested that because: A. sediba and H. erectus both had more ape-like tendencies in terms of the timing of the appearance of adult teeth, they may not have evolved the longer development of our species in childhood. If that idea is correct, then the fact is gay nalediThe timing of tooth eruption is more like ours, which can mean: gay naledi children, like H. sapiens and Neanderthals, took longer to grow up.
And that raises questions for future research. “The contrast in brain size between [these species] is striking,” Bolter and her colleagues wrote, “raising the possibility of testing whether the maturation pattern in these immature skeletons is related to brain development.”
PLOS2020 DOI: 10.1371/journal.pone.0230440 (About DOIs).