en right-handed, from the orientation of cut marks that they accidentally made sometimes on their teeth when they were chewing and cutting meat. But in Homo naledi, we have not yet found any evidence of this kind.”
The occipital lobes also provide a clue about Homo naledi’s visual system. The key is a structure called the lunate sulcus, which is part of the primary visual cortex. In nonhuman apes, it’s larger and farther forward than in humans. That’s because our visual cortex is arranged differently from other apes’. The human visual cortex is larger overall, but a lot of that expanded area is focused on association, and the primary visual cortex ends up being a bit smaller than it is in chimpanzees, our closest living relative.
That difference affects the size and position of the lunate sulcus. And in Homo naledi, it’s smaller and farther back—more like a human brain than a chimpanzee brain. (There aren’t any good, universally accepted endocasts of a lunate sulcus from other extinct hominins for comparison yet.)
Structure first, then size
The presence of these humanlike traits in Homo naledi’s brain structure means that these traits were probably present in our last shared ancestor with the Australopithecines and might, in fact, be part of what it means to be a hominin.
“Brain size was once commonly viewed as one of the most important distinguishing features of the genus Homo,” wrote Holloway and his colleagues in their paper. But it turns out that the same brain structures that show up in humans and in the Homo naledi endocasts show up in several other members of the genus Homo, even though brain size varies widely from around 450 to 600 cubic cm in Homo naledi to 1,450 cubic cm in modern humans. Many of those structures are more thoroughly developed in humans than in our extinct hominin relatives, but they were present at the beginning of our lineage.
That means that the development of those structures wasn’t simply a consequence of evolving bigger brains—although increased brain size did eventually enable us to better develop those abilities. Language, social behavior, and motor planning may have been important criteria for natural selection around the time Homo branched off from Australopithecus, the researchers suggest.
Where does Homo naledi fit?
So Homo naledi has a brain about the size of an Australopithecine but built (more or less) like a human’s. Where does it fit into the hominin family tree? That has been a puzzle since its discovery in 2015, when bones from 14 individuals revealed a mixture of modern and archaic qualities. The mystery got even more interesting in 2017 when uranium-series dating put Homo naledi on the scene at around the same time as early modern humans. To draw more specific conclusions about our relationship to our recently discovered cousin, paleoanthropologists need more information—and that means more fossils.
“We have so little evidence. It sounds great when you say that we have this skull and that portion of the skull and so forth, but we really need an awful lot more. Our picture is very, very foggy,” said Holloway. He is especially interested in getting a slightly earlier glimpse at Homo naledi's history.
"If we get more fossils that are coming out of roughly 400,000 to 600,000 years ago, that would be very helpful," Holloway told Ars. "Unfortunately you can’t do DNA analysis at that age right now, which is what you would really need to clarify [phylogenetic relationships], but I think if some more fossils come out of the zone that is between 600,000 to 400,000 years old, it would be very helpful."
And the same goes for other hominin species, many of whom we know even less about than Homo naledi, according to Hawks.
“We badly need more evidence from across the skeleton of many other hominin species. We literally know nothing about the body of Homo rudolfensis, for example,” Hawks told Ars. “Right now, we know more about Homo naledi, Neanderthals, and modern humans than about any of the other fossil hominin species, and we’ve got to build those other samples.”