acity for at least rudimentary language may stretch back around 1.8 million years in the human lineage.
But it’s still hard to say exactly how “smart” Homo naledi may have been.
Hawks told us:
The best way to answer this question is to build a better picture that draws in many kinds of evidence. We have debated how smart Neanderthals were for 150 years—and now that scientists are bringing together more evidence from archaeology, from pigments, from the brain and genetics, it is clear that they were not so different from us. We don’t have that breadth of evidence yet for Homo naledi, and that is our challenge, to find these other sources of evidence about behavior.
What’s in the back of your mind
The casts also show that Homo naledi’s occipital lobes were larger on one side than on the other—another humanlike trait. In humans, that asymmetry is probably associated with language-processing and producing centers of our left hemispheres being more developed. (That asymmetry might, as a side effect, be the reason most of us tend to have a dominant right hand.)
The Homo naledi casts suggest that asymmetry but aren't conclusive. Thanks to greater capacity for language, this may be a common trait shared by hominins but probably not our last common ancestor with the other great apes.
“We do know that apes, chimpanzees, [and] gorillas do have some handedness, but they don't show the same level or more consistent pattern that you find in hominins,” said Holloway.
Two of the skulls had larger left occipital lobes, but that doesn’t necessarily mean that those Homo naledi individuals were right-handed. Typically in humans, a larger left occipital lobe and right frontal lobe are associated with right-handedness, but none of the Homo naledi skulls preserves details of the right side of the frontal lobes, which limits what we can say about handedness.
“We can only speculate about handedness in Homo naledi, unfortunately,” said Hawks. “In some other hominins, like Neanderthals, scientists have some evidence that they were more often 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