Impressions on the inner surface of skull fragments reveal parts of Homo naledi's brain

Kiona N. Smith

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H naledi 800x426Hawks et al 2017

Homo naledi’s brain may have been small, but it looked surprisingly similar to ours, according to a new study that suggests that structure may have come before size in the evolution of hominin brains.

Measurements of skull fragments indicate that Homo naledi’s brain was about the same size as that of an Australopithecine—the genus of primates that lived in Africa 2 to 4 million years ago and may be among our early ancestors. Yet the diminutive species was present in Africa long after the Homo lineage appeared and may have overlapped with modern humans. So how it fits into our family tree is not clear.

A new study reveals that, despite the size, Homo naledi’s brain looked quite different from Australopithecus’ and much more like ours, at least in some very important areas.

In some key ways, the structure of an Australopithecine’s brain looked a lot like that of modern great apes. Human brains are different mostly because of structures that have to do with language and social skills. The fact that Homo naledi’s looks so much more like ours hints that those changes happened early in the history of our lineage—closer to the base of our branch of the family tree.

Fancier frontal lobes

How do we know what extinct hominins’ brains looked like? The inside of the skull sometimes preserves an impression of the outer surface of the brain, so if you make a cast of the inner surface of the skull (paleoanthropologists call this an endocast), you’ll get a decent look at the shape of an animal’s brain. Paleoanthropologists have studied endocasts from two Australopithecine species and a handful of Homo species—and now they have expanded that list to include Homo naledi.

In casts from Australopithecus sediba (2 million years ago) and Australopithecus africanus (3.3 to 2.1 million years ago) skulls, we can see a structure at the front of the brain called the fronto-orbital sulcus. But in modern humans, the frontal lobes of the brain have expanded toward the back and sides, creating structures called the frontal opercula.

The frontal opercula contain several structures with roles in language, social behavior, and the kinds of motor planning you’d need in order to (for instance) make stone tools. Those structures include Broca’s area, a structure on the left side of the brain that’s strongly associated with the ability to produce speech.

Frontal opercula show up in endocasts from some early Neanderthal relatives from Sima de los Huesos in Spain and even in some Homo erectus (1.5 million years ago) endocasts. And we can say, at least, that no fronto-orbital sulcus—the structure found in Australopithecines—shows up on endocasts from earlier hominins like Homo rudolfensis (1.8 million years ago) and Homo habilis(2.5 to 1.5 million years ago), although researchers are still debating whether those casts show signs of frontal opercula.

We think that another look at these specimens would be really useful, to see if the evolution of this brain region happened all in one step or whether there were different stages that these older fossils might represent,” University of Wisconsin paleoanthropologist John Hawks told Ars.

Naledi’s brain

In the Homo naledi endocasts, Columbia University anthropologist Ralph Holloway and his colleagues found clear signs of frontal opercula. In casts from one skull, Brodmann’s area 47—a part of the frontal lobe associated with processing syntax and recognizing social emotions—is especially clearly visible.

BrainsA: 3D model of a Chimpanzee brain; B: 3D model of a human brain; C: endocast of an Australopithecus sediba skull; D: endocast of a Homo naledi skull. Hawks et al 2018

Does that mean Homo naledi had a language?

What exactly the span of the communicative abilities would be—there's no way of knowing just looking at an endocast,” Holloway told Ars. But since parts of Broca’s region are so clearly visible on the casts, “It really does suggest that their communicative abilities probably included some rudimentary language. Certainly that’s where my speculation would go.”

In fact, based on endocasts belonging to other hominin species from the 1.8 million-year-old Homo rudolfensis to the 400,000-600,000-year-old Homo heidelbergensis (this is including Homo erectus at about 1.5 million years old), the capacity 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 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.”

PNAS, 2018. DOI: 10.1073/pnas.1720842115  (About DOIs).