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.
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.
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 cap