Homo Naledi, an Ancient Hominin, Had Human-Like Features In Its Brain

In recent years, scientists have shaken off the myth that the size of the human brain rocketed us to the top of the food chain. While the human brain is three times larger than that of chimpanzees and bonobos, it’s increasingly understood that it’s the brain’s morphology and molecular activity — not size — that dictates intelligence. A study on the ancient hominin Homo naledi, released Monday in Proceedings of the National Academy of Sciences, presents evidence supporting this theory, showing that it’s the design of the Homo brain that set humans and our prehistoric relatives apart as a genus.

The new study, which used H. naledi remains between 236,000 and 335,000 years old, suggests that while the hominins had small brains and primitive bodies compared to that of other Homo species (like humans, the Neanderthals, and the “hobbit” H. floresiensis), their cranial anatomy was actually quite similar. That similarity lives on in human brains today and suggests that this brain organization was likely present in the common ancestor we share with the H. naledi.

“The fact that Homo naledi’s brain had characteristics consistent with modern human brains despite its small size reconfirms that brain organization is likely more important than brain size — and that the two, brain size and morphology, are not necessarily linked,” study co-author and Des Moines University associated professor Heaver Garvin, Ph.D. tells Inverse. “This is all despite the fact that H. naledi also exhibits some anatomical features consistent with climbing and a brain size about one-third of ours.”

Garvin and her colleagues, led by Columbia University anthropologist Ralph Holloway, Ph.D. compared H. naledi’s brain morphology to that of other hominids — a group that includes all modern and extinct great apes — by creating models from four H. naledi cranial fragments. These “endocasts” are digital impressions of the interior of the skull, which allowed the scientists to study the brain’s cranial surface features.

The details they were able to extract, says Gavin, were “extraordinary.” Like that of modern humans, the H. naledi brain contained a frontal operculum but no fronto-orbial sulcus, a feature found in the brains of apes and Australopithecus, an early human species that lived between 3.86 and 2.95 million years ago that includes the infamous Lucy. Like H. naledi, humans and other Homo species also lack a fronto-orbial sulcus. The endocasts also showed that H. naledi had asymmetry in its occipital lobe and lunate sulcus morphology resembling that of humans.

That the brain of H. naledi differs from that of Australopithecus is key. Discovered in 2013 within the Rising Star Cave system outside Johannesburg, South Africa, H. naledi is characterized as a curious mix of the Homo and Australopithecus genera. This analysis confirms that H. naledi was more similar to the early ancestors of modern humans, which were living in continental Africa at the same time. There’s not enough evidence to show whether they actually interacted with humans, but Gavin says “it’s always possible.”

Though their brains were more like ours than scientists first expected, it’s also difficult to determine whether their behavior was similar, too.

“There is a lot that we need to figure out even in regards to even our own brain and behaviors, so I think it’s a little early to make any concrete connections between H. naledi’s brain morphology and specific behavioral characteristics,” says Gavin. “That being said, the fact that H. naledi endocasts share features with other Homo species, features that are different from the Australopithecines, suggests that H. naledi likely exhibited more Homo-like behaviors.”

These behaviors are connected to the brain features highlighted in this paper. Their ubiquity in Homo brains, Gavin and her team write in the paper, “suggests a behavioral niche with serialized communication, planning, and complex action sequences that underlie tool production as well as an increased display of prosocial emotions.” The team hypothesizes that the increase in size of the hominin brain reflects an evolutionary adaptation to the structures demonstrated across Homo specimens. Our brains are a link to hominin cousins like the H. naledi, and in turn, to the first of our genus. Whether or not that initial Homo individual also had a small brain is another question that we can’t answer without more data from our family tree.