Researchers pieced together traces of Homo naledi's brain shape from an extraordinary collection of skull fragments and partial crania, from at least five adult individuals. One of these bore a very clear imprint of the convolutions on the surface of the brain's left frontal lobe. The recently-discovered species Homo naledi may have had a pint-sized brain, but that brain packed a big punch. A new study by researchers that include the imprints of the brain upon the skulls of this species, called endocasts. The study was published in the Proceedings of the National Academy of Sciences.
The research highlights the humanlike shape of naledi's tiny brain, surprising scientists who studied the fossils. These findings draw further into question the long-held belief that human evolution was an inevitable march towards bigger, more complex brains. In 2017, geologists demonstrated that this species existed in southern Africa between 236,000 and 335,000 years ago potentially at the same time that modern humans first emerged in Africa.
This is a puzzle to scientists, who long held that there were only one species in Africa at this late period Homo sapiens. The new study suggests that naledi's behavior may have reflected the shape and structure of the brain more than its size.
The researchers pieced together traces of Homo naledi's brain shape from an extraordinary collection of skull fragments and partial crania, from at least five adult individuals. One of these bore a very clear imprint of the convolutions on the surface of the brain's left frontal lobe.
The anatomy of naledi's frontal lobe was similar to humans and very different from great apes. Naledi wasn't alone. Other members of our genus, from Homo erectus to Homo habilis and the small-brained "hobbits," Homo floresiensis, also share features of the frontal lobe with living humans. But earlier human relatives, like Australopithecus africanus, had a much more apelike shape in this part of the brain, suggesting that functional changes in this brain region emerged with Homo.
The back of the brain also showed humanlike changes in Naledi compared to more primitive hominins like Australopithecus. Human brains are usually asymmetrical, with the left brain displaced forward relative to the right. The team found signs of this asymmetry in one of the complete Naledi skull fragments. The small brains of Homo naledi raise new questions about the evolution of human brain size. Big brains were costly to human ancestors, and some species may have paid the costs with richer diets, hunting and gathering, and longer childhoods.
A humanlike brain organization might mean that Naledi shared some behaviors with humans despite having a much smaller brain size. Lee Berger, a co-author on the paper, suggests that the recognition of Naledi's small but complex brain will also have a significant impact on the study of African archaeology.
Archaeologists have been too quick to assume that modern humans made complex stone tool industries. With Naledi being found in southern Africa, at the same time and place that the Middle Stone Age industry emerged, maybe they have had the story wrong the whole time.