Where hominid brains are concerned, size doesn’t matter

Image of three of the endocast specimens used in the PNAS study to evaluate Homo naledi’s brain morphology placed in the approximate position in the cranial vault of the Homo naledi reconstruction. Image created by Heather Garvin using 3D scans.

The recently discovered species Homo naledi may have had a pint-sized brain, but that brain packed a big punch. New research by a group of scientists including Heather Garvin, Ph.D., D-ABFA, associate professor of anatomy at DMU, examines the imprints of the brain upon the skulls of this species, called endocasts. Published in the Proceedings of the National Academy of Sciences (PNAS), 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 toward bigger, more complex brains.

The discovery of Homo naledi in 2013 was one of the largest hominin discoveries ever made and hailed as one of the most significant hominid discoveries of the 21st century. In 2017, geologists demonstrated that this species existed in southern Africa between 236,000 and 335,000 years ago—potentially the same time that modern humans first emerged in Africa. This is a puzzle to scientists, who long believed only one species existed in Africa at this late time period—Homo sapiens. How did this species exist alongside others with brains three times its size? The new study suggests that naledi’s behavior may have reflected the shape and structure of the brain more than its size.

The researchers—who, in addition to DMU, represent Columbia University, Indiana University, the University of the Witwatersrand and the University of Wisconsin-Madison—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.

Homo naledi endocast (top) with a curvature map (bottom) highlighting the sulci that are visible. The frontal of naledi’s brain looked very human-like despite its small size. Image: Heather Garvin, DMU

Garvin’s role was working with the 3D scans of the specimens, extracting the endocast surfaces and provided the imaging used for making comparisons, including the curvature maps. “In previous publications of Homo naledi, I was responsible for estimating the brain size and body size of this new species of human ancestor,” she says. “I was on the original team that described and analyzed the remains in 2014 that led to the announcement of a new species.”

This new research revealed that 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 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,” Garvin told the science website 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.”

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 most complete naledi skull fragments. They also found hints that the visual area of the brain, in the back of the cortex, was relatively smaller in naledi than in chimpanzees—another humanlike trait.

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. But that scenario doesn’t seem to work well for Homo naledi, which had hands well-suited for toolmaking, long legs, humanlike feet and teeth suggesting a high-quality diet.

A humanlike brain organization might mean that naledi shared some behaviors with humans despite having a much smaller brain size.

“There is a lot that we need to figure out in regard 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,” Garvin told Inverse. “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.”

In addition to Garvin, authors of the research are Ralph Holloway, Columbia University; Shawn Hurst, Indiana University; P. Thomas Schoenemann, Indiana University; William Vanti, Columbia University; Lee Berger, the University of Witwatersrand and the National Geographic Society; and John Hawks, University of Wisconsin-Madison and Witwatersrand. The work was funded by the National Geographic Society, the Lyda Hill Foundation, the National Research Foundation of South Africa, the Vilas Trust, the Fulbright Scholar Program and the Wisconsin Alumni Research Foundation. It has been published in several media outlets, including the New York Times, Discover Magazine and Science Daily.