Carl Sagan observed that the frontal lobe of the human brain, comprising more than two-thirds of our brain mass, is where “matter is transformed into consciousness.” Maybe, suggest scientists, we’ve had the story of human evolution wrong: that language evolved before our brains started getting larger (we have brains 3x the size of apes), and language led to brain size increase instead of being a result of it?
An extraordinary discovery in 2017 suggests, perhaps yes, when paleoanthropologists demonstrated that the species Homo naledi existed in southern Africa between 236,000 and 335,000 years ago–potentially the same time that modern humans first emerged in Africa.
This discovery presented a puzzle for scientists, who long held that there was only one species in Africa at this late time period – Homo sapiens.
How did this species exist alongside others with brains three times its size? A 2018 study suggests that naledi’s behavior may have reflected the shape and structure of the brain more than its size.
The small brains of Homo naledi raise new questions about the evolution of human brain size (image above). 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.
According to study coauthor John Hawks, a paleoanthropologist at the University of Wisconsin–Madison. “Naledi’s brain seems like one you might predict for Homo habilis, two million years ago. But habilis didn’t have such a tiny brain–naledi did.”
“Maybe brain size isn’t all it’s cracked up to be,” said Hawks. “It opens the door for us to say that maybe they were more capable than we might assume; maybe it isn’t just (brain) size.”
The research shows that the more complex structural features of brains may not solely be a consequence of size, and it suggests that modern humans, Neanderthals and Homo naledi may have a common ancestor.
Tiny Brain Packed a ‘Big Punch’
Homo naledi may have had a pint-sized brain, but that brain packed a big punch. Research by Ralph Holloway and colleagues – that include researchers from the University of the Witwatersrand, Johannesburg, South Africa – published in the Proceedings of the National Academy of Sciences examines the imprints of the brain upon the skulls of this species, called endocasts. The research highlights the humanlike shape of naledi’s tiny brain, surprising scientists who studied the fossils.
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. (Heather Garvin, Des Moines University).
Hawks and study co-author Shawn Hurst, then at Indiana University Bloomington, presented some of the size and structure data at a meeting in April 2017, but the most recent study includes new comparative data that examines differences between Homo naledi and its Australopithecus cousins — sediba and africanus
In these cousins and in apes, a groove called the fronto-orbital sulcus is visible. In humans and other Homo species, this feature isn’t there, because two neighboring areas of the frontal lobe have expanded, partly covering over another area known as the insula. The best-preserved endocast from Homo naledi also lacks evidence of the fronto-orbital sulcus and mirrors the human form despite its small size.
In these cousins and in apes, reported the University of Wisconsin, “a groove called the fronto-orbital sulcus is visible. In humans and other Homo species, this feature isn’t there, because two neighboring areas of the frontal lobe have expanded, partly covering over another area known as the insula. The best-preserved endocast from Homo naledi also lacks evidence of the fronto-orbital sulcus and mirrors the human form despite its small size.”
Questions a Long-held belief
These findings draw further into question the long-held belief that human evolution was an inevitable march towards bigger, more complex brains.
The Rising Star Caves Fragments
The discovery of Homo naledi by Lee Berger of Wits University Centre for Exploration of the Deep Human Journey and his team at the Rising Star caves in the Cradle of Human Kind 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. Berger and John Hawks who was also part of the original Rising Star team who made the naledi discovery, as well as as well as Heather Garvin from Des Moines University in the US, are associated with the Evolutionary Studies Institute (ESI), based at Wits University.
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.
“This is the skull I’ve been waiting for my whole career,” said lead author Ralph Holloway, of Columbia University.
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.
“It’s too soon to speculate about language or communication in Homo naledi,” said coauthor Shawn Hurst, “but today human language relies upon this brain region.”
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.
Language may have led to brain size increase
“But we did not find asymmetry in the frontal lobe (asymmetry would be if the left and right sides are different) because we didn’t have a skull with both left and right sides of the frontal, not because we looked and it wasn’t there,” said Hurst.
“We’re now studying a more complete skull found later, but we couldn’t determine if there was asymmetry in the Rising Star caves fragments, not that it didn’t have any. Instead we found that the frontal lobe of Homo naledi, despite being similar in size to a chimpanzee frontal lobe, looked like a human frontal lobe in form, particularly in the area where humans produce speech (Broca’s language area),” Hurst told The Daily Galaxy.
“This suggests,” he said, “some sort of language evolved before our brains started getting larger (we have brains 3x the size of apes) and that language may have led to brain size increase instead of being a result of it.
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 complex stone tool industries were made by modern humans,” said Berger. “With naledi being found in southern Africa, at the same time and place that the Middle Stone Age industry emerged, maybe we’ve had the story wrong the whole time.”
Image credit: Top of page,. with thanks to DepositPhotos
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