Our brains don’t look that special when looking at their relative size compared to our closest animal relatives. To understand human intelligence, scientists are now looking deeper.
Arthur Keith was one of those misbegotten researchers who have turned out to be wrong in many of the things they said. A prominent anatomist and anthropologist in the early 20th Century, he was a proponent of scientific racism and opposed racial mixing. At least partly because of his racial views, he was convinced humans originated in Europe, not Africa as is now universally accepted. And he was a strong supporter of Piltdown Man, a notorious hoax involving fake fossils.
Keith also described a notion that became known as the cerebral Rubicon. Noting that humans have larger brains than other primates, he argued that human intelligence only became possible once our brains reached a particular threshold size. For Homo, the genus to which we belong, he thought the minimum volume was around 600-750 cubic cm (37-46 cubic inches). For our species Homo sapiens, it was 900 cubic cm (55 cubic inches). Any smaller, the argument went, and the brain wouldn’t have enough computational power to support human reasoning.
It’s certainly true that Homo sapiens, as a species, have large brains. But what this means is increasingly murky. Evidence from palaeoanthropology suggests that some species, such as the “hobbits” Homo floresiensis and Homo naledi, performed complex behaviours despite having fairly small brains. These reports are contentious. However, there is also gathering evidence from genetics and neuroscience that brain size is far from the be-all-and-end-all of intelligence.
Instead, changes to the brain’s wiring diagram, to the shapes of neurons, and even to when and where certain genes are turned on, are all equally if not more important. Size, as we might have guessed, isn’t everything.
Small-brained smarts
It’s certainly true that the human brain is unusually large. This remains true even if you look at brain size relative to the size of our bodies. “Humans are by far the primates with the largest brain,” says neuroscientist Martijn van den Heuvel of the Free University Amsterdam in the Netherlands.
It’s also true that if you look at the last six million years of human evolution, there is a trend towards increasing brain size. Early hominins like Sahelanthropus and Australopithecus have relatively small brains, but the first Homo species have bigger ones, and Homo sapiens brains are bigger still.
However, when you look more closely at the details, the story isn’t so simple. Two species stand out for their unusually small brains: Homo floresiensis, also known as the real-life “hobbit”, and Homo naledi. Both are 21st-Century discoveries.
H. floresiensis was first described in 2004. They were just 1m (3ft) tall and lived on the island of Flores in Indonesia within the last few hundred thousand years. They died out at least 50,000 years ago. The first specimen had a brain measuring just 380 cubic cm (23 cubic inches) or perhaps 426 cubic cm (26 cubic inches), putting her on a par with chimpanzees.
There is solid evidence that H. floresiensis made and used stone tools, much as other Homo species did. Early studies also reported evidence of burning, suggesting the hobbits had control of fire. However, later re-analyses suggested the fires were all lit more recently than 41,000 years ago – suggesting they were made by modern humans, not the hobbits. Nevertheless, the stone tools alone are evidence that hobbits behaved in ways that chimpanzees cannot.
A decade later, researchers in South Africa described Homo naledi. The remains were found deep in the Rising Star cave system, which only experienced cavers can reach. Like the hobbits, H. naledi had small brains – but they also lived recently, between 200,000 and 300,000 years ago.
Lead researcher Lee Berger and his colleagues have described soot marks on the cave ceilings, which they interpret as evidence H. naledi had control of fire. It’s thought that they may have lit torches to navigate in the darkness of the deep caves.
In 2021, Berger’s team described the skull of a H. naledi child, which seemed to have been placed on a shelf-like formation in an extremely inaccessible chamber. They interpreted this as a deliberate burial. In July, they released a follow-up claiming that several skeletons had been interred in the floor of the cave, adding to the evidence for funerary behaviour.
This latest study caused a furore among palaeoanthropologists, partly because Berger announced his results before the paper had gone through the usual scientific process of peer review – including in a high-profile Netflix documentary called Unknown: Cave of Bones. When other researchers did peer review the study, some were extremely critical, saying the study “does not meet the standards of our field” and that “there is a significant amount of missing information”.
Connection patterns found in humans but not chimpanzees were often associated with a higher risk of schizophrenia
The debate over the behaviours and capabilities of H. floresiensis and H. naledi, along with their implications for the role of brain size, will likely continue for years to come. Meanwhile, another set of researchers have tackled human brain evolution in a different way: instead of examining fossilised bones, they study actual brains.
Anatomy of the mind
The first thing to note is that, although on average humans have unusually large brains, size does vary. “There are patients that have smaller brain size,” says neurobiologist Debra Silver at Duke University in Durham, North Carolina. People with microcephaly – where their head is abnormally small – often have intellectual disabilities and other symptoms. Nevertheless, says Silver, “they’re still human”. There are also cases where people are missing large chunks of their brains, and show relatively few ill effects
Clearly, something else is going on. One possibility is the brain’s wiring diagram or “connectome”. The human brain contains around 86 billion specialised cells called neurons, which connect to each other and send signals back and forth. Many neuroscientists suspect that changes to the pattern of connections are more important for the development of human cognition than anything as crude as the brain’s volume.
“Even small changes in connectivity, especially in long range connectivity, it really leads to profound cognitive and behavioural changes,” says neuroscientist Nenad Sestan at Yale University in New Haven, Connecticut.
In particular, some parts of the human brain receive inputs from many other regions. This allows them to integrate multiple pieces of information, and make decisions accordingly. The prefrontal cortex, at the outermost front of the brain, is one such region. Sestan calls it “the CEO of the brain”.
“A little bit more of this integrative circuitry is really beneficial for human cognitive abilities,” agrees van den Heuvel. In a study published in May, his team showed that human and chimpanzee brains share many patterns of connectivity, but humans have stronger connectivity between regions involved in language.
These integrated areas of the brain have also been associated with psychiatric disorders. For example, in 2019 van den Heuvel’s team showed that connection patterns found in humans but not chimpanzees were often associated with a higher risk of schizophrenia. This suggests humans have made an evolutionary trade-off: greater intelligence in exchange for a higher risk of poor mental health.
Evidence like this suggests the connectome is important. But what about the neurons themselves: are human neurons different to chimpanzees’?
