Human brains are larger than those of our primate relatives, but evidence from extinct human ancestors suggests that size isn't everything.
To understand human intelligence, scientists now have to dig deeper.
Arthur Keith was one of those notorious explorers who turned out to be wrong about many of the things they claimed.
A prominent anatomist and anthropologist from the beginning of the 20th century, he was a supporter of scientific racism and an opponent of racial mixing.
At least partly because of his racist views, he was convinced that humans originated in Europe, not Africa, as is now universally accepted.
And he was a strong supporter of Piltdown Man, the notorious fake fossil hoax.
Keith also described an idea that became known as the cerebral Rubicon.
Pointing out that humans have larger brains than other primates, he argued that human intelligence became possible only when our brains reached a certain threshold in size.
For Homo, the genus we belong to, he thought the minimum volume was around 600-750 cubic centimeters.
For our species, Homo sapiens, it was 900 cubic centimeters.
Any less, the argument went, and the brain would not have enough computing power to support human reasoning.
In any case, it is true that Homo sapiens, as a species, has a larger brain.
But what exactly that means is less and less clear after that.
Evidence from paleoanthropology suggests that some species, such as the "hobbits" Homo floresiensis and Homo naledi, exhibit complex behavior despite having relatively small brains.
Those reports are hotly contested.
However, there is also increasing evidence from genetics and neuroscience that brain size is by no means the key and definitive factor in intelligence.
Instead, changes in the brain's connectivity diagram, the shape of neurons, and even when and where certain genes are activated are just as important, if not more so.
As we could have guessed, size is not everything.
The wisdom of small brains
It is certainly true that the human brain is unusually large.
This remains the case even if you compare the size of the brain to the size of our body.
"Humans are the primates with by far the largest brains," says neuroscientist Martin Van Den Hovel of the Free University of 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 such as Sahelanthropus and Australopithecus have relatively small brains, but the first Homo species have larger ones, and since the appearance of Homo sapiens the brain has been even larger.
However, if you dig into the details, you will see that the story is not 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 discoveries from the 21st century.
H. floresiensis was first described in 2004.
They were only a meter tall and lived on the island of Flores in Indonesia for the last few hundred thousand years.
They died out at least 50.000 years ago.
The first specimen had a brain volume of only 380 cubic centimeters or perhaps 426 cubic centimeters, which was at the level of chimpanzees.
There is strong evidence that H. floresiensis made and used stone tools, just like most other Homos.
Early studies also reported evidence of burning, suggesting hobbits had control over fire.
However, later repeated analyzes showed that all the fires were lit earlier than 41.000 years ago - suggesting that they were lit by modern humans and not hobbits.
Still, the stone tools alone are proof enough that hobbits behaved in ways that chimpanzees could not.
A decade later, researchers in South Africa described Homo naledi.
Their remains were found deep in the Rising Star cave system, which can only be reached by experienced cavers.
Like the hobbits, H. naledi had a small brain - but they also lived earlier, between 200.000 and 300.000 years ago.
Lead researcher Lee Berger and his colleagues described traces of soot on the ceilings of the caves, which they interpret as evidence that H. naledi had control over fire.
It is believed that they may have lit torches to make it easier for them to navigate in the dark in the deep caves.
In 2021, Berger's team described the skull of a child H. naledi, which appeared to have been placed on a shelf-like scalameria in an extremely inaccessible chamber.
They interpreted this as a tendentious burial.
In July, they published an addendum claiming that several skeletons had been laid in the cave floor, adding evidence of burial customs.
The latest study caused a stir among paleoanthropologists, in part because Berger published his results before the work went through the normal scientific process of peer-review — among them the popular Netflix documentary "The Unknown: Cave of Bones."
When other researchers did a peer review of the study, some were extremely critical, saying that the study "does not meet the standards of our field" and that "there is a significant amount of missing information."
Connectivity patterns found in humans but not in chimpanzees are often associated with higher risk of schizophrenia
The debate over the behavior and abilities of H. floriensis 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 tackled the evolution of the human brain in a different way: instead of studying the fossil remains of bones, they studied the brain itself.
Anatomy of the mind
The first thing to point out is that, although on average people have a large brain, its size does vary.
"There are patients with smaller brains," says neurobiologist Debra Silver of Duke University in Durham, North Carolina.
People with microcephaly - when their head is abnormally small - often have intellectual disabilities and other symptoms.
Still, says Silver, "they're still human."
There are also cases when people are missing large pieces of the brain and show relatively few defects.
Ljlimb brain in numbers
At about 1,5 kilograms, the human brain is about two to three times smaller than an elephant's.
It is up to six times smaller than the brain of some whales and dolphins.
The human brain contains 86 billion neurons and 85 billion non-neuronal cells.
But despite the fact that it takes up about 2 percent of the body mass of an average adult, the human brain burns about 20 percent of the calories we consume.
Apparently something else is going on there.
One possibility is a brain connectivity diagram or "connectome".
The human brain contains about 86 billion specialized cells called neurons, which connect to each other and send signals back and forth.
Many neuroscientists suspect that changes in connection patterns are more important to the development of human cognition than anything as crude as brain volume.
"Even small changes in connectivity, especially long-range connectivity, really lead to serious cognitive and behavioral changes," says neuroscientist Nenad Sestan of Yale University in New Haven, Connecticut.
Some parts of the human brain especially receive information from many other regions.
This enables them to integrate various information and make decisions accordingly.
The prefrontal cortex, at the very front of the brain, is one such region.
Šestan calls her the "executive director of the brain".
"A little more of this interactive circuitry is really beneficial for human cognitive abilities," agrees Van Den Huvel.
In a study published in May, his team showed that human and chimpanzee brains share many patterns in connectivity, but that humans have stronger connectivity between regions involved in language.
These integrated areas of the brain are also associated with psychiatric disorders.
For example, in 2019, Van Den Hoovel's team showed that connectivity patterns found in humans but not in chimpanzees are often associated with a higher risk of schizophrenia.
This suggests that humans have made an evolutionary trade-off: higher intelligence in exchange for a higher risk of poor mental health.
Evidence like this suggests that the "connectome" is important.
But what about the neurons themselves: are human neurons different from chimpanzee neurons?
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Changed cells
"There's a long history of people looking for specific, unique neurons in the human brain," says Van Den Huvel.
One of the first attempts was by Konstantin For Ekonomo, an Austrian neurologist active in the early 20th century.
He identified spindle neurons in the human spinal cord: they are sometimes called "von Econom neurons".
At first they were thought to be unique to humans, says Von Den Huvel, "but later they found Von Econom neurons in other brains."
More recently, in 2022, Šestan and his colleagues studied cells in a part of the brain known as the dorsolateral prefrontal cortex in humans, chimpanzees and monkeys.
They could only find one cell type that was unique to humans.
It was not a neuron, but a microglial cell: part of the immune system of the brain.
The cells looked normal on the outside, but they activated a unique set of genes.
Šestan does not want to exaggerate the importance of these discoveries.
"I don't think this is crucial," he says.
"There is no reason to believe that microglia give us cognitive abilities."
Neurons unique to humans may be hard to find, but it is clear that the percentage of different cell types has changed during our evolution.
Silver says that Von Econom neurons are more common in humans and great apes than in other primates.
They "could help take on new tasks," she suggests.
If we have cortical neurons that make dopamine, they could be a 'thinking-only reward system'
Understanding the modified neurons in the human brain requires understanding how cells develop and grow.
We can't study this in living embryos for obvious reasons, but researchers can study neurons grown in the lab.
In recent years, they have also grown "organoids": clusters of cells that mimic the structure and behavior of developing parts of the brain.
The field has produced a veritable blizzard of discoveries, most of which are not fully understood, says Barbara Troitlein, a developmental neurobiologist at the ETH in Zurich, Switzerland.
However, one pattern clearly stands out.
"It takes longer for people to make neurons and for the neurons to actually mature," she says.
"In chimpanzees, neurons mature faster than in humans."
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Troitline tentatively links this slow maturation of neurons to the relatively longer time period required for human babies to develop compared to chimpanzees.
However, she also says that we cannot yet make strong connections between her studies of neuron development - which never mimic anything after the second trimester of pregnancy - and the behavior of adults.
There is another factor to consider - the human genome and its influence on our brain.
Expressive genes
It is known that humans and chimpanzees share 99 percent of our DNA.
"But the point is that we are not only 1 percent different from chimpanzees," says Šestan.
The difference is obviously more drastic than that.
Geneticists have discovered parts of the genome that are unique to humans, and many of them appear to play a role in the brain.
For example, a 2019 study looked at parts of DNA specific to humans and found that many of them have effects on cells known to be involved in brain expansion.
Similarly, a gene named SRGAP2C is unique to the genus Homo.
In a 2019 study, researchers discovered this hominin gene in mice and found that it alters their "connectome," creating additional connections between certain levels of the cortex.
"It changes neuronal activity and neuronal morphology at the circuit level," says Silver.
During the long period of human evolution, many genes have changed.
In February, Von Den Huvel's team published a timeline of 13.5 million mutations unique to humans over the past five million years - dating back to before the Homo branch of the evolutionary tree.
They found two bursts of mutations unique to humans.
The first one happened around 1,9 million years ago, around the time when the species Homo erectus developed.
The second was between 62.000 and 1.500 years ago.
Mutations associated with cognition were often relatively young, says Van Den Huvel.
It is not about the DNA sequence itself: as Šestan's study of microglia shows, the essence is also in which genes are activated in each cell.
Changes in "gene expression" can give cells fundamentally different shapes and behaviors, despite having the same genome.
The complexity here is dizzying.
A 2021 gene expression study showed that some genes important in the brain can make 100 proteins each, depending on how they are expressed.
One gene expressed in developing humans but not in chimpanzees controls a whole network of other genes believed to be involved in human brain development.
Some changes in gene expression are intriguing.
In a 2017 study, Šestan's team compared gene expression in the brains of humans, chimpanzees, and monkeys.
They found that some neurons in one region of the human brain expressed genes active in making dopamine, a brain chemical involved in feelings of reward.
Equivalent cells in chimpanzees and monkeys did not contain these genes.
"We grew these neurons," says Šestan.
"They can make dopamine 'in a test tube'."
"If this were true in the real brain, it means that we humans could produce dopamine internally in the cortex," says Sestan.
He has an intriguing speculation about what that might mean.
People can feel satisfaction just from thinking and solving problems, which could be really unique to them.
If we have cortical neurons that make dopamine, they could be a "thinking-only reward system."
He points out, however, that for now this is just speculation.
We have come a long way from simply comparing the brain sizes of different primates.
Scientists are now tracking changes in genome sequence, changes in gene expression, changes in both the shape and behavior of cells, and changes in the brain's connectivity diagram.
What we lack is "an understanding of how all these elements, as an interplay, become a system and how this system shapes our behavior," says Van Den Huvel.
Troitline and her colleagues took a major step in this direction in 2019, publishing an "atlas" of every cell in the human brain at an early stage of development.
In 2023, a team of 500 researchers from across Europe announced the completion of the Human Brain Project, a ten-year project dealing with the complex structure and function of the brain.
A huge ongoing project called the Human Cell Atlas wants to build on the insights gained so far.
Its members want to map every type of cell in the human body: their location, shape, gene expression, and more.
"There are so many cell types in the brain," Troitlein says.
The challenge will be to interpret that huge data set.
Although the project will take decades, it is already possible to draw some conclusions about the size of the brain.
"I think she's just one of many factors," says Silver.
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