Our modern civilization may be one of the most advanced ever on Earth, but about 100 generations ago, our ancestors' brains were larger than our own.
Your ancestor's brain was bigger than yours. Thousands of years ago, humans reached a milestone in their history - the first known complex civilization began to emerge. The people who walked around and met in the world's earliest cities were in many ways familiar to today's modern urbanites. But since then, the human brain has actually shrunk slightly.
Jeremy DeSilva, an anthropologist at Dartmouth College in the United States, says that on average, the volume lost is about the size of four ping pong balls. This shrinkage began 3,000 years ago, according to an analysis of fossilized skulls that he and his colleagues published last year.
"It's much later than we expected," DeSilva said. "We were expecting something closer to 30,000 years ago."
Agriculture emerged between 10,000 and 5,000 years ago, although there is evidence that plant cultivation may have begun as early as 23,000 years ago. A vast civilization of buildings and machinery soon followed. The first texts appeared around the same time. Why did the human brain start getting smaller in this age of rapid technological development?
It's a question that has puzzled researchers. It also raises the question of what the size of the brain really reveals about an animal's intelligence or cognitive ability. Many species have much larger brains than ours, but their intelligence - as far as we know - is quite different. Thus, the relationship between brain size and the way humans think cannot be straightforward. There must be other factors.
Exactly what drives the brains of specific species to become larger or smaller over time is also often difficult to know. deSilva and his colleagues note that the human body has become smaller over time, but not enough to explain the reduction in our brain volume. Questions about why such changes occur remain unanswered. So, in a recent paper, they looked to an unlikely source - the humble ant - for inspiration.
At first glance, or should I say squinting, the ant's brain appears to be completely different from ours. They are about one-tenth of a cubic millimeter in size - or about one-third the size of a grain of salt - and contain only 250,000 neurons. By comparison, the human brain has about 86 billion.
But some ant societies bear a striking resemblance to our own. Surprisingly, there are even ants that are engaging in a form of agriculture where they grow large amounts of fungi in their nests. These ants collect leaves and other plant material for use on their farms before harvesting the fungi. When DeSilva's team compared the brain sizes of various ants, they found that sometimes those with large societies evolved larger brains - unless they also evolved this hobby of growing fungi.
This suggests that, at least for ants, having larger brains is important for performing well in large societies - but in contrast, more complex social systems with a greater division of labor may contribute to the shrinking of their large brains. This may be because cognitive abilities are divided and distributed among many members of the colony, who play different roles.
In other words, wisdom is collective.
"What if this happened to a human?" De Silva said. "What if, in humans, we reach the threshold of population size where individuals share information in the brains of others and externalize it?"
Another possibility is that the advent of writing - which occurred about 2000 years before the size of the human brain was reduced - also had an impact. Writing is one of the relatively few things that separates us from all other species, and DeSilva questions whether this affects brain capacity by "externalizing information in written form and being able to communicate ideas by accessing information outside of your own brain.
The many differences between ant and human brains mean that we should be careful not to draw parallels too hastily. That said, DeSilva thinks this possibility is a useful starting point for thinking about what causes the dramatic and relatively recent reduction in human brain size.
These ideas remain hypothetical for now. There are many other theories that attempt to explain the shrinking size of the human brain. However, if brain shrinkage really existed 3000 years ago, then a significant part of it would become unbelievable. A good example is domestication. Dozens of different animals that have been domesticated, including dogs, have smaller brains than their wild ancestors. But it is estimated that self-domestication in humans occurred tens or even hundreds of thousands of years ago - well before brain atrophy.
But does a smaller brain mean that humans as individuals are becoming more stupid?
Not really, unless you're talking about subtle differences between large populations. 2018 years ago, a group of researchers analyzed a huge amount of data from the UK Biobank, a vast biomedical database containing brain scans and IQ test results of thousands of people, among other things.
Philipp Koellinger, a behavioral geneticist at the Free University of Amsterdam in the Netherlands and co-author of the study, said the study's sample size of 13,600 people is larger than all previous studies on brain size and IQ combined.
The study found that, on average, having a larger brain was associated with slightly better performance on IQ tests, but crucially, the relationship was inconclusive. This means that some people perform very well on the test despite having relatively small brains, and vice versa.
"There really aren't very strong ties," Kollinger said. "It's all over the place."
In terms of a person's general cognitive ability, structural differences may be more meaningful than the absolute size of the brain
This is important, in part because people have historically tried to classify and categorize individuals based on factors such as the size or shape of the head.
"The Western world has a very ugly history of the eugenics movement and all of that based on these ideas about biological determinism," Kollinger said. "The relevance of our report does not imply any form of biological determinism."
Because brain scans also reveal certain information about the structure of people's brains, not just their size, the study was able to detect what else might be going on. It found a relationship between the volume of gray matter (the outer layer of the brain with a particularly large number of neurons) and performance on IQ tests.
In fact, such structural differences may be more meaningful than the size of brain organs in terms of one's general cognitive abilities.
Simon Cox, who studies brain aging at the University of Edinburgh, said, "It would be crazy to think that volume could explain the whole difference." He adds that this may be one of the least important factors.
When you consider it, it makes sense. Because of their larger size, male brains are typically 11 percent larger in volume than female brains. But studies have found that, on average, women have an advantage in some cognitive abilities, while men have an advantage in others.
Cox noted that other studies he has been involved in have revealed how the female brain can compensate for getting smaller through structural differences. For example, on average, women have a thicker cortex (containing the gray matter stratum).
Many features and aspects of the brain appear to affect cognitive abilities. Another example is myelin formation. This refers to the sheath of material surrounding axons, the long, thin "cables" that allow neurons to connect to other cells and form neural networks.
As people get older, their myelin is broken down, which reduces the efficiency of the brain. This change can be detected by studying the ease with which water diffuses through brain tissue. As myelin decreases, water flows more easily. This indicates a decrease in cognitive ability.
Cox said the brain is still "extremely complex" and it is difficult to know exactly how the structural makeup of a particular brain affects a person's intelligence. It's also worth noting that some people who have parts of their brains due to injury or developmental abnormalities don't seem to be affected. In France, a successful civil servant was found to have lost 90% of his brain, but had an IQ score of 75 and a verbal IQ of 84, only slightly below the French average of 97.
However, exceptions can never be interpreted as rules. Ultimately, several studies do show that there is a statistically significant and subtle link between brain volume, structure, and intelligence.
All of this becomes more interesting when you consider the different brains of the animal kingdom. We've explored comparisons between human and ant brains, but what about other species? What drives the evolution of large - or small - brains?
Amy Balanoff, who studies brain evolution at Johns Hopkins University in Baltimore, Maryland, says brain tissue requires a lot of energy to grow and maintain, so it's unlikely a species would evolve a large brain unless it needed to. Think of parasitic organisms that depend on a relatively stable environment and resources, she suggests. Seven-gill eels, for example, have very small brains, only a few millimeters long.
Human infants are actually born with over 100 billion neurons, and as they develop, this number decreases
"They really don't need to spend extra energy on the neural tissue, which is really expensive metabolically," Balanoff said.
In addition, over time, the brains of some animals appear to develop larger relative to their body size - but their brains don't actually change, their bodies just get smaller. This applies to birds, Balanoff explains.
Other animals appear to have evolved specialized brain regions that have larger overall brain size compared to similar species. Take mormyrid fish, for example, whose brains are quite large relative to their size - in fact, their proportions are similar to those of humans. These fish use electrical charges to communicate with each other and detect prey, and in 2018, researchers found that a specific part of their brain - the cerebellum - was unusually heavy. No one is entirely sure why this is, but the study's authors speculate that it helps the fish process electrically sensed information.
In humans, one area of the brain that marks us is the neocortex, which is involved in higher cognitive functions - conscious thought, language processing, etc. There is no doubt that we rely heavily on these things, so it makes sense that our brains will be tailored to our needs.
Anjali Goswami, a paleontologist at the Natural History Museum in London, said that given the amount of energy required to keep the gears turning, it is interesting to note that animals with larger brains have evolved to get a lot of energy early in life.
Think of the nutritional boost birds even get in eggs, or the nutrition mammals get through the placenta or breast milk. Human infants are actually born with over 100 billion neurons, and that number decreases as they develop. This is because the brain fine-tunes itself based on development and individual circumstances. As they age, only the parts of the neural network that are truly necessary are preserved, but this can be done if the brain has an abundance of neurons to begin with.
Goswami says that mammals evolved in the shadow of dinosaurs. They needed excellent sensory abilities to survive, which is probably why they developed nocturnal habits and night vision. This almost certainly had an impact on neurodevelopment. Primates, including our ancestors, also needed to develop the specialized motor skills needed to swing from tree to tree.
Thus, the environment puts pressure on the mammalian brain to evolve the ability to help us get out of trouble. In a challenging world, cognitive abilities must be improved, and many animals may have benefited from this.
Birds that settle on oceanic islands and therefore have to adapt to unpredictable new territories have larger brains than their mainland counterparts, a study has found.
However, it should now be clear that you cannot just measure the size of an animal's brain, compare it to the size of its body, and then draw hard and fast conclusions about how intelligent that animal is. Size is only one part of the puzzle.
In any case, what is smarter to think or survive? Humans like to think, but as Goswami says, when you consider our current efforts to deal with long-standing problems like the climate crisis, our ability to plan looks very poor indeed.
Cox makes another point: "There's a lot more to life than having higher cognitive abilities or a high IQ."
It almost makes you wish our brains were smaller.