Our big brains have shrunk. Scientists might know why.
Summary
The average human brain size has decreased by the size of a lime in the last few millennia, research says.The development of bigger brains has long been considered a hallmark of our species’ increased intelligence and subsequent dominance on this planet. The last two million years of our evolution were marked by a nearly fourfold increase in brain volume.
But a growing body of evidence suggests our brains recently changed in an unexpected way: They diminished in size sometime following the end of the last Ice Age.
“Most people think of brain evolution happening in this linear way. It grows, plateaus and stops," said Jeremy DeSilva, a professor of paleoanthropology at Dartmouth College. “But we’ve lost brain tissue equal to the volume of a lime—it isn’t a tiny little sliver we’re talking about."
The precise timing of that post-Ice Age brain shrink has remained a mystery until now. A group of researchers led by DeSilva used a mixture of fossil and modern specimen data to pinpoint that this loss of gray matter happened between 3,000 to 5,000 years ago, according to research published in June in the journal Frontiers in Ecology and Evolution.
Many anthropologists had initially posited the changes coincided with the advent of agricultural practices around 10,000 years ago, and a global shift away from hunting and gathering.
The more-recent dates from DeSilva’s group point to booming eras for ancient civilizations in North Africa, the Middle East and South America—complex societies that they think may have played a role in the shrinkage.
They hypothesized that human societies got so cooperatively organized in the past 3,000 years that we began relying on what researchers call collective intelligence.
“It is the idea that a group of people is smarter than the smartest person in the group," said James Traniello, a biology professor at Boston University and one of DeSilva’s co-authors. “So basically, if you live in a group, you solve problems more rapidly, more efficiently and more accurately than what’s possible for any individual."
Traniello said the inspiration for applying this idea to why human brains may have shrunk came from “ultrasocial" insects such as ants. Ants form highly cooperative societies in which division of labor has favored smaller-brained individuals due to an advanced level of social organization.
The researchers suggested that perhaps our need to maintain a large brain—to keep track of information about food, social relationships, predators and our environment—has also relaxed in the past few millennia because we could store information externally in other members of our social circles, towns and groups.
“We’re so social that we don’t have to know everything anymore," DeSilva said. “And we collectively then operate as a pretty functional society."
That trend is likely bolstered by our use of books, personal devices and the internet as similar information sinks, according to Chris Stringer, a paleoanthropologist with the Natural History Museum in London and Christof Koch, a neuroscientist at the Allen Institute, a Seattle-based bioscience research nonprofit. Neither were involved in the recent research.
“Our brains don’t have to work as hard," Stringer said.
DeSilva’s group calculated that human brains had remained roughly the same size in average volume, about 1,450 cubic centimeters, for roughly the past 150,000 years. That average rapidly dropped by around 10%, or up to 150 cubic centimeters, over the course of the last few millennia.
Though average human height appears to have increased in the last few centuries, Stringer said our species had gotten noticeably shorter, lighter and smaller-boned in the past 10,000 years after the climate warmed—and brain sizes scaled down accordingly. “But that is probably not the whole story," he added.
DeSilva’s group found that not only did the humanbrain size shrink in general, but also it decreased relative to our body size—suggesting that brain size reduction isn’t just a byproduct of our shrinking bodies. The group first suggested this in a 2021 paper they published in the same journal, using cranial capacity data from nearly 1,000 specimens that included individuals spanning the past 10 million years of evolution—from fossil apes to modern humans only a century old.
Koch and Lars Chittka, a professor of behavioral and sensory ecology at Queen Mary University of London, said that it is important not to conflate brain size with smarts.
“We shouldn’t be too quick to say that, ‘If there was a reduction in brain size, that must have meant our ancestors 3,000 years back would have been much more intelligent,’ " said Chittka, who wasn’t involved in the research.
It is possible, Chittka and DeSilva said, that our mental circuitry evolved to be more optimized—with improved neural connections in a smaller package.
“Like, computers used to be the size of a room, and now they fit in your pocket," DeSilva said.
How well our neurons pass along information between themselves and the complexity of those neural networks—essentially, how robust your gray matter is—are more important than brain size, according to scientists. And those elements of our neural tissue aren’t preserved in the fossil record.
“There is a correlation between IQ and thickness of your brain, not overall volume of the brain in general," Koch said.
Bigger isn’t always better, anthropologists say. Our brains, which represent about 2% of our body mass, eat up a disproportionate amount of the body’s energy—consuming roughly 20%. Nutritionally sustaining a large brain can be challenging.
“Evolution works to make things economical. It will not maintain an expensive tissue if it doesn’t need to be there for survival," Stringer said. “There is no doubt our large brains have been important to our survival and success, but if you can get away with a smaller brain, then you do."
Not all scientists are convinced our brains have shrunk. Brian Villmoare, a biological anthropologist at the University of Nevada, Las Vegas, co-wrote a paper last year responding to some of the work from DeSilva’s group. He said that a limited sample size—particularly of fossil skulls—may have skewed calculations with regards to brain size averages over time.
“You’re sometimes talking about one skull every 10,000 years, and that skull is supposed to represent the entire human population worldwide," Villmoare said.
A limited fossil record does make it challenging to identify subtle changes in brain size over time, according to DeSilva and Traniello. But the duo said their group’s newest work looked at a sample size that was an order of magnitude greater than their 2021 paper and further honed the statistical analysis.
“Now we’ve got tens of thousands of samples and the pattern still holds," according to DeSilva. He’s even added himself to the data—DeSilva had his brain digitally isolated using magnetic resonance imaging, and then printed a 3-D version. “It is 1,325 cubic centimeters, which is about average," he said, holding it up. “Einstein’s is smaller than that and, well, he was Einstein."
Write to Aylin Woodward at aylin.woodward@wsj.com