The mystery of why human brains shrink | Sciences

Modern human brains are about 13% smaller than those of modern humans. Homo sapiens Who lived 100 thousand years ago. Why exactly does this still interest researchers?

Traditionally, our “big brain” is thought to be just that It distinguishes our species from other animals. Humans' ability to think and innovate is what enabled us to create the first art, invent the wheel, and even land on the moon.

Certainly, when compared to other animals of similar size, our brains are huge. The human brain has nearly quadrupled in size in the six million years since our species last shared a common ancestor with chimpanzees. However, studies suggest that this trend toward larger brains has reversed Homo sapiens. In our species, average brain size has declined over the past 100,000 years.

For example, in a recent 2023 study, Ian Tattersall, a human paleontologist and curator emeritus of the American Museum of Natural History in New York City, tracked volumes of ancient human brains over time. It began with the oldest known species and ended with modern humans.

The researcher discovered that the rapid expansion of the brain It has occurred independently in different human species and at different times in Asia, Europe and Africa. Includes species whose brains have grown over time Australopithecus afarensis, Homo erectus, Homo heidelbergensis that it Neanderthal.

However, the trend of brain expansion over time was turned upside down with the arrival of modern humans. The skulls of men and women today are on average 12.7% smaller than those of the rest of the world Homo sapiens Who lived during the last ice age.

“We have very strangely shaped skulls, so it's very easy to recognize early humans — and early humans had very large brains,” Tattersall says.

Tattersall's discovery replicates the discoveries of others. For example, in 1934, Gerhard von Bonin, a German scientist affiliated with the University of Chicago in Illinois, wrote that “there is a clear indication of a decline [no cérebro humano]”At least in Europe for the last 10 or 20 thousand years.”

So how can we explain this impressive decline? Tattersall suggests that the decline in brain size began about 100,000 years ago, which corresponds to a time period in which humans shifted from a more intuitive way of thinking to what he calls “symbolic information processing” – or thinking in a more abstract way to better understand your surroundings.

“This was the time when humans began producing symbolic artefacts and prints with meaningful geometric images,” says Tattersall.

The researcher believes that the motivating factor that brought about the change in thinking style was the spontaneous invention of language. This has led to the reorganization of neural pathways in the brain in a more metabolically efficient manner, allowing humans to get “more bang for their buck.”

In other words, as smaller, better-organized brains were able to perform more complex calculations, larger, metabolically expensive brains became unnecessary.

“It appears that our ancestors may have processed information through some kind of brute force, and intelligence, in this context, was measured according to brain size. The bigger your brain is, the more you can get out of it,” says Tattersall.

“But our way of thinking is different. We deconstruct the world around us into a vocabulary of abstract symbols, and we reassemble those symbols to ask questions like 'What if?'

“This kind of symbolic thinking must require a more complex set of connections within the brain than existed previously. My suggestion is that having these additional connections allows the brain to function in a more energy-efficient way.”

However, other paleontologists argue that the fossil record shows that brains began to shrink sooner than Tattersall suggests, meaning the change cannot be linked to language. The date at which Tattersall estimates the language was acquired (100,000 years ago) is also disputed.

“I love this theory, and I think it's really cool,” says cognitive scientist Jeff Morgan Staple of the California Museum of Natural History.

“But we haven't seen data that shows that there was a decline 100,000 years ago that didn't lead, at some point, to a reversal, where brain size started to increase again. There were declines at that time. But then, the brain has grown back.” “In other words, the data still do not agree with this hypothesis.”

Stibel believes that climate change, not language, can explain our tiny brains. In a study conducted in 2023, he analyzed the skulls of 298 skulls Homo sapiens In the past fifty thousand years. It was found that human brains have been shrinking for about 17,000 years, since the end of the last ice age. When the researcher carefully examined the climate record, he discovered that decreased brain size was associated with periods of global warming.

“What we saw is that the hotter the climate, the smaller the human brain is. The colder it is, the larger the brain is,” Stibel says.

Smaller brains would have allowed humans to cool quickly. It is known that humans in hot climates developed thinner and taller bodies to maximize heat loss. It is possible that our brains evolved in a similar way.

“Nowadays, if it's hot, we can put on a shirt, jump in the pool, or turn on the air conditioning, but 15,000 years ago those options weren't available to us,” Stibel says.

“The brain is the largest energy consumer of all the body's organs, weighing about 2% of our body mass, but it consumes more than 20% of our metabolic energy at rest. It should probably also adapt to the climate. Our theory is that smaller brains dissipate heat better.” It also reduces heat production.”

This discovery suggests that the rapid warming of today's planet could cause our brains to shrink even further.

The emergence of complex civilizations

Perhaps the most prominent theory put forward to explain our shrinking brains is that it all began when our ancestors stopped hunting, gathering, and establishing roots and began building complex societies.

In 2021, Jeremy De Silva, an anthropologist at Dartmouth College in the USA, analyzed skull fossils ranging from hominid to hominid. Miocene rhudapithecus (approximately 10 million years ago) to modern humans (300,000 to 100 years ago). He calculated that our brains began to shrink only 3,000 years ago, around the same time that complex civilizations began to emerge (although he has since revised his estimates, claiming that the decline in brain size occurred between 20 and 5 years ago). thousand years).

De Silva suggests that the birth of complex societies and empires meant the possibility of the spread of knowledge and tasks. People no longer had to know everything, and since individuals no longer had to think as much to survive, their brains shrank in size.

However, this theory is also disputed.

“Not all hunter-gatherer societies became complex in the same way as the Egyptians became 3,000 years ago, for example, but brain size also decreased in these societies,” says Eva Jablonka, professor emeritus at the Cohen Institute for History, Philosophy, Science, and Science. Ideas at Tel Aviv University, Israel.

Jablonka argues that even if brains became smaller with the emergence of more complex societies, this did not necessarily mean that smaller brains were necessarily an adaptive response.

“If larger, more complex societies arose 3,000 years ago, this could be associated with much greater differences in social class. As a result, if most people were poor, we know that poverty and malnutrition would harm brain development.”

Marta Lahr, from the Leverhulme Center for Human Evolutionary Studies at the University of Cambridge, suggested that nutrient deficiencies could explain the shrinkage of our skulls. In 2013, she analyzed bones and heads from across Europe, Africa and Asia. And I discovered that Homo sapiens The largest brain lived between 20 and 30 thousand years ago, and human brains began to shrink 10 thousand years ago.

This is within the time frame in which it is believed that our ancestors stopped being hunter-gatherers and began devoting themselves to agriculture. She believes that dependence on agriculture may have led to a lack of vitamins and minerals, which led to stunted growth.

However, some scientists argue that human skulls became smaller as a result of domesticity, based on the fact that domesticated species such as dogs and cats (which are bred to be friendly) have brains that are 10 to 15% smaller than those of other species. Animals have wild ancestors. If more friendly and sociable humans were more evolutionarily successful, then brains might have shrunk over time. But not everyone was convinced.

“I don't believe in this theory of self-domestication,” Jablonka says. “If this happened, it must have happened about 800,000 years ago, and there is no evidence that the human brain shrank at that time.”

So where does this leave us? Unfortunately, to understand why brains shrink, we have to determine exactly when the shrinkage began. But the fossil record makes this task almost impossible. Older fossils are difficult to find, so the record is heavily skewed toward newer species. For some species that are not well preserved, we currently rely on a few or even a single skull.

“What we know is that in the Ice Age, human brains were about the same size as Neanderthal brains, which is slightly larger than the average size of modern human brains,” Tattersall says.

“The average of all minds in Homo sapiens The age of more than 20 thousand years is also high. But exactly when the decline began is not entirely clear, because the record is not good. “All we know is that back then, brains were big, but today they are about 13% smaller.”

Are we becoming less intelligent?

If brains are shrinking, what does this mean for human intelligence? Depending on which theory you believe, small brains can make us smarter, dumber, or have no effect on intelligence at all.

It's true that brain size isn't everything. Men's brains are about 11% larger than men's Those for women because of their body size, which is also larger. However, research has shown that women and men have similar cognitive abilities. There is some disputed evidence that hominin species had smaller brains, such as: Homofluorescens And the Homo nalediThey were able to perform complex behaviors, suggesting that it is the way the brain is wired that determines intelligence. However, in general, having a larger brain relative to body size is associated with intelligence.

“The fact that our brains are shrinking so dramatically right now leads to the logical conclusion that our ability to become smarter is also shrinking, or at least not growing,” Stibel says.

“However, what we have done in the last 10,000 years is create tools and technologies that allow us to offload cognition into artifacts. We are able to store information in computers and use machines to calculate things for us. So our brains may also be offering less capacity for intelligence and power.” Intellectualism, but that does not mean that we, as a species, have become collectively less intelligent.