“Why Are Our Brains Shrinking?”

As promised, here is our first post in “The Student of Life” series. This essay, focusing on the fascinating subject of the shrinkage of our brain over the last 70,000 years, was masterfully penned by USF student Savannah Robison during the Fall 2017 semester of BIOL100. It is a wonderful example of student research, blending subjects Savannah learned in “The Science of Life” classroom with her own interest in cognition as a Psychology major. Congratulations Savannah!

Why Are Our Brains Shrinking?

Flashback to six million years ago: Since then, 19 species of hominids have roamed the earth, and Homo sapiens are just one of them. We first diverged from the other hominids 250,000 years ago, and for most of our existence, humans were no smarter or more significant than any other animal. We lived in harmony and equality with all the rest of earth’s inhabitants, but through a series of events over the course of millions of years, humans have arisen as the most powerful species on this planet. We harnessed the ability to use fire, we began to walk upright and we developed elaborate speech. We also have advanced cultures, and amazingly complex brains capable of planning for the future, processing the past and navigating the present with astonishing grace. These are the things that now separate us as unique from the rest of the animal kingdom. We have been evolving, most would say for the better, for millions of years, but 70,000 years ago we see something start to switch.

Although our brains were getting bigger progressively, around 70,000 years ago they plateaued, and have been shrinking ever since. John Hawks, an anthropologist at the University of Wisconsin, explains how, over the last 20,000 years alone, human brains have shrunk from 1,500 cubic centimeters (cc) to 1,350 cc, roughly the size of a tennis ball.

Although our brains were getting bigger progressively, around 70,000 years ago they plateaued, and have been shrinking ever since.

There are many possible explanations for this, but no one can pinpoint exactly why and how this is happening. The type of matter within the brain that we are losing paints an even more perplexing picture. We see an erosion of grey matter, not white matter. Grey matter contains the cell bodies of our neurons, they are the backbone of our brain. This grey matter accounts for our complex behaviors and abilities that are unique to humans such as language, culture, and writing. White matter is the fatty myelin sheath that covers the axons of these neurons, this fatty substance is a support structure but nonetheless essential to proper brain function.  Christopher Stringer, a paleoanthropologist and expert on human origins at the Natural History Museum in London says that “Scientists haven’t given the matter the attention it deserves. Many ignore it or consider it an insignificant detail.”
There are many different schools of thought around the matter. Some scientists think that humans are in fact getting dumber. Some think that the shrinkage is due to more efficient wiring of our brain. Then there are some scientists that think we have simply been domesticated. Domesticated sheep and dogs, for example, have smaller brains than their wild ancestors. Some credit environmental factors, like the heating of the earth, or physical factors, like the decrease in female hip size, which means babies with smaller skulls and smaller brains. Scientists have flipped over every stone to develop different hypotheses but fitting them all together into a narrative that makes sense is the hard part. One might ask, but we are getting smarter, right? We have to be! And my answer is that humans may be the most powerful apex predators on the face of the earth, but we are not above the forces of nature nor evolution, yet.

Brain Versus Body

Scientists say that “As a general rule…The more meat on your bones the more brain you need to control massive muscle blocks” (Stringer). So, it would make sense then that when humans started to evolve to have bigger brains, our muscles evolved as well. Stringer gives the example of the Neanderthals and Cro-Magnons, two species of hominids that roamed the earth at the same time as Homo Sapiens. These robust, big species of humans had big brains, bigger brains than ours, but we consider them less intelligent. This is because of the issue of scaling. Their EQ was smaller than that of humans. The encephalization quotient, EQ, is the proportion of brain size compared to body size. The higher the brain mass to body size ratio is, the more intelligent a species is said to be. For instance, dolphins have a relatively high EQ (5.6), while animals with large bodies but smaller brains, like killer whales, have a lower EQ (2.9). Humans are said to have an EQ of 7.4. Because of the general rule of scaling, it would make sense that as our bodies shrank, so did our brains. But this transformation would have taken much longer if it were due solely to a shrinking body. Over time, we see brain shrinkage moving much faster than our body shrinkage.

Over time, we see brain shrinkage moving much faster than our body shrinkage.

To support this, Hawks claims, “For a brain as small as that found in the average European male today, the body would have to shrink to the size of a pygmy” to maintain proportional scaling. Clearly, we have not been evolving proportionally, so the issue of brain volume is not explained. And yet, it has been shown that over the period of time that our brain has been shrinking, it has also transformed, possibly changing the way we think and feel. This may be evidence that as our brains shrank, they got better.


Yuval Noah Harari, an Israeli historian and a professor at the Hebrew University of Jerusalem, outlines how big brains are not necessarily better brains in his book Sapiens: A Brief History of Humankind. Our brains use around 25% of the calories we eat a day but account for only 2-3% of our body weight (Harari). As Homo sapiens moved from the forests to the savannas, our brains grew and we developed complex planning abilities, societies, and language, but with this new-found brain power and the caloric intake it required, our muscles shrunk. A chimpanzee today could tear off the limbs of a human with no problem at all, but a human has the abilities to avoid that fight in the first place. It is a game of “you give some, you take some”, and in the narrative of Homo sapiens evolution, we chose our brain size over our body size.

What we want to see is a brain that provides the most intelligence for the least energy.

Brains need a lot of fuel, and the bigger the brain, the more fuel it demands and the longer it takes to develop. What we want to see is a brain that provides the most intelligence for the least energy. It is important to note that brain shrinkage is not just phenotypic, but genotypic. Our genetic code was altered, which led to our increased brain size, not the other way around. But, Hawks says that in order for this to have happened, mutations that took place during the period of the brain shrinkage would have all had to have been beneficial, and that would have been extremely unlikely. Hawks says that this isn’t a long shot, though. A huge population boom 10-20,000 years ago coincides with our brain shrinkage. The bigger the population, the more gene mutations we see in the gene pool. Hawks thinks that as our brains became wired more efficiently, the neurochemistry of the organ shifted. This hypothesis is valid because it follows the definition of evolution, which is the change in the frequency of alleles over time. The hypothesis that humans had changes in allele frequencies that led to brain wiring efficiency is just one piece of the puzzle though.


Synaptic pruning is the process of synapse elimination that occurs from the time one is born until their mid 20’s. Human babies are born with many more neurons than they need to survive in the world and in the process of pruning the excess will be “trimmed” and die. Pruning has a strong correlation with the environment one grows up in. These effects can be seen when one compares the languages of people from different environments around the world. Studying language can give a more tangible look inside the inner workings of our brains and the pruning that happens as a result of our experiences. If one grows up in Antarctica surrounded by snow, they will have many more neural pathways related to that subject than if one grows up in California. Eskimos have 50 words for snow whereas the average Californian has possibly five. The more one’s brain is pruned, the faster and more efficiently one can react to their environment and process their surroundings. In opposition to hunter-gatherers, modern humans live in environments that pretty much remain constant throughout their lifetimes.

Human babies are born with many more neurons than they need to survive in the world and in the process of pruning the excess will be “trimmed” and die.

Our brains are pruned to respond to only these environments, whereas our hunter-gatherer ancestors were nomadic. They could tell you every river, mountain and plant species in a 15-mile radius. Not only is that not our reality, we simply do not need to know these things because we rely on other people, past scholars, and the internet to fill in necessary blanks.
Our knowledge base and the things we value as important are, by nature, different due to our different environment. But is it farfetched to assume that our brains simply don’t need to process as much information as the hunter-gatherer’s? Are our brains pruned and fundamentally organized differently because of this?

Heating Hypothesis

Periods of heating and cooling have been a normal occurrence since the genesis of our earth, but how these abiotic factors have affected humans is a more recent question. Over the last 20,000 years of history, our world has been in a warming period. It has been shown that people of small stature are able to release heat more efficiently and may be better equipped to the climate of a warmer world, and as our bodies have shrunk, so have our skulls and brains. This may be one part of the equation, but Stringer points out that over the last 2 million years as Homo Sapiens have evolved, our world has had comparable cooling and warming periods but our brains steadily kept getting bigger. Although the heating hypothesis is a question worth asking and exploring, there may be more fitting explanations.

Agricultural Revolution

Yuval Harari says that the worst choice in the history of human societies is when humans made the transition from nomadic hunter-gatherers to sedentary farmers. With this, humans became much more susceptible to malnutrition, disease, and famine. Their grain heavy diets lacked critical nutrients that are essential to proper brain function. But, we see brain shrinkage in the aboriginal people of Australia and South Africans, who never succumbed to agriculture until recent times, and we see the same amount of shrinkage in these populations as well. For this reason, the agricultural revolution is likely not the key player in the brain shrinkage question.

The Idiocracy Theory

A cognitive scientist, David Geary, believes that humans are just getting dumber.  He conducted a study that aimed to gain more knowledge in the relationship between decreasing brain size and increased social behavior that happened 1.9 million to 10,000 years ago. What Geary found was that as human societies population densities started to increase, cranial size decreased. Around 15-10,000 years ago he saw a 3-4% drop in EQ worldwide. The implications of this finding are huge because it suggests that as population density increases, humans don’t have to be as smart to stay alive because they can rely on other people to support and protect them. Geary says that “During this time, humans that would not have been able to survive by their wits alone could scrape by with the help of others.”

As population density increases, humans don’t have to be as smart to stay alive because they can rely on other people to support and protect them.

A question that is important to ask in support of Geary’s argument is how this societal support influenced sexual selection. Picking a mate that was smart enough to dodge predators, find food and protect their young used to be a matter of life or death, but it gradually became less and less vital for this to be the most important trait in a partner. When you have a band of 10 people willing to protect you, the cognitive ability of a mate is not as important as say their physical health. Another factor that may be at play has to do with the correlation between education and birth rate. Uneducated women have kids earlier and have more of them. According to the CDC, education and birth rate are inversely related. It has also been shown that as women start to become more educated in scholastic environments, the age that they decide to have kids keeps being pushed back, and that is assuming that they want to have children in the first place. Women with only some high-school education have the highest birthrate and as one continues to get more education that number declines dramatically. What this means is that the most educated, assuming they are the smartest, women are contributing less to the gene pool, whereas the least educated, assuming they are the least intelligent, contribute to it the most. This may mean that smaller brains are being naturally selected for. Of course, this hypothesis relies heavily on premises that are both not confirmed and highly controversial.


Domestication of species is linked to decreased brain size. We have seen it in the 30 or so animals that have been domesticated. Every single one of them has a brain 10-15% smaller than their wild ancestors. We also see certain phenotypes arise with domestication. Domesticated dogs, for instance, have flatter faces, smaller teeth and a wide array of coloration and hair types. We see this in humans as well. Richard Wrangham, a primatologist at Harvard University, says that the easiest way we select against aggression is choosing individuals whose brains develop slowly compared to their bodies. This makes it so adults have more juvenile brains and are therefore less aggressive. Wrangham says that this form of natural selection is an easy target for the reasoning of decreased brain size because it only would count on one or two genes being introduced to take effect. We see the effects of this when we look at how helpless human babies are compared to other species of animals. Human beings are born prematurely, our brains are unmyelinated which means we cannot stand or walk. In a perfect world, babies would have another three months in their mother’s bellies to continue developing, but as we adapted to life in the Savannas with an upright posture, our hips became smaller so only the smaller, premature babies survived. The only set of skills we are born with are a few reflexes that have helped us to stay alive despite a lack of any other skills whatsoever. Human babies rely on their parents for everything. This may be the “juvenile” brain that Wrangham was referring to. We are born without defenses of our own and we grow up extraordinarily slowly compared to other animals.

Every single domesticated animal has a brain 10-15% smaller than their wild ancestors.

Although humans love to try and separate themselves from other animals, we are animals in the same way that apes are. So, why then should we follow different rules than the rest? We shouldn’t, so the domestication theory is a really good one. It is quantifiable and qualitative as well. Not only can we measure the atrophy exactly, we can observe the fact that humans differ from chimpanzees in that we can talk through disagreements rather than kill each other over them. A former graduate student of Wrangham, Brian Hare, is a professor of evolutionary anthropology at Duke University and has compelling evidence to support the theory that humans are simply the domesticated version of our ancestors. But the theory doesn’t involve humans at all, it has to do with chimpanzees and bonobos. Hare suspects that bonobos are the domesticated version of chimpanzees. He highlights how bonobos have a build that resembles domesticated species and how they are much less aggressive and do not kill each other. He says that they also have brains that are 20% smaller than chimpanzees. This percentage decrease is in the same ballpark as the ones we see across the board of domesticated species.

Wrangham also offers another piece of evidence to support the domestication theory. After the dawn of human domestication and traveling back to our time as hunter-gatherers, there is evidence that humans performed capital punishment. When a person was causing trouble, being a bully, or was a repeat offender in a tribe, tribe members would get together and discuss the problem and would decide whether or not to get rid of that problem. Tribes in New Guinea today kill 10% of their population through capital punishment like this. This type of selection against aggression can explain why individuals with brains much bigger, and therefore much more aggressive, were weeded out of the population and the gene pool around the world.

Final Thoughts

Why human brains are getting smaller when it is widely believed we are getting smarter has clearly befuddled modern scientists. The theories that have been put forth so far, some better than others, have probably only begun to scratch the surface. As we continue to evolve into an age that resembles nothing we have ever seen or done before, questions will continue to arise about what this means for human societies and our world. But, some theories may allow us to explain some of the past and prepare for the future better than others. The domestication theory is the backbone of the answer to the brain shrinkage question, but it is supported by other forms of natural selection that build off of it. By virtue of the fact that humans are trying to study humans, we are biased, but the domestication theory bypasses this by putting forth a rule, or a foundation of knowledge one might say. If we see a substantial amount of brain shrinkage in every other domesticated species, humans should follow the same rule, and I believe they do. If domestication and smaller brains are correlated with less aggression, I am all for us continuing down a path of brain volume atrophy. Human beings cause the most damage to our earth and are the most violent species using many other mediums than just brute muscle power. We have developed such destructive war tactics and weapons that we do not need to rely on our bodies as a defense anymore. If smaller brains mean humans become more peaceful creatures, I cross my fingers that we continue on this trajectory.

If smaller brains mean humans become more peaceful creatures, I cross my fingers that we continue on this trajectory.

I do believe that if we are becoming more harmonious animals, that means we are evolving for the better, despite all the other implications that a smaller brain suggests. As for whether or not we are getting more or less intelligent, it is too soon to tell. If our brains continue to decrease in size below the normal percent associated with domestication, the latter may be true. But, it is important to differentiate between intelligence intended as how much information one can store, versus intelligence intended as how well someone can make meaning out of that information. Hunter-gatherers probably had more rote memorization knowledge, whereas we may have better problem-solving abilities, better ability to assign connotations to what we know or have the capability of knowing through resources like libraries and internet databases. The modern human brain may be smaller, but we have more information available to us than ever before. What we choose to do with this information may either filter us onto a path of prosperity or onto a path of hardship. Our brains will either be our biggest blessing or our most detrimental asset, and only time will tell.

Savannah Robison

San Francisco, 29 November 2017


Works Cited

Bichell, R. E. (2016, January 14). Average Age Of First-Time Moms Keeps Climbing In The U.S. Retrieved December 03, 2017, from https://www.npr.org/sections/health-shots/2016/01/14/462816458/average-age-of-first-time-moms-keeps-climbing-in-the-u-s

Flinn, M. V., Geary, D. C., & Ward, C. V. (2005). Ecological dominance, social competition, and coalitionary arms races. Evolution and Human Behavior, 26(1), 10-46. doi:10.1016/j.evolhumbehav.2004.08.005

MacAuliffe, K. (2011, January 20). If Modern Humans Are So Smart, Why Are Our Brains Shrinking? Retrieved December 03, 2017, from http://discovermagazine.com/2010/sep/25-modern-humans-smart-why-brain-shrinking

Harari, Y. N., Purcell, J., & Watzman, H. (2015). Sapiens: a brief history of mankind. London: Vintage Books.


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