Is Human Evolution an Inevitable Race Toward Bigger Brains?

Is Human Evolution an Inevitable Race Toward Bigger Brains?

“Some scholars believe,” writes Yuval Noah Harari in his book Sapiens, “there is a direct link between the advent of cooking, the shortening of the human intestinal tract, and the growth of the human brain.” Harari argues that technologies around cooking allowed human intestinal tracts to evolve toward simplicity. Cooking food broke down compounds in foods that were harder to digest and neutralized pathogens that could have made us sick. As we learned to use fire, boil water, and create stone or dirt ovens, we made food more healthy, safer, and easier on our digestive systems which meant that we didn’t need to have such a robust digestive tract.
Harari continues, “Since long intestines and large brains are both massive energy consumers, it’s hard to have both.” When our intestines didn’t need to be so beefy, it was advantageous for humans to evolve with shorter, more streamlined guts. The energy saved in the gut could go toward other organs. Specifically, in the view Harari explains, the extra energy could be used to maintain a larger brain.
This makes me wonder, is all of human evolution a race toward a bigger brain? It is true that taller men are more likely to be elected president in the United States and that the typical image of a sexy man is a taller and more muscular individual (like Thor or Captain America), but for how long in human evolutionary history have tall bodies and large biceps been the most advantageous features for survival? Perhaps our desire for big brawny genes is leftover from our super quick ascendancy to the top of the food chain. Perhaps, as Harari’s quote eludes to, bigger brains have been the most advantageous feature for human survival for most of our history. Perhaps that truly is still the case.
An argument that Harari makes throughout the book is that humans have come to dominate the planet through our improved cognitive, reasoning, and social skills, which are all dependent on our brains. In this sense, evolutionary pressure has been toward larger brains, so all of human evolution is in some ways a race toward bigger brains. Shortening our gut allowed for bigger brains, giving up musculature allowed for more brain energy, standing on two feet allowed us to better survey the land – to provide our big brains with more data. We are not evolving to be better fighters, faster runners, or to physically occupy new niches. We are (and have been) evolving to better support better brains.
Early Births and Socialization

Early Births & Socialization

“Compared to other animals,” writes Yuval Noah Harari in his book Sapiens, “humans are born prematurely, when many of their vital systems are still under-developed.” Harari explains that humans are born so early because humans walk upright. As humans evolved, we began to stand and walk on two legs, rather than walk on four legs or primarily walk on two legs while greatly supporting ourselves with our arms. A consequence is that female hips narrowed to support an upright gait, which in turn restricted the birth canal, meaning that infants born sooner were more likely to survive because they were smaller and less of a risk to a female human with a relatively more narrow birth canal than other species.
In evolutionary terms, earlier births had additional consequences for the direction in which humans evolved. Harari continues, “since humans are born underdeveloped, they can be educated and socialized to a far greater extent than any other animal.” Raising a child is difficult work, and raising infants who are born under-developed and then develop slowly takes the work of many. Harari’s suggestion is that raising human infants was not possible for our female ancestors to do on their own – at least it was incredibly difficult compared to females raising their infants within a tribe. It would have been hard for a lone female to forage for sufficient food and to provide defense for herself and the infant. As a result, “evolution thus favored those capable of forming strong social ties,” writes Harari.
I find this explanation of the evolution of human social skills pretty convincing, even if I can imagine particular instances where it may not necessarily fit. I think it is reasonable to assume that there was a dual feedback mechanism, where evolving brains shaped our physiology, and where our evolved physiology in turn shaped our brains. As we got smarter and as evolution favored larger brains and increased cognitive capabilities, our bodies changed to adapt to our brain, and certain physiological traits were more advantageous to a species with increased cognitive abilities. Those physiological traits and characteristics, like narrow hips and birth canals, changed the size of the brain at birth, changing the cognitive capabilities of human infants. This allowed infants to be socialized at a younger age and favored individuals who were better at being part of a community, changing the dynamics for what type of cognitive abilities were favored by natural selection.
There was no clear evolutionary path and no way to predict where the future would go based on the past. Social factors, physiological factors, environmental factors, and other factors we have not identified all likely shaped the evolution of humans. We adapted to fit the circumstances and evolution favored different ways of being human at different times. I think this is a lesson we can still learn lessons from today. We can learn that we will evolve and that different ways of being human will be favored at different times based on many factors unique to our time, space, cultures, and environments.


Across her numerous books, Mary Roach is not afraid to ask the questions most of us probably want to ignore and never think about. For example, in the book Grunt, about soldiers and warfare, Roach asks how the army addresses diarrhea for soldiers in the field. The question is gross and Roach makes it a little funny, but for the army it is not a laughing matter, and it is a serious concern for soldiers.
Speaking with Captain Mark Riddle, Roach learns that soldiers’ gastrointestinal tracts have indeed kept them off certain missions. Riddle explains that one soldier was unfortunate enough to experience diarrhea almost every time he was on a mission, and as a result he was limited to shorter missions closer to army bases. He could not be used for long-term missions that took him deep into enemy territory. You need to be able to process the food and water in underdeveloped or un-sanitized areas if you are going to successfully execute a mission deep into dangerous enemy territory.
At one point, Roach asks a “Special Operations mechanic whether he knows of a vital operation that might have been compromised because someone got a vicious case of food poisoning. He dismisses the very idea. The guys they select for this type of work? They don’t have these types of problems. They’re selected for a reason.”
Roach continues, “20 percent of the population are what Riddle calls nongetters: people who can eat ceviche from street vendors, drink the water, never get sick.” For a whole host of reasons, some probably genetic, some the result of a fortunate set of gut bacteria, and some probably psychological and stress management related, some soldiers simply don’t get sick. During the age of COVID, this is certainly an enviable 20 percent of the population. These nongetters are the ones who are selected for long-term missions deep into enemy territory. They are the ones who can go on foot patrol, mingle with the locals, eat the food they provide, and manage the end results of that food and water. If they constantly got sick, they could not handle the mission, and they wouldn’t find themselves on lengthy deployments. In the army, a strong gut is as important as a strong mind and strong biceps. Nongetters are the ones our army relies upon for crucial missions that put our soldiers’ gastrointestinal tracts in the microbiological firing lines.
Cooling Through Sweat

Cooling Through Sweat

The physics of how sweat cools us down is pretty interesting. I can’t remember the specific physics formula that calculates the cooling effect of evaporation, but as water molecules leave the skin, they take some heat energy with them, leaving the body slightly cooler. But how well this system works is also dependent on the weather and air conditions that the sweat would potentially evaporate away into.
In Gulp, Mary Roach describes some of the important factors for this process. “When the air around you is saturated with moisture, your sweat – most of it, anyway – has no where to evaporate to. It beads on your skin and beads down your face and back. More to the point, it doesn’t cool you.” The water in your sweat has to evaporate away for cooling to take place. When the air is too humid for the water to evaporate away into (an over simplification of the physics I’m sure) then you can’t take advantage of the cooling potential of sweat.
Roach continues, “It’s the humidity, but it’s also the heat. When the air is cooler than 92 degrees Fahrenheit, the body can cool itself by radiating heat into the cooler air. Over 92 – no go.” Hot air rises through convection, allowing cooler air to replace the hotter air, cooling you off as slightly cooler air replaces the hotter air around you. At a certain point however, there is no cooler air moving in to replace the hot air coming from your body. Roach also writes, “a breeze cools you by blowing away the penumbra of swampy air created by your body. If the air that moves in to take its place is cooler and drier, so, then, are you.”
Sweating is an incredible ability that helps keep us cool, but its efficiency is dependent on the weather outside our body. We can sweat all we want, but if the air that is around us isn’t cooler and drier than we are, we won’t enjoy the benefits of sweating. We won’t dry off in the air, we won’t cool down, and we will be gross and swampy.
Passing Out In The Heat

passing Out In The Heat

I have never over-heated to the point of passing out or having any real problems or long term consequences. I’m lucky that I have not because I do a lot of running and have had a few extremely hot runs in the past. I live in Reno, NV and our high temperatures in the summer can reach into the triple digits and cause problems. When I was in high school the dangers of heat and athletics had become more of a national focus because several high school football players across the country had recently died from heat exhaustion during practices. Possibly one reason why I never had too terrible an experience of running in the heat was because there was an increased focus on keeping high school athletes cool in the during summer practices when I was competing.
In her book Grunt, Mary Roach examines what heat exhaustion means for members of the military. It can be a serious problem for service members in full military gear on patrol in the hot deserts of Iraq or Afghanistan. Luckily, for soldiers and high school athletes, the body has a quick solution, “heat exhaustion is embarrassing but not particularly dangerous. Fainting is both symptom and cure. Once you’re horizontal on the ground, the blood flows back into your head and you come to. Someone brings you water and escorts you to the shade and you’re fine.” Roach continues to explain that heatstroke is what can kill, going a step beyond heat exhaustion.
It is important to be aware of how hot it is, how much water people are able to take in, and how much rest and shade is available as well. It is often not when people are in the middle of exercise that overheating becomes apparent. Roach writes, “counterintuitively, overheated people sometimes pass out not in the midst of their exertions but when they stop and stand still; this is because contracting the leg muscles helps keep blood from pooling.” Breaks and cool down points have to come before people are soo hot that stopping exercise will cause them to faint. Hopefully the fainting an lying flat helps get the blood out of the legs to cool the rest of the body, but if they have gone too far, then they are already in more serious trouble. This was a painful lesson that coaches were learning at the time I was in high school, and hopefully drill-sergeant high school coaches are more aware of the importance of water breaks today.