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What Caused Human Brain Size to Increase?

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People talk a lot about intelligence and brain size. Something that’s most always brought up is how the human brain increased in size the past 4 million years. According to PP, the trend for bigger brains in hominins is proof that evolution is “progressive”. However, people never talk about a major event in human history that caused our brains to suddenly increase: the advent of fire. When our ancestors mastered fire, it was then possible for the brain to get important nutrients that influenced growth. People say that “Intelligence is the precursor to tools”, but what if fire itself is the main cause for the increase in brain size in hominins the past 4 million or so years? If this is the case, then fire is, in effect, the ultimate cause of everything that occurred after its use.

The human brain consumes 20-25 percent of our daily caloric intake. How could such a metabolically expensive organ have evolved? The first hominin to master fire was H. erectus. There is evidence of this occurring 1-1.5 mya. Not coincidentally, brain size began to tick upward after the advent of fire by H. erectus. Erectus was now able to consume more kcal, which in turn led to a bigger brain and the beginnings of a decrease in body size. The mastery and use of fire drove our evolution as a species, keeping us warm and allowing us to cook our food, which made eating and digestion easier. Erectus’s ability to use fire allowed for the biggest, in my opinion, most important event in human history: cooking.

With control of fire, Erectus could now cook its foods. Along with pulverizing plants, it was possible for erectus to get better nutrition by ‘pre-digesting’ the food outside of the body so it’s easier to digest. The advent of cooking allowed for a bigger brain and with it, more neurons to power the brain and the body. However, looking at other primates you see that they either have brains that are bigger than their bodies, or bodies that are bigger than their brains, why is this? One reason: there is a trade-off between brain size and body size and the type of diet the primate consumes. Thinking about this from an evolutionary perspective along with what differing primates eat and how they prepare (if they do) their food will show whether or not they have big brains or big bodies. How big an organism’s brain gets is directly correlated with the amount and quality of the energy consumed.

There is a metabolic limitation that results from the number of hours available to feed and the low caloric yield of raw foods which then impose a trade-off between the body size and number of neurons which explains why great apes have small brains in comparison to their bodies. Metabolically speaking, a body can only handle one or the other: a big brain or a big body. This metabolic disadvantage is why great apes did increase their brain size, because their raw-food diet is not enough, nutritionally speaking, to cause an increase in brain size (Azevedo and Herculano-Houzel, 2016). Can you imagine spending what amounts to one work day eating just to power the brain you currently have? I can’t.

Energy availability and quality dictates brain size. A brain can only reach maximum size if adequate kcal and nutrients are available for it.

Total brain metabolism scales linearly with the number of neurons (Herculano-Houzel, 2011). The absolute number of neurons, not brain size, dictates a “metabolic constraint on human evolution”, since people with more neurons need to sustain them, which calls for eating more kcal. Mammals with more neurons need to eat more kcal per day just to power those brains. For instance, the human brain needs 519 kcal to run, which comes out to 6 kcal per neuron. The brain is hugely metabolically expensive, and only the highest quality nutrients can sustain such an organ. The advent of fire and along with it cooking is one of, if not the most important reason why our brains are large (compared to our bodies) and why we have so many neurons compared to other species. It allowed us to power the neurons we have, 86 billion in all (with 16 billion in the cerebral cortex which is why we are more intelligent than other animals, number of neurons, of course being lower for our ancestors) which power human thought.

The Expensive Tissue Hypothesis (ETA) explains the metabolic trade-off between brain and gut, showing that the stomach is dependent on body size as well as the quality of the diet (Aiello, 1996). As noted above, there is good evidence that erectus began cooking, which coincides with the increase in brain size. As Man began to consume meat around 1.5 million years ago, this allowed for the gut to get smaller in response. If you think about it, it makes sense. A large stomach would be needed if you’re eating a plant-based diet, but as a species begins to eat meat, they don’t need to eat as much to get the adequate amount of kcal to fuel bodily functions. This lead to the stomach getting smaller, and along with it so did our jaws.

So brain tissue is metabolically expensive but there is no significant correlation between brain size and BMR in humans or any other encephalized mammal, the metabolic requirements of relatively large brains are offset by a corresponding gut reduction (Aiello and Wheeler, 1995). This is the cause for the low, insignificant correlation between BMR and our (relatively large brains, which correlates to the amount of neurons we have since our brains are just linearly scaled-up primate brains).

Evidence for the ETA can be seen in nature as well. Tsuboi et al (2015) tested the hypothesis in the cichlid fished of Lake Victoria. After they controlled for the effect of shared ancestry and other ecological variables, they noted that brain size was inversely correlated with gut size. Perhaps more interestingly, they also noticed that when the fish’s’ brain size increased, increased investment and paternal care occurred. Moreover, more evidence for the ETA was found by Liao et al (2015) who found a negative correlation between brain mass and the length of the digestive tract within 30 species of Anurans. They also found, just like Tsuboi et al (2015), that brain size increase accompanied an increase in female reproductive investment into egg size.

Moreover, another cause for the increase in brain size is our jaw size decreasing. This mutation occurred around 2.4 million years ago, right around the time frame that erectus discovered fire and began cooking. This is also consistent with, of course, the rapid increase in brain size which was occurring around that time. The room has to come from somewhere, and with the advent of cooking and meat eating, the jaw was, therefore, able to get smaller along with the stomach which increased brain size due to the trade-off between gut size and brain size. Morphological changes occurred exactly at the same time changes in brain size occurred which coincides with the advent of fire, cooking, and meat eating. Coincidence? I think the evidence strongly points that this is the case, the rapid increase in brain size was driven by fire, cooking, and meat eating.

The rise of bipedalism also coincided with the brain size increase and nutritional changes. Bipedalism freed the hands so tools could be made and used which eventually led to the control of fire. Lending more credence to the hypothesis of bipedalism/tools/brain size is the fact that there is evidence that the first signs of bipedalism occurred in Lucy, our Australopithecine ancestor who had pelvic architecture that showed she was clearly on the way to bipedalism. There is more evidence for bipedalism in fossilized footprints of australopithecines around 3 mya, coinciding with Lucy, tool use and eventually the advent and use of fire as a tool to cook and ward off predators. Ancient hominids could then better protect their kin, have higher quality food to eat and use the fire to scare off predators with.

The nutritional aspect of evolution and how it co-evolved with us driving our evolution in brain size which eventually led to us is extremely interesting. Without proper nutrients, it’s not metabolically viable to have such a large brain, as whatever kcal you do eat will need to go towards other bodily functions. Moreover, diet quality is highly correlated with brain size. Great apes can never get to the brain size that we humans have, and their diet is the main cause. The discovery and control of fire, the advent of cooking and then meat eating was what mainly drove the rapid increase of brain size starting 4 mya.

In a way, you can think of the passing down of the skill of fire-making to kin as one of the first acts of cultural transference to kin. It’s one of the first means of Lamarckian cultural transference in our history. Useful skills for survival will get passed down to the next generation, and fire is arguably the most useful skill we’ve ever come across since it’s had so many future implications for our evolution. The ability to create and control fire is one of the most important skills as it can ward off predators, cook meat, be used to keep warm, etc. When you think about how much time was freed up upon the advent of cooking, you can see the huge effect the control of fire first had for our species. Then think about how we could only control fire if our hands were freed. Then human evolution begins to make a lot more sense when put into this point of view.

When thinking about brain size evolution as well as the rapid expansion of brain size evolution, nutrition should be right up there with it. People may talk about things like the cold winter hypothesis and intelligence ad nauseam (which I don’t doubt plays a part, but I believe other factors are more important), but meat-eating along with a low waist-to-hip ratio, which bipedalism is needed for all are much more interesting when talking about the evolution of brain size than cold winters. All of this wouldn’t be possible without bipedalism, without it, we’d still be monkey-like eating plant-based diets. We’d have bigger bodies but smaller brains due to the metabolic cost of the plant-based diet since we wouldn’t have fire to cook and tools to use as we would have still been quadrupeds. The evolution of hominin intelligence is much more interesting from a musculoskeletal, physiological and nutritional point of view than any simplistic cold winter theory.

What caused human brain size to increase is simple: bipedalism, tools, fire, cooking, meat eating which then led to big brains. The first sign of big brains were noticed right around the time erectus had control of fire. This is no coincidence.

Bipedalism, cooking, and food drove the evolution of the human brain. Climate only has an effect on it insofar as certain foods will be available at certain latitudes. These three events in human history were the most important for the evolution of our brains. When thinking about what was happening physiologically and nutritionally around that time, the rebuttal to the statement of “Intelligence requires tools” is tools require bipedalism and further tools require bigger brains as human brains may have evolved to increase expertise capacity and not IQ (more on that in the future), which coincides with the three events outlined here. Whatever the case may be, the evolution of human intelligence is extremely interesting and is most definitely multifaceted.



  1. infowarrior1 says:

    Is the evolution of big brains also the reason why humans are physically weaker and slower than our relatives the chimpanzee and gorilla? Where energy that would otherwise go towards muscles and bones went to the brain instead?


    • RaceRealist says:

      It’s sorta like that.

      It’s commonly said that humans have brains that are 3 times bigger than their bodies. However, neuronal scaling holds for all primates. When looking at the neuronal composition of human and gorilla brains, each brain had the same amount of neurons expected for its brain size. Looking at it in this way, the human brain is not special, it is not the outlier. The true outlier is the gorilla, the gorilla evolved a bigger body than expected for its brain size.

      Humans can afford such a big brain with so many neurons (each billion neurons costs 6 kcal, so the brain uses 519 kcal) because we started cooking meat. That allowed us to spend less time eating and get more nourishment. Gorillas eat for 8bto 10 hours a day to power their body. If they had a brain our size, they’d need to eat for 3 more hours to get what they’d need to power a brain our size.

      A big brain doesn’t work with a plant-based diet. A big brain also needs a lot of kcal. Without meat and adequate kcal, Man would not have evolved. The plant-based diet is why gorillas have bodies bigger than their brains; they are the outliers not us.

      Read this paper.

      Gorilla and Orangutan Brains Conform to the Primate Cellular Scaling Rules: Implications for Human Evolution


    • infowarrior1 says:

      I understand the point of your article and agree. Although I am going a bit off topic.

      Why are humans weaker and slower than our primate cousins in your opinion?

      Is it the atrophy that occurs when we become bipedal? Or is it reallocation of resources plus all the factors you have described.


    • RaceRealist says:

      You’re not going off-topic.

      Why are humans weaker and slower than our primate cousins in your opinion?

      You can either have brains or brawns, you can’t have both. Gorillas and orangutans have bigger bodies due eating a plant-based diet. Gorillas have been documented feeding for up to ten hours per day. Though the most viable number of hours a primate can forage and eat is around 8 hours. It would take us around 9.3 hours to eat enough food eating the plant-based diet of gorillas and other primates.

      It’s about getting enough energy to power the brain. Herculano-Houzel and Kaas (2011) write:

      Therefore, the unusually large and late-grown body size of the great apes appears to be a consequence of a focus on feeding on relatively abundant but less nutritious food under conditions of competition for food and territory as well as considerable male-male competition for mates.

      They have bigger bodies because they need them for mate competition as well as the plant-based diet they eat. They cannot get enough kcal and nutrients to power a bigger brain with more neurons. So what they eat goes towards a bigger body, while they get enough nourishment for the amount of neurons their brains have.


  2. Oprah's gonna pop! says:

    “a calorie is a calorie” does not violate any law of thermodynamics. i don’t know what the authors meant by that.

    their point was that food energy sources vary in how efficiently they are converted into energy for the body and perhaps energy stores as well, yet the amount of energy the body needs is assumed not to vary. this means that if one maintains his weight on a high carb diet, he may have to consume more calories of a low carb diet just to prevent his body’s spending its energy stores.

    the confusion comes about because it is tacitly assumed that the body employs all the energy in its food from carbs, fat, ethanol, and protein or there is an excess which accumulates in some form, fat or muscle or whatever. the reality being that 60% of the calories in carbs are expended as heat.

    so this is a measurable phenomenon. do isocaloric diets differ in the amount of heat they generate? does a low carb diet result in greater heat dissipation than a high carb diet?


    • RaceRealist says:

      Good post. There’s nothing really wrong with what you said.

      do Isocaloric diets differ in the amount of heat they generate?

      If I recall correctly they do. I can get you more information tomorrow.

      Here is a direct quote from Feinman on the metabolic advantage:

      What Figure 1 of the paper shows is that metabolic advantage must exist between systems that rely to different degrees on gluconeogenesis. You learn this in biochemistry: it costs you 6 ATP to obtain glucose from GNG but, of course nothing if you start with glucose. So, there is a built in metabolic advantage. Not could be. Not debatable. It is there. Period. That is an absolute biochemical fact. So just as people thought metabolic advantage was excluded by the “laws” of thermodynamics (by which they meant the first law), “a calorie is a calorie” is excluded by the combined first and second law. (To try to use the first law in the absence of the second law is like, actually exactly like, using gravity without considering friction).

      Thermodynamics and the metabolic advantage


  3. Oprah's gonna pop! says:

    but iirc the authors say that whatever the difference in efficiency it’s a small difference. the authors are apparently unaware that should one diet require more calories than another to satisfy the body, the dieter will eat more of it.

    but they don’t. that’s the mystery. whatever the weight loss effect of low carb it has absolutely nothing to do with metabolic efficiency. it has to do with behavior and the brain, gut-brain. low carb is more satiating.


    • RaceRealist says:

      Yes the effect is small, but it is still there. Does it make any sense to you that all macros would do the same thing once ingested into the body like the body is a bomb calirometer?

      The thing is, the weight loss effect from low carb diets cannot be overstated. When insulin is spiked, fat cannot be unlocked from the adipocyte. I’m concerned with weight loss for diabetics, personally, and a they show more quality weight loss on very low carb ketogenic diets. At least as far as type II diabetics are concerned, that’s the best diet for them.

      And low carb diets are most definitely satiating. So along with the metabolic advantage (however small of may be) along with more satiating protein and fat, that’s why low carb diets are superior over high carb diets.

      Also see this.

      How Kevin Hall Tried to Kill the Insulin Hypothesis with Pure Spin


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