• Your life and health are your own responsibility.
• Your decisions to act (or not act) based on information or advice anyone provides you—including me—are your own responsibility.


Big Brains Require An Explanation, Part I: Why Did Humans Become Smarter, Not Just More Numerous?

(This is a multi-part series. For the index, click here.)

How did we get from this:

Australopithecus afarensis reconstruction

Australopithecus afarensis (reconstruction)

To both this…

Hadzabe hunting Maribou storks on the shore of Lake Eyasi, Tanzania.

Hadzabe hunting Marabou storks

And this?

Shibuya Crossing 163

Shibuya Crossing, Tokyo

That’s more than a tripling of brain size—and an astounding increase in cultural complexity—in under 3 million years.

I’ve previously written about the currently accepted explanation, in this article: “Why Humans Crave Fat.” Here are a few bullet points:

  • Chimpanzees consume about one McDonalds hamburger worth of meat each day during the dry season—mostly from colobus monkeys, which they hunt with great excitement and relish.
  • Kleiber’s Law states that all animals of similar body mass have similar metabolic rates, and that this rate scales at only the 3/4 power of size. Therefore, in order for our brains to grow and use more energy, something else had to shrink and use less energy.
  • It takes a much larger gut, and much more energy, to digest plant matter than it does to digest meat and fat. This is why herbivores have large, complicated guts with extra chambers (e.g. the rumen and abomasum), and carnivores have smaller, shorter, less complicated guts.
  • The caloric and nutritional density of meat allowed our mostly-frugivorous guts to shrink so that our brains could expand—and our larger brains allowed us to become better at hunting, scavenging, and making tools to help us hunt and scavenge. This positive feedback loop allowed our brains to grow from perhaps 400cc (“Lucy”, Australopithecus afarensis) to over 1500cc (late Pleistocene hunters).
  • In support of this theory, the brains of modern humans, eating a grain-based agricultural diet, have shrunk by 10% or more as compared to late Pleistocene hunters and fishers.

(For a more detailed explanation, including links, references, and illustrations, read the original article.)

The Teleological Error

When discussing human evolution, it’s easy to fall into the error of teleology—the idea that evolution has a purpose, of which intelligence (specifically, self-conscious intelligence recognizable to our modern philosophical traditions, and producing something recognizable to us as ‘civilization’) is the inevitable expression and end result.

Geology and archaeology proves this is not so. For instance, 140 million years of saurian dominance (far more than the 65 million years mammals have so far enjoyed) apparently failed to produce any dinosaur civilizations: they simply became bigger, faster, and meaner until the K-T asteroid hit.

Thus endeth the reign of the dinosaurs.

Thus endeth the reign of the dinosaurs.

Likewise, the increased availability of rich, fatty, nutrient- and calorie-dense meat (enabled in large part by the usage of stone tools to deflesh bones, first practiced by our ancestors at least 2.6 million year ago, or MYA) does not, by itself, explain the over threefold increase in human brain size which began with the Pleistocene era, 2.6 MYA. When a climate shift brings more rain and higher, lusher grass to the African savanna, we don’t get smarter wildebeest, or even larger wildebeest. We get more wildebeest. Neither does this increase in the prey population seem to produce smarter hyenas and lions…it produces more hyenas and lions.

Contrary to their reputation, spotted hyenas are excellent hunters, and kill more of their own prey than lions do. (Many “lion kills” were actually killed by hyenas during the night—whereupon the lions steal the kill, gorge themselves, and daybreak finds the hyenas “scavenging” the carcass they killed themselves.) One 140-pound hyena is quite capable of taking down a wildebeest by itself.

So: if the ability to deflesh bones with stone tools allowed australopithecines to obtain more food, why didn’t that simply result in an increase in the Australopithecus population? Why would our ancestors have become smarter, instead of just more numerous?

The answer, of course, lies in natural selection.

Natural Selection Requires Selection Pressure

I don’t like the phrase “survival of the fittest”, because it implies some sort of independent judging. (“Congratulations, you’re the fittest of your generation! Please accept this medal from the Darwinian Enforcement Society.”)

“Natural selection” is a more useful and accurate term, because it makes no explicit judgment of how the selection occurs, or what characteristics are selected for. Some animals live, some animals die…and of those that live, some produce more offspring than others. This is a simple description of reality: it doesn’t require anyone to provide direction or purpose, nor to judge what constitutes “fitness”.

“Natural selection” still implies some sort of active agency performing the selection (I picture a giant Mother Nature squashing the slow and stupid with her thumb)—but it’s very difficult to completely avoid intentional language when discussing natural phenomena, because otherwise we’re forced into into clumsy circumlocutions and continual use of the passive voice.

(And yes, natural selection operates on plants, bacteria, and Archaea as well as on animals…it’s just clumsy to enumerate all the categories each time.)

Finally, I’m roughly equating brain size with intelligence throughout this article. This is a meaningless comparison across species, and not very meaningful for comparing individuals at a single point in time…but as behavioral complexity seems to correlate well with brain size for our ancestors throughout the Pleistocene, we can infer a meaningful relationship.

Therefore, we can see that “The availability of calorie- and nutrient-rich meat allowed our ancestors’ brains to increase in size” is not the entire story. The additional calories and nutrients could just as well have allowed us to become faster, stronger, or more numerous. For our ancestors’ brain size to increase, there must have been positive selection pressure for big brains, because big brains are metabolically expensive.

While at rest, our brains use roughly 20% of the energy required by our entire body!

In other words, the hominids with smaller brains were more likely to die, or to not leave descendants, than the hominids with larger brains.

What could have caused this selection pressure?

Ratcheting Up Selection Pressure: Climate Change and Prey Extinction

Just as “natural selection” is simply a description of reality, “selection pressure” is also a description of reality. It’s the combination of constraints that cause natural selection—by which some animals live, some die, and some reproduce more often and more successfully than others.

The selection pressure applied by one’s own species to reproductive choices—usually mate choice by females—is often called “sexual selection.” Sexual selection is, strictly speaking, part of natural selection, but it’s frequently discussed on its own because it’s so interesting and complex.

In this essay, I’m speaking primarily of the non-sexual selection parts of natural selection, for two reasons. First, because this article would expand to an unreadable size, and second, because understanding the influence of sexual selection in the Pleistocene would require an observational knowledge of behavior. Lacking time machines, anything we write is necessarily speculation.

In order for selection pressure to change, the environment of a species must change. I believe there are two strong candidate forces that would have selected for intelligence during the Pleistocene: climate change and prey extinction.

The Incredible Oscillating Polar Ice Caps: Understanding Pleistocene Climate

I’ve discussed Pleistocene climate change at length before. (Note: the Pleistocene epoch began approximately 2.6 MYa.)

“Unlike the long and consistently warm eons of the Jurassic and Cretaceous (and the Paleocene/Eocene), the Pleistocene was defined by massive climactic fluctuations, with repeated cyclic “ice ages” that pushed glaciers all the way into southern Illinois and caused sea level to rise and fall by over 100 meters, exposing and hiding several important bridges between major land masses.” –“How Glaciers Might Have Made Us Human”

Here is a chart of the estimated average surface temperature of the Earth, starting 500 MYA and ending today. Note the logarithmic time scale!

Click image for larger version.

To appreciate the magnitude and severity of Pleistocene climactic oscillation, note the tiny dip in temperature towards the right labeled “Little Ice Age”. This minor shift froze over the Baltic Sea and the Thames River, caused Swiss villages to be destroyed by glaciers, wiped out the Greenland Norse colonies, and caused famines in Europe which killed from 10% to 33% of the population, depending on the country.

Furthermore, the climate was changing very quickly by geological standards. Let’s zoom in on the Quaternary period (2.6 MYA – present), of which the Pleistocene forms the overwhelming majority (up to 11,800 years ago):

5 million years of temperature estimates from ice cores.  Cool!

Click image for larger version.

Note that massive 41,000 year climactic oscillations, each far greater than the Little Ice Age, began approximately 2.7 MYA—and the first known stone tools made by hominids (the Oldowan industry) are dated to 2.6 MYA.

Coincidence? Perhaps not.

Genetic Vs. Cultural Change

The behavior of most animals (and all plants) is primarily determined by genetic factors (“instinct”, “innate behavior”)—so in order to adapt to a changing environment, selection pressure must be exerted over many generations. For a short-lived species which reproduces a new generation ever year, or every few years, it might be possible to adapt to a 41,000 year climate cycle via natural selection.

However, for a long-lived species like humans, with generations measured in decades, genetic change is most likely too slow to fully adapt. We would have had to move in search of conditions that remained as we were adapted to…

…or we would have had to alter our behavior in cultural time, not genetic time.

Culture is the ability to transfer knowledge between generations, without waiting for natural selection to kill off those unable to adapt—and it requires both general-purpose intelligence and the ability to learn and teach. While space does not permit a full discussion of these issues, I recommend the PBS documentary “Ape Genius” for an entertaining look at the differences between modern human and modern chimpanzee intelligence and learning. (And I can’t resist noting that spotted hyenas outperform chimpanzees on intelligence tests that require cooperation: more information here and here, abstract of original paper here.)

You can watch the full video of “Ape Genius” here if you are a US resident. (If not, you’ll have to find a US-based proxy server.)

However, climate change is insufficient by itself to cause the required selection pressure. The overwhelming majority of known species survived these changes—including the glacial cycles of the past 740,000 years which scoured North America down to southern Illinois on eight separate occasions—because they could approximate their usual habitat by moving. Even plants can usually disperse their seeds over enough distance to keep ahead of glaciers.

Therefore, to fully explain the selection pressures that led to modern intelligence, we must look farther…to the consequences of intelligence itself.

Live in freedom, live in beauty.


This series continues! Click here to read Part II.

Since my last update, The Gnoll Credo received yet another stellar review:

“A tale told with simple words that are beautifully put together…The most scathing yet beautiful insights into “civilized” humanity that I have ever seen…This novel made me reconsider my life and make serious, long-term changes that have brought nothing but positive results. That is the sign of a truly powerful book. Reading this novel, you will see the names of fictional characters and places, but you are not reading about them. You are reading about yourself.

My conclusion: must read.” –Steven Gray, “Book Review: The Gnoll Credo”

I don’t advertise or have a donation button: sales of TGC keep gnolls.org alive and updated with fresh, meaty content. In addition to the usual online retailers (Amazon.com, Barnes & Noble), US readers can buy signed copies directly from 100 Watt Press. (Outside the US? Click here for a list of international retailers.)


Permalink: Big Brains Require An Explanation, Part I: Why Did Humans Become Smarter, Not Just More Numerous?
  • Juan

    As always, JS, your posts are welcome islands of clarity. I’ve been reading and re-reading all of your previous posts, although hadn’t quite finished the lot of them. Now, there’s more!! Whooop!

  • Neal Matheson

    Hey he’s back and on my Birthday too!
    Thanks J

  • Jan's Sushi Bar

    It’s good to have you back, JS! I’m looking forward to next week’s post.

  • tess

    …what they said! SO glad you’re writing again; i learn so much from your blog!

  • Asclepius

    Welcome back. Looking forwards to this series. I’m figuring in the course of these posts you’ll cover sexual selection as well? It seems to be a powerful driver of evolution…

  • Doug

    Hi J.
    Wondering what you make of this. It seems the Expensive Tissue Hypothesis may not be as rock solid as we all have been led to believe. The vegans are rejoicing:


    “The key way they tested the overall hypothesis across various mammal groups was controlling for adipose tissue deposits in their calculation of a given animal’s mass. In short, they omitted fat deposit mass from all specimens, eliminating it as a variable. This was an important control tactic (and one not used by Aiello & Wheeler in their original paper), because adipose mass varies by season and habitat among many species, and can thus be a major confounding variable… Under these conditions, no negative correlation between brain size and digestive tract mass was found. In fact, no negative correlation was found between brain size and the mass of any expensive tissue. The authors did, however, uncover a tight negative correlation between brain size and adipose tissue depots: the fattest species had the smallest brains.”

  • Octavian @ Full Fat

    Good to see you back, I’ve been craving an article for a while. I’m really looking forward to Part 2 of this one.

  • Fmgd

    Hey, it’s nice to have gnolls rolling again.

    Looking forward to part 2.

  • Fmgd

    Btw, that hyena trial is pretty interesting.

  • anand srivastava

    Great to see this article.
    I hope that you will answer my question in your next article.

    Why don’t carnivorous cats have bigger brains than chimpanzees or even humans.

    I believe that ETH will cause a increased brain size if there is available energy.

    My belief is that carnivores do not have extra available energy because they convert protein to glucose, which is a very thermogenic process and wastes almost 30% of the energy. Which is only slightly more efficient than fermenting fiber.

    Chimpanzees and gorilla on the other hand can use freely available sugars from fruits, which provides them slightly more energy than cats, allowing them to have bigger brain. Chimpanzees does one better by able to find and eat meat.

    Humans have the biggest brain because we can find and utilize fat and starch, due to our ability to handle weapons and fire.

  • Sam K

    Why did we not get technologically advanced when our brains were 1500cc. Why did the current human have to wait till his brain ‘shrank’ by 10% before he could begin to unravel the mysteries of the universe. This question begs an answers. ..

  • Sean

    Interesting stuff, hope you enjoyed your sabbatical 😉

  • Nance

    I was tickled to see your email that you were back! It’s so welcome to have some new brain food.

  • Fmgd

    Sam K, I don’t think we can make too strong an inference on iteligence levels then and now based on this data, but technological advance depends on so much more than just inteligence.

    It’s really more about culture, civilization, big numbers and previous knowlodge, so one doesn’t just get technology advanced, it’s a lenghty precess. You could compare technology (let alone science) now to 150 yeas ago to 300 years ago and so on and it’s clearly not linear at all.

    Besides, those guys with the supposedly bigger brains were the ones to get the ball rolling for these conditions to thrive by introducing agriculture, so that’s arguably a big deal.

    Again, not that they necessarily were, all I’m saying is that I see no reason to believe they couldn’t be as inteligent as the average human or even a bit above that.

  • Bodhi

    Great article, I can’t wait to read the next part. I just finished reading “Catching Fire, How Cooking Made Us Human” I’ll be interested in seeing how the two theories dovetail together, or not.

  • John

    Good to have you back, and good to see that you’re delving into evolutionary psychology as well. I’ve understood Natural Selection to refer more to traits that enhance survival, and Sexual Selection to refer more to traits that enhance mating potential. The two can go together, but are sometimes at odd (like a peacock tail- it can lead to a shorter lifespan as a handicap, but lead to better mating cause it impresses more peahens). Wondering if you have read The Mating Mind by Geoffrey Miller, as he argues that our minds were more a product of Sexual Selection than Natural, and that there was indeed an intellegence that helped design us, but that intelligence and design came from the species choices itself.

  • Timothy

    Welcome back, JS! Once again cutting to the chase on the big questions.

    Genetic bottlenecks are apparently common in human evolution. My favorite is the eruption of the Toba volcano about 70,000 years ago, with the human race being reduced to only several thousand indviduals. Our species seems to have come within a hair’s breadth of extinction many times. Was it big brains that preserved our lineage while so many others died?

    And how did Homo Sapiens end up the last hominan species alive? We were only one of many until very recently. I particularly miss the Neanderthals, who would have produced some awesome weightlifting videos for Youtube.

  • Peggy The Primal Par

    I saw your comment on my website the other day and thought, my god, he’s alive! yippee! And now you’re back here too. I can’t wait to read your posts again. You’re one of my favorite bloggers. Keep up the good work!

    PS. I will read this post later this week. I have been swamped with book deadlines…

    Welcome back!

  • Juan:

    This one’s pretty dense, but it’s a necessary theoretical underpinning for the next one — and for any questions relating to evolution.

    We can’t just posit that something “allowed” evolution to occur: we must also propose how conditions caused the organisms without that evolved characteristic to die more often, or to reproduce less often, than the organisms with the evolved characteristic.


    Congratulations on surviving another one!

    Jan, tess, Octavian:

    I’m glad to be back.


    I’m going to avoid the topic of sexual selection in the Pleistocene for now.  As I said above, “Lacking time machines, anything we write is necessarily speculation,” and I don’t believe it’s necessary to invoke it for the point I’m making.


    The demise of the Expensive Tissue Hypothesis appears to be greatly exaggerated. 

    First, the ETH is only about the great apes — specifically the gorilla/chimp/bonobo/human continuum.  Yet the authors of “Energetics and the evolution of human brain size” include no data at all from apes, while claiming their data disproves human divergence from chimps and bonobos!  Furthermore, they note that their conclusion doesn't even hold true for the primates for which they gathered data.

    In other words, the authors are deliberately trumpeting a result they don’t have in order to cause controversy.  And, as expected, the vegans are seizing on it.

    (I can see other problems with the paper, even though I don’t have fulltext.  If anyone has access, please contact me…I might be able to write an entire article about it.)


    Spotted hyenas are amazing creatures…which remained completely unstudied and unknown until the last few decades.  I have a book from a well-known African naturalist, published in the 1960s, which still claims hyenas are hermaphroditic!


    As I’ve explained, increased energy availability isn’t enough by itself to cause increased brain size — if more food is available, you get more animals, not smarter animals.  There must be advantages to increased intelligence which outweigh the costs in sufficient measure to cause selection pressure — by which the smarter animals survive and reproduce more often than the dumber ones. 

    I’ll talk about that subject next week.  Do stick around!

    Sam K:

    The average Late Pleistocene human was far more technologically advanced than the average human today.  Just because someone can use an iPhone doesn’t mean they could build one, or even write an app for it.

    In contrast, hunters had to make all their own weapons and tools out of dead animals, wood, and rocks, not to mention the mental and physical challenges of finding, tracking, and killing animals much faster than they were — a far greater challenge than pushing a plow up and down a field, working as a retail clerk, or cleaning houses.  Recall that hunter-gathers invented agriculture, and human progress stopped dead for about 7,000 years once that happened…

    Read my review of Jack Brink’s “Imagining Head-Smashed-In” for a better perspective on the skills required to survive as a big-game hunter.


    More soon!


  • Christopher Sturdy

    Nice to have you back posting! Great review of the Credo, by the way. It is a great read.

  • Sean, Nance:

    Glad to be back!  It’ll get even more interesting.


    Absolutely.  See my comment above to Sam…


    I’ll get to “Catching Fire” soon.  I may even have to devote an entire article to it, since it’s apparently being read quite a bit in the paleosphere.


    Sexual selection enhances mating potential, which enhances future survival.  I’m not spending much time on it in this series for reasons I’ve already explained…and which I believe have good support in the literature.  Specifically, in all the writing I’ve read about the remaining hunter-gatherer tribes in Africa (the Hadza, the various tribes of Bushmen, etc.), women don’t prefer husbands based on their physical attractiveness or any other secondary sexual characteristic, nor for their ability to string beads or tell stories — they prefer husbands who are the best hunters and bring home the most food calories.

    Ergo, sexual selection was most likely selecting for the same traits as natural selection.

    Mainly, though, I don’t like writing articles on evolutionary psychology because it’s almost entirely unsupported speculation.  Entertaining speculation, but speculation nonetheless — and while it’s great for causing arguments and controversy, it doesn’t necessarily lead to useful empirical knowledge.


    Most likely.  As I said above, general-purpose intelligence allows humans to survive changes much too quick and severe for natural selection.  (Although the Toba catastrophe theory is not, to my knowledge, proven at this point in time.)

    The Neandertal vs. human question is extremely entertaining, and beyond the scope of this series.  Maybe I’ll write about it in the future.


    Awww, you’re making me blush.


    I'm glad TGC spoke to you.  Thanks for the vote of confidence! 

    Keep spreading the word…since small presses don't have the marketing budget to buy end caps in Barnes and Noble, I'm dependent on fans like yourself to let others know about the book.


    Thanks for the warm welcome back, everyone!  I always appreciate the ideas and questions: they give me directions for future articles.


  • Whooop! J! Glad you’re grinning again … show me them teeth … cackle 🙂

    We hear you loud and clear! FAT = BRAINS. Damn good comeback!

  • Geoff Hetherington

    What a find you are… I enjoy Sisson and some others but Damn it I’ll find a week (or two) just to read the archives.
    Erudite, wry and informative I am going to enjoy this ride!!!
    Thank you.

  • Kate

    “First, because this article would expand to an unreadable size….”

    I completely doubt that any article of yours, however long, could be unreadable. “Why We Get Hungry” isn’t long enough, in my opinion, though I know I’d be tired of writing by the end of all you’ve done with it. Glad you’re back! I’ve had to resort to studying actual schoolwork instead of all the rabbit holes your links send me to.

  • Mich

    OH wow. This is awesome. I had no idea that temp. changes were so wild before. Up and down, makes me sea-sick.

  • anand srivastava


    I had not read the article. I had assumed that coming from you it will be very well reasoned.

    I see a very basic flaw in understanding. Kleiber’s Law on which ETH is based, has nothing to do with reproduction. It is relevant to an animal, AT REST, AT ALL TIMES. An animal can use much more energy than it uses at rest, during working time. I hope Michael Phelps is a good example. This law will apply during pregnency and it will apply during youth, it will also apply during old age, always only during rest periods.

    So when wildebeast gets more grass they get more food more nutrients and they are able to reproduce. This has nothing to do with Kleiber’s law. This has to do with other bodily processes. It has nothing to do with energy utilization.

    I will try to explain what ETH means to me.

    Kleiber’s law defines how much energy an animal AT REST can use. The AT REST is critical. This probably happens because of the limit on blood circulation. So it wouldn’t be efficient to shunt more/less energy than the given value. This is the base value of energy utilization. Remember this law is only related to energy not nutrient or food.

    ETH says that the energy that is available must be divided between all tissues. Rest of the tissues remain fairly equivalent in all animals, except gut and brains. The trade-off is between the gut and the brain. The more the gut the less the brain, and the less the gut the more the brain.

    I make a simple modification to the concept of gut here to mean the energy efficiency of the digestive process. Ultimately the energy is being extracted by the gut from food. So we would call the energy used by the gut to be equal to all the energy that is made available by the gut minus the energy used by all the processes that happen before that energy becomes available, not necessarily only the energy used by the gut. ie

    Egut = Einput – Eoutput.

    Saying that Carnivores have a higher energy but they use this energy to somehow reproduce more doesn’t work. If you want to channelize that energy the animal must store it somewhere. What you are saying is that the animal will keep getting fatter till it produces a new offspring. Wouldn’t that point to a dis-function in energy balance processes. How does this work for a male? Do we see this in the wild for any carnivores?
    How many cubs does a lion produce compared to say a deer?

    The ETH doesn’t work this way at all.

    Saying that natural selection will be required to produce a bigger brain also doesn’t work.

    Energy = Ebrain + Egut + Erest.

    The above equation is obvious, and I would think not a speculation. Kleiber’s law says that Energy is constant and ETH says that Erest is constant. So we have
    Egut + Ebrain = constant.

    Egut is inversely proportional to Ebrain.

    If this equation is correct and the Kleiber’s law and ETH is correct, you cannot reduce gut without increasing brain. Both will happen via the same evolutionary mechanism. Any diet change that causes a large change in gut’s energy efficiency will cause brain size to change. The diet change is the basic evolutionary change and it will cause the gut to change and will cause the cranial capacity to change to modify the brain.

    How can it be anything else? Where will the extra energy flow?

    Now ETH only talks about brain size. There may be other selection pressures involved in utilizing that extra brain. But would probably happen together.

    So the question still remains, why do Carnivores have a small brain.

    The answer relies on the fact that the energy contained in proteins is not usable. It must be first converted to glucose. This conversion is very expensive. Losing as much as 30% of the energy to heat. ie the efficiency of the gut goes down by 30%. And 30% less energy for use for feeding the brain. This means that any carnivore that relies on lots of muscle as food will not have a big brain. This includes all carnivores, except carnivorous humans :-), they do eat a lot of fat by necessity, enforced by this equation.

    This means that the only way to increase brain is to eat things that can be digested easily, without much losses. Fat is one very obvious food. The other can be starch provided the animal can get access to easily digested starch and has amylase enzyme and do not require expensive bacteria. I would think that cooking would have provided the easily digested starch.

    I guess the stages in human evolution would be
    fruit consumption (easily available sugars) allowed chimps to become intelligent enough to be able to do some basic hunting (some fat with the meat ??) which increased brain size. Next evolution came when humans discovered fatty brain and bone marrow. Next evolution would be cooked food allowing more fat and easier digestion. The next evolution would be based on cooked starch as there are limits on how much fat you can get in the wild. Next would be amylase increase for utilizing starch without those wasteful bacteria.

    A couple of open points. Why would humans lose the brain size after agriculture. Also Europeans have a smaller gut size compared to africans. It points to Erest or Energy not being completely constant. They are both supposed to be more or less constant. Probably we deviated in both Erest and Energy so as to absorb the 10% plus the gut size change. Maybe grains and/or civilization has something to do with this deviation.

    One point about human evolution, which is kind of ignored in this article. The Turkana basin has been continually hot for the past 4 million years. Humans have possibly evolved in this region. The last exodus from Africa (ie our species) was only 60Kya. Our evolution may not have been affected by the glacial periods. So those speculations are probably moot. If we did evolve in cold climates we should have had a lot more hair on our bodies, and we wouldn’t have so many sweat glands.

    Now for some speculation here.

    If Richard Wrangham is correct humans were cooking since 1.8mya, but we got amylase only around 200Kya. We know that cooking makes starch available. If cooking was there since 1.8mya then why did it take till 200kya for us to get amylase. I would think that plants at that time did not have much starchy components. That might mean that humans learnt plant growing sometime before the 200Kya. This allowed them to actually select plants for more starch. This would also explain why sorghum starch was found on neanderthals teeth some 50kya. They might have been growing some sorghum, although only for emergency purposes. This would mean the technology for agriculture was already there, it just got used when over population forced people to start it.

    The availability of wide variety of starchy foods might be due to human intervention :-). Humans are probably the only animals that can consume and use a large quantities of starch. What would be the selection pressure for plants to produce starch rich storage organs before humans. Sugar rich fruits yes, but starch rich, which requires cooking to digest easily.

    Happy speculation :-).

  • anand srivastava

    I am pretty sure the paper predicting the demise of ETH, did not consider Egut as I have done (Einput-Eoutput). If they had done it this way, the data would apply on all mammals.

    If you treat Egut to be only Gut size then obviously it will apply to animals with similar guts, varying only in size. It will not apply to herbivores, carnivores, and omnivores at the same time.

    ETH is very simple in design, and based on common sense. It just needs a refinement of what gut means.

  • Neal Matheson

    Carnivores have access to higher energy and therefore don’t eat as much or particularly do anything for most of the day. I don’t think it follows that access to more calories means that an animal will neccesarily become smarter especially if there is no selection pressure to do so. if we are talking about smarts a huge powerful animal (lion) needs to be only slightly smarter (if that) than the thing it eats, a less powerful animal (wolf) needs to be considerably smarter, an even less powerful animal (H.sapiens) needs to be much smarter. I always thought ETH attempted to explain changes seen in homind bones over time. Debunking ETH won’t change the morphological changes seen in our genus.

  • anand srivastava

    The major problem with the Navarette et al paper seems to be is that they changed the whole premise by getting rid of the fat. Kleiber’s law applies to whole animals. I guess Navarette did not understand ETH.

  • anand srivastava

    One question. How do you utilize protein for energy, without converting to glucose?

    If you do that conversion most of the energy will go to waste as heat. Then you have to take out that heat. I think Kleiber’s law has something to do with heat sinking rather than energy utilization. Maybe the network of blood vessels is required to carry the heat out to the extremities or other transportation mechanism. If this is the case then you cannot really use more energy than the limit.

    So in case of a carnivore, it will not be able to have bigger brains because the energy requirement will go above the limit, which is required to cool the system down.

    Ofcourse there is a possibility of using less energy than required, but I think that the system will optimize the energy usage, so that all animals will be at the limit, rather than below it.

  • Neal Matheson

    Hi Anand,m I don’t know is the short answer. There are plenty of carnivores with big brains though (whales for one). I was more interested in making the point that the increasing brain size and increased carnivory in our genus are not dependent on the ETH being correct. ETH helps explain some changes seen in the archaeological record so any debunking of it is essentially inconsequential to anyone following (forgive me for using the word) a paleo-diet or evolutionary eating model.

  • Fmgd


    Ha, of course hyenas are awesome. Specially the ones of the bipedal variety.

    The idea of hyenas being hermaphroditic is hilarious. I wonder how could anyone conclude that. It may be hard to take a peek but then shouldn’t they just say “I don’t know”, or suppose they’re not since, well, that’s the general rule? I’m sure there’s a wacky story behind it all.

  • jesse

    “In support of this theory, the brains of modern humans, eating a grain-based agricultural diet, have shrunk by 10% or more as compared to late Pleistocene hunters and fishers.”

    Hi J. Thanks for the wonderful articles. For the above bullet point to hold wouldn’t you have to show an increase in gut size (or some other useful and selected for tissue) over the same time period as well? Is there one?


  • jesse

    Anand. Since I’m new here I’ll preface that this is not a refutation but an exploration. I question a few parts of your hypothesis that the expense of thermogenesis is what necessitates smaller brain size in strict carnivores. I did some searching to validate my thoughts that some aminos could be burned directly by muscle tissue, something that I have read before. This link shows that some can be converted to glucose and others to ketones.


    I’m assuming that carnivores are very efficient at running on ketones, as are humans (omnivores). I would *guess* that carnivores are *more* efficient than omnivores. So a human doesn’t need much glucose, something like 5-10% of calories for some picky parts of the brain and some red blood cells. Of course we do better with more, why stress the body. But for the sake of argument the 30% overhead for thermogenesis:

    a) would only apply to the absolute bare minimum of glucose needs, not all energy production in general
    b) may or may not be as costly when converting to ketones, an efficient fuel for much of the body

    (side note, I’ve had a wonderful journey into diet/metabolism in the last year and it never ceases to amaze me when I find out about a new fuel that the brain uses. For example, lactate and pyruvate)

    OTOH, about ETH in general why is it always gut vs. brain? I didn’t see why lung tissue, heart tissue, kidneys etc… wouldn’t come into play? For example if kidney size went up then wouldn’t that support your argument, as in it was needed in order to provide enough bandwidth of gluconeogenesis and would be expensive tissue such that more brains was not needed?

    Thanks, looking forward to responses.


  • jesse

    Wish I could edit above post with a further reference:

    “Amino acids are either used to synthesize proteins and other biomolecules, or oxidized to urea and carbon dioxide as a source of energy.[35] The oxidation pathway starts with the removal of the amino group by a transaminase. The amino group is fed into the urea cycle, leaving a deaminated carbon skeleton in the form of a keto acid. Several of these keto acids are intermediates in the citric acid cycle, for example the deamination of glutamate forms α-ketoglutarate.[36] The glucogenic amino acids can also be converted into glucose, through gluconeogenesis (discussed below).[37]”

    from: http://en.wikipedia.org/wiki/Metabolism

  • […] started a series on his blog about different things that made us humans. You should check it out. Big Brains Require An Explanation, Part I: Why Did Humans Become Smarter, Not Just More Numerous? &a… …how do you look, feel, and perform? — Robb Wolf My Blog. Reply With Quote […]

  • anand srivastava

    Jesse, agreed. It could be that the Erest may not be constant as ETH implies. It looks to be constant superficially but there may be a lot of variations on the Erest. And without Erest being constant ETH is not valid. Then we cannot conclude anything.

    On the point of protein being burnt directly and efficiently then we have to find out why they don’t have extra energy for the brains. Where is the extra inefficiency in energy extraction. Could it be explained by Erest being much higher for them? Doesn’t look very likely.

    It maybe that the Dolphin’s have bigger brains because they do burn proteins more efficiently, and thereby are able to have a bigger brain.

  • jesse

    Anand. Thanks for the response. I’m out of my league here and will go back to lurking 😉 …. in a moment. If (big if) the protein burning efficiency is higher in obligate carnivores wouldn’t there still need to be selective pressure for larger brains, else they may just hunt less? I will go read about ETH as I have a very naive understanding of it. For example, my understanding of it doesn’t justify all the niches that have been exploited by organisms from bacteria to dinosaur. Certainly all that dinosaur tissue was expensive. Again, I’m sure it’s just my ignorance so I’ll go read up on it. In fact if anyone has a definitive pointer to the theory I would read that first.


  • […] Big brains require an explanation. Why did humans become smarter, not just numerous? […]

  • eddie watts

    yay you’re back!

    the neanderthals weightlifting comment made me laugh, have you not seen magnusson? 1015 DL that looked like 100 pounds!

    very interesting read and looking forward to next installment.
    saw a few documentaries that states evidence shows that cro magnon and neanderthals may have interbred with modern humans rather than being wiped out in warfare but were bred out effectively.
    saw this on BBC so presumably this is actually old news

  • Vizeet

    Great article. I am waiting more in this series and on catching fire and Neanderthals.
    My thoughts:
    1. It is wrong to say that humans started using tools 2.6 Mya because even great apes are known to use plant stems and leafs to push insects out and eat them.
    2. You already mentioned that climate change is insufficient to cause selection pressure. I think it is not about selection pressure but more about advantage. Evolution from 10Mya to 5Mya created great apes who could walk on 2 legs making them more capable to survive both in savanna and forest because there was selection pressure. But I think this is not the case why humans evolved to more intelligent specie. Big brain is consequence of having simpler gut, and simpler gut is consequence of having lower fiber diet. It also helped humans to get more intelligent, walk faster and see predator from longer distance.

    Humans started eating bone marrow and brain which was mostly left by other species by using tools which started the process of improvement in intelligence level/simplification of gut and other benefits.

    I need to read catching fire but I think fire helped humans to cook starchy tubers and was not much of advantage before 250Kya. Because average tribe size used to be 150 people (Dunbar’s Number) while one ox can feed 400 people at one time which means even internal organs would have been sufficient which does not require cooking. And humans were mostly scavenging not hunting.

  • pam

    ok! this article finally broke me down down so just ordered your book. i wish there was a Kindle version tho.

    this reminds me what Vernon Vinge writes in “Marooned in Realtime;” which i read way before i switched diet.

    it is something like “evolution favors local optimization, not intelligence.” & “intelligence is a very rare thing.”


  • Paul:

    It’s good to be back. 


    Much appreciated!  I’ve gone to some effort to keep the archives organized…stop by anytime.


    “Why Are We Hungry” isn’t done yet…I’m just taking time to explore other topics.  Frankly, I could write an entire book about hunger, and perhaps I should.


    Pleistocene climate was indeed a wild ride…and there’s no reason to think it’s over.  Food for thought.


    Aiello and Wheeler are clear, in the original paper, that the ETH only applies as stated to the great apes.  They don’t have solid data for other apes, let alone the rest of the animal kingdom, and they don’t attempt to apply the ETH to the rest of the animal kingdom.

    Why not?  Because it’s clear that, as you mention, there are many more possible energetic tradeoffs between different body tissues.  Brain vs. gut was the biggest tradeoff for great apes, but muscle mass and bone mass can also be traded.  Then there is general activity level: you can have lots of muscle that spends most of its time at rest, or less muscle that spends more of its time in motion.

    “Why do carnivores have a small brain?”  Answer: it’s as big as it needs to be.  Larger-brained lions (for instance) have no survival advantage.  I’ll discuss the reasons for this at greater length in next week’s article.

    It’s very interesting that you’re attempting to expand the ETH to apply to all mammals, not just the great apes, and it’s a worthwhile exercise.  Unfortunately, evaluating your hypothesis is beyond the scope of this series at this time…that’s a whole another series right there.

    And while it’s true that the ancestors of modern humans only left Africa c. 60 KYA, it’s also true that Pleistocene climate shifts dramatically affected Africa even if there were no glaciers there.  There were permanent snowfields in Morocco even during the Little Ice Age, and there is snow on Kilimanjaro now.  More importantly, Ice Age climate is much cooler and drier than interglacial climate, and it would have featured much more savanna, grassland, and temperate forest than tropical forest and desert.  (We find all kinds of archaeological remains in the Sahara.)


    Absolutely correct, and that’s a big piece of the puzzle.  In addition, larger predators spend more time scavenging, while smaller predators spend more time hunting.  I’ll bring that up next week.


    That’s one problem with Navarrete et.al.  There are a lot of problems with it, and I’ll discuss them in the future when I devote an entire article to the subject.

    Also keep in mind Kleiber’s Law is both an empirical observation and an approximate rule of thumb.  There is still much debate as to the best value of the exponent (2/3 or 3/4?), and as to which biological constraints cause it to be a useful rule of thumb.


    Yes, spotted hyenas are awesome creatures, especially the bipedal ones…hence the name of this site! I’m glad you found TGC worthwhile.

    I’ll make fun of the lazy, stupid prejudice of Hemingway and Disney all day, but I understand the sex role confusion: when females are just as well-hung as males and socially dominant, it’s easy to make that mistake.


    The ETH tradeoff doesn’t necessarily go the other way.  Assuming constant income, if your rent goes up you have to spend less money on something else, like food…but if your rent goes down, you’re not required to spend the surplus on anything in particular.

    You’re absolutely correct that aminos can enter directly into the Krebs cycle and be burned for energy: though each amino enters at a different place, it averages out to the published energy figure of about 4 kcal/gram for “protein”.  This pathway is unjustifiably ignored by most because it’s difficult to measure the degree to which it’s occurring.

    I’m looking for papers on carnivore physiology, but they’re rather rare!

    jesse, anand:

    The “inefficiency” is more in the fact that we don’t have a dedicated storage organ for protein as we do for fat and (limited amounts of) glucose: building lean tissue is much slower than fat storage in adipocytes or glycogenesis in muscles and the liver.  So it’s not that protein is an intrinsically poor energy source, it’s that we have a limited ability to store and retrieve it.

    More soon!


  • eddie:

    Glad to be back!

    It's amazing watching anyone lift over half a ton.  My excuse (and I'm sticking to it) is that Magnusson is short and stocky, so I have to pull the bar about three times higher than he does when I DL. 

    Yes, it's reasonably well-accepted now that Neandertals contributed perhaps 1% to the African genome and up to 4% to the European and Asian genomes.  There even appears to be a small contribution from the Cro-Magnons to some populations, though I think we're still arguing about which and how much.


    Absolutely.  2.6 MYA is the first traces of stone tools, not tools in general.  The Schoningen spears didn't come out of nowhere.  It's too bad that we'll probably never know the real history of wooden tool use.

    Keep in mind that a significant survival advantage is a selection pressure.  It's not just about not dying, it's about having more offspring.  The ability to procure more surplus high-quality calories (e.g. from fat) would have allowed those hominids to reproduce more often and provide their offspring with better nutrition.

    In general, I agree with your interpretation of the evidence…more on that subject next week.


    Thank you!  Sales of TGC are what keeps this website running and updated without lots of ads or donation buttons.  (That, and Amazon referrals…if you buy things through this link (or the link on the sidebar), I get a small spiff from your purchases at no cost to you.) 

    Vernor Vinge is right: general-purpose intelligence is far more expensive than local optimization, and is only selected for under rare circumstances.


    Thanks, everyone, for your contributions!  You've given me ideas for future articles, and you're helping keep gnolls.org a civil, troll- and drama-free place for high-level discussions, whether of evolutionary discordance or otherwise.


  • Elenor

    Instead of trying to shoehorn larger brains into natural selection — which never quite works — let me recommend very highly this book:

    The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature by Geoffrey Miller

    EXCELLENT exploration of the other half of Darwin’s wisdom — it’s not survival (of the fittest) that “forced” the growth of the human brain, but sexual selection — mate choice! Really well written, good read, very provocative and makes good sense!

  • Elenor

    “The idea of hyenas being hermaphroditic is hilarious. I wonder how could anyone conclude that.”

    It’s because the female hyena APPEARS to have a penis…
    Leolupus on Yahoo questions writes:
    In spotted hyenas (Crocuta crocuta), females are larger than males and have an enlarged clitoris that resembles a penis (it’s often called a ‘pseudopenis’). They also have ‘pseudotestes’ – fatty lumps in the lips of the vagina that resemble testes. These fuse the lips together, and females of this species are obliged to urinate, mate and give birth through the clitoris. These unusual adaptations led to people once thinking these animals were hermaphrodites, but this is not the case.

    Note that the other two hyena species, the striped and brown hyenas, and their relative the aardwolf, do not have these unusual features. In these species, males are larger than females, and females have ‘normal’ genitalia.

    Photo here: http://scientopia.org/blogs/scicurious/2009/09/25/friday-weird-science-the-hyena-mating-game/

  • eddie watts

    as for Benedikt Magnusson, he is 6 foot tall and i think the majority of impressive deadlifters are over 6 foot.
    he is not short and stocky, just looks that way because he is so damn big!
    (of course i don’t know how tall you are, i’m at 6’4″ so can’t use my height as an excuse!)

  • Elenor:

    Aren't spotted hyenas fascinating? 

    That's a good article on the problems of hyena reproduction, but I have one bone of contention: it claims erroneously that “They hunt for themselves sometimes, but do a lot of scavenger work”, when the extensive existing literature is clear that they hunt the majority of their food — far more than lions, who in some locations (like Ngorongoro Crater) subsist almost entirely on what they steal from hyenas!

    Re: sexual selection, books like “The Mating Mind” concentrate on “culture” — by which they mean its modern manifestations.  Miller: “Even if the survivalist theory could take us from the world of natural history to our capacities for invention, commerce, and knowledge, it cannot account for the more ornamental and enjoyable aspects of human culture: art, music, sports, drama, comedy, and political ideals.”

    Given that the first known manifestations of art were created well after our ancestors became anatomically modern, I'm more interested in the question of how we got to that point in the first place.  More in my upcoming article!


    You're right…I had no idea he was 6 feet tall.  When someone weighs 380 pounds it kind of throws off your sense of scale.


  • Fmgd

    @ JS, Elenor

    Oh, I had no idea. That makes sense. I knew there had to be an (wacky) explanation behind it all 🙂

  • Vizeet

    Your answer got me thinking. Aren’t humans capable of reproducing more then other great apes?
    Some changes that happened during human evolution:
    1. All human babies are born premature.
    2. Human females are not seasonally receptive.
    3. Humans have shorter nursing age then other primates.

    Isn’t that human evolution not just favored increased brain size but also better ability to reproduce? May be it was easier to increase brain size then to become numerous. It might be evolution complexity involved between the two choices that favored increased brain size.

  • Vizeet:

    Gorillas, chimpanzees, bonobos, and human hunter-gatherers all have approximately four-year interbirth intervals.  Having more than 5-6 offspring is a primarily Neolithic behavior, because it's only made possible by sedentism.  (If you're a hunter-gatherer mother, you can only realistically carry around one small child at a time.  H-G mothers nurse for several years vs. the one year or less common with agriculturalists.)

    Also, none of the great apes have seasonal estrus, though it isn't concealed to the degree it is in humans.

    However, you are correct AFAIK that human babies are indeed 'premature' (helpless for longer than chimp/gorilla babies), most likely because it's difficult to fit that big head through the hips.  Thus the flared hips of the human female.


  • Sean

    I always figured “premature” human babies is just another aspect of human neotony. As far as the difficulties of childbirth go, I’m convinced a lot of this is related to the modern hospitalized process–going back to doctors replacing midwives. I picked this up from Desmond Morris’ “Babywatching” and my own experiences with the birthing process.

  • anand srivastava


    Good point. Dolphins might have a larger capacity for storing proteins directly, so that they don’t need to convert as much protein to glucose.

    Or they may eat small fishes constantly. That way they don’t need to store much. But this rule would apply more to blue whales, which eat very small animals.

    Unfortunately I don’t know much about their anatomy or their energy utilization :-(.

  • Sean:

    I don't know whether early birth was part of the neoteny package or not…big heads are definitely part of it, though.


    That's how you know you're doing science: there isn't already a paper (or body of work) out there that answers all your questions!


  • Honora Renwick

    Regarding using tools to extract bone marrow. When I was horse trekking with Mongolians, they roasted the bones in the fire, then smashed them on rocks and sucked out the bone marrow. I guess that could be done with a skull too. So I infer we can add the technological advance of cooking with fire for enabling more consumption of fat as well as starch.

  • Honora:

    Very interesting…and delicious, I'm sure!

    It's no problem to eat bone marrow raw, so I don't think fire is necessary: you can just smash the bone open and dig in.  However, you can boil bones in water to extract otherwise-unavailable fat out of the extracellular matrix…the Plains Indians did that with the bison they killed. 

    Unfortunately the boiling technique was hot rocks dropped into a pit lined with bison skin, which doesn't fossilize, so we don't know for how long this technique was used.


  • […] to eat meat. Over at Hunt, Gather, Love – a review of Why Women Need Fat. Gnolls.org examines why humans became smarter rather than just more […]

  • […] Big Brains Require An Explanation, Part I: Why Did Humans Become Smarter, Not Just More Numerous? Gnolls.org starts a three part series on humans and how/why we developed big brains – pretty cool, I particularly enjoyed the climate change section. […]

  • […] I’ve previously written about the currently accepted explanation, in this article: “Why Humans Crave Fat.” Here are a few bullet points:  Read More » […]

  • […] In contrast to archaeological ages, the Pleistocene is a geological term (an “epoch”), defined precisely in time as beginning 2.588 MYA and ending 11,700 BP. It’s preceded by the Pliocene epoch (5.332 to 2.588 MYA) and followed by the Holocene epoch (11,700 BP – present). You’ll see a lot of sources that claim the Pleistocene began 1.6 or 1.8 MYA. This is because the definition was changed in 2009 to its present date of 2.588 MYA, so as to include all of the glaciations to which I referred in Part I. […]

  • jffryz

    we were created smarter

  • jffryz:

    I don’t find any compelling evidence that we were “created” in any meaningful sense of the term, and quite a bit of evidence to the contrary.  All the “but you haven’t found the missing link!” canards have been definitively put to rest by finds like Ardipithecus and Orrorin.

    That being said, you’re welcome to take the dietary information from my other articles and discard the anthropology.  However, I have no interest in debating established scientific fact.


  • Elton

    @Sam K:

    Uhm, we were. There is strong evidence that Man had been on the Earth as h. sapiens 300 mya. There are ruins in Africa that support this hypothesis, as well as digs in North America (Mexico) that have tools dating to abt 250 mya. There have also been some anomalous artifacts that turn up every now and then that stumps science. They aren’t in the right strata, in fact they are in older strata than they are supposed to be. We had a very high degree of technology then, perhaps ST: TNG tech. The Mahabarata and the other Rig Vedas say we have been here for longer than Paleo-anthropologists say we have.

    Typically, I don’t know what the impetus was that drove us to till the ground and to start Agriculture from a Paleo-Anthropological perspective. The reason why has not been explained by Scientists, they can only give dates. However, there is a reason why. Were we meant to eat seeds? Certainly not, not at the degree it’s been pushed into our heads since school (the food pyramid is all wrong).

  • Elton

    oops, I made a mistake. It’s 300,000 years ago, not 300 mya. Sorry everyone. I’m an Atlanteologist, so I have license to exaggerate. 🙂 But not by that much.

  • Elton:

    If you can point to reliable evidence of human occupation of the Americas at 250 KYA, you'll overturn the entire field of archaeology.  Even the most aggressive interpretation of sites like Pedra Furada only goes back perhaps 48 KYA — and those interpretations are both doubtful and hotly contested.  The earliest uncontested occupation of the Americas is about 14,800 BP at Monte Verde.

    Frankly, I find assertions of high-tech in layers where people are chipping stone points to be utter bunk.  All the “mysteries” of how the pyramids or Stonehenge were built turn out to be lots and lots of people rolling things around on logs and understanding the principle of leverage.  My mind isn't closed — but there's a very large burden of proof to meet.


  • Kenneth Shonk

    Maybe as global warming represents anther oscillaction, it will increase selection pressure to take humans to the next stage.

  • Kenneth:

    That's a specific case of the general problem, which is that humans are changing our environment much more quickly than we can adapt to it.


Leave a Reply

You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>




Subscribe me to the sporadic yet informative gnolls.org newsletter! (Your email will not be sold or distributed.)