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Big Brains Require An Explanation, Part I: Why Did Humans Become Smarter, Not Just More Numerous?
sp_BlogLink Read the original blog post
February 8, 2012
2:14 pm
Christopher Sturdy

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

February 8, 2012
2:31 pm
Forum Posts: 2045
Member Since:
February 22, 2010
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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.


February 8, 2012
3:04 pm
Halifax, UK
Forum Posts: 364
Member Since:
June 5, 2011
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Whooop! J! Glad you're grinning again ... show me them teeth ... cackle 🙂

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

Living in the Ice Age

February 8, 2012
3:13 pm
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.

February 8, 2012
10:03 pm

"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.

February 8, 2012
11:43 pm

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

February 9, 2012
12:57 am
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 :-).

February 9, 2012
1:05 am
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.

February 9, 2012
1:40 am
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.

February 9, 2012
3:49 am
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.

February 9, 2012
4:36 am
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.

February 9, 2012
7:38 am
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.

February 9, 2012
3:08 pm


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.

February 9, 2012
3:39 pm

"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?


February 9, 2012
4:57 pm

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.


February 9, 2012
5:23 pm

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]"


February 9, 2012
10:15 pm
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.

February 10, 2012
11:19 am

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.


February 10, 2012
11:22 am
February 11, 2012
5:28 am
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

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