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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
4:18 am
Forum Posts: 2045
Member Since:
February 22, 2010
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(This is a multi-part series. For the index, click here.)

How did we get from this:

To both this...

And this?

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…
February 8, 2012
6:12 am

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!

February 8, 2012
6:18 am
Neal Matheson

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

February 8, 2012
6:42 am
Jan's Sushi Bar

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

February 8, 2012
7:43 am

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

February 8, 2012
8:00 am

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...

February 8, 2012
8:06 am

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."

February 8, 2012
9:12 am
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.

February 8, 2012
9:31 am

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

Looking forward to part 2.

February 8, 2012
9:32 am

Btw, that hyena trial is pretty interesting.

February 8, 2012
9:45 am
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.

February 8, 2012
9:51 am
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. ..

February 8, 2012
10:09 am

Interesting stuff, hope you enjoyed your sabbatical 😉

February 8, 2012
10:10 am

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

February 8, 2012
10:12 am

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.

February 8, 2012
10:28 am

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.

February 8, 2012
10:44 am

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.

February 8, 2012
11:21 am

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.

February 8, 2012
12:34 pm
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!

February 8, 2012
2:06 pm
Forum Posts: 2045
Member Since:
February 22, 2010
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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!


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