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Big Brains Require An Explanation, Part II: Sexual Selection, and What Does “Paleolithic” Mean, Anyway?

Upon writing Part I of this article, it expanded to two parts…and now it’s expanded to three parts! So if an issue you were hoping to learn about hasn’t yet been covered, rest assured I’ll get to it.

(Or, go back to Part I.)

Let’s Get Oriented In Time: What Does “Paleolithic” Mean?

Since we’ve talking about the "paleo diet" for years, and this series explores the increased brain size and behavioral complexity that took place during the Paleolithic, I think it’s important to understand exactly what the term “Paleolithic” means. Yes, everyone knows that it happened a long time ago—but how long? And how is the Paleolithic different from the Pleistocene? What do all these terms mean, anyway?

First, Some Common Archaeology Terms And Abbreviations

BP = years Before Present. “The artifact was dated to 6200 BP.”
KYA (or ka) = thousands of years Before Present. “The bones were dated to 70 KYA.”
MYA (or ma) = millions of years Before Present. “The Permo-Triassic extinction occurred 250 MYA.”
industry = a technique that produced distinct and consistent tools throughout a span of archaeological time. Examples: the Acheulean industry, the Mousterian industry.

Oldowan choppers

They don't look like much—but they were much better than fingernails or teeth at scraping meat off of bones.

The word itself is a straightforward derivation from Greek. “Paleo-” means “ancient”, and “-lithic” means “of or relating to stone”, so “Paleolithic” is just a sophisticated way to say “old rocks”. Its beginning is defined by the first stone tools known to be made by hominids, dated to approximately 2.6 MYA—the Oldowan industry—and it ends between 20,000 and 5,000 BP, with technology generally agreed to be transitional towards agriculture (the “Mesolithic” industries).

The Paleolithic age is further divided:

  • Lower Paleolithic: 2.6 MYA – 300 KYA. Defined by the Oldowan and Acheulean industries.
  • Middle Paleolithic: 300 KYA – 30 KYA. Defined primarily by the Mousterian and Aterian industries.
  • Upper Paleolithic: 50 KYA – between 20 and 5 KYA. Defined by a host of complex industries.
  • (Click here for more information, including links to all the above terms.)

The reason for the imprecise ending of the Upper Paleolithic (and the overlap between Paleolithic stages) is not because there is doubt about the dates of such recent artifacts…it is because the Paleolithic is a technological boundary, not a temporal boundary, and is defined by the suite of tools in use. So for the first cultures to transition towards agriculture, the Paleolithic ended approximately 20 KYA (and was succeeded by the Mesolithic), whereas other cultures used Paleolithic technology until perhaps 5000 BP.

It’s also important to keep in mind that there are continuing definitional squabbles, particularly with the Mesolithic and Neolithic. What constitutes a Mesolithic culture vs. an Epipaleolithic culture? If a culture never takes up farming, is it still Neolithic if it uses similar tools and technology?

I don’t like to spend too much time in this morass, because it’s not an interesting argument—it’s just a failure to agree on definitions. However, it is always true that Paleolithic cultures were hunter-gatherers. Furthermore, it is almost always true that Neolithic cultures were farmers. (There are a few cases where nomadic cultures adopted Neolithic technology, such as pottery.)

So when we are speaking of a “Paleolithic diet”, we are speaking of a diet nutritionally analogous to the diet we ate during the Paleolithic age—the age during which selection pressure caused our ancestors to evolve from 3’6″, 65# australopithecines with 400cc brains into tall, gracile, big-brained, anatomically modern humans with 1400cc brains. (A figure which has decreased by roughly 10% during the last 5000 years.)

No, we can’t just ‘eat like a caveman’: the animals are mostly extinct and the plants have been bred into different forms. I discuss the issue at length in this article: The Paleo Identity Crisis: What Is The Paleo Diet, Anyway?

Now Let’s Orient Ourselves In Geological Time

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.

(More specifically, geological time divisions are defined by a “type section”, which is a specific place in a specific rock formation, and which is dated as precisely as possible given available technology.)

Remember, these are all just names…changing the name doesn’t alter the events of the past.

To give some idea of the time scales involved, our last common ancestor with chimps and bonobos lived perhaps 6.5 MYA, the dinosaurs died out 65.5 MYA, and Pangaea broke up 200 MYA.

Note that the middle timeline of the illustration below zooms in on the end of the top timeline, and the bottom timeline zooms in on the end of the middle timeline. Also note that the time period we’re exploring takes up one tiny box in the lower right, so small that the word “Pleistocene” doesn’t even fit inside it!

Geological timeline of the Earth, from The Economist

Click the image for a larger and more legible version, and an interesting article from The Economist.

For a slightly deeper look into the significance of each geological period, I highly recommend you click here for a graphical, interactive timeline. And here’s a long explanation of the terminology: ages, epochs, eons, and so on.

Summary: Paleolithic or Pleistocene?

The Paleolithic began approximately 2.6 MYA, with the first known stone tools, and ended between 20 KYA and 5 KYA, depending on when the local culture adopted a Mesolithic or Neolithic industry. Since it’s defined by our knowledge of hominid tool use, these dates could change in the future.

The Pleistocene began exactly 2.588 MYA and ended 11,700 BP. These dates are defined by our best estimates of the age of two specific pieces of rock (or ice) somewhere on the Earth.

So though the two terms are measuring nearly identical spans of time, they’re defined by two completely different phenomena…and since we’re speaking of human development, it is appropriate to use the term defined by human artifacts—the Paleolithic age.

Did Sexual Selection Drive The Australopithecus -> Homo Transition?

Evolutionary psychology is great fun to read about…but the problem with extrapolating it back into the Lower and Middle Paleolithic is that it’s pure speculation. The entire fossil record of this era of hominids can be itemized on one Wikipedia page, and I think it’s extremely risky to draw behavioral conclusions so far beyond the physical evidence.

More importantly, though, it’s unnecessary to invoke sexual selection in order to explain the growth in human brain size.

“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.”
-Geoffrey Miller, “The Mating Mind”

While this may very well be true, the first known archaeological evidence of art (blocks of ocher engraved with abstract designs) is dated to just 75,000 years ago, at Blombos Cave in South Africa—long after our ancestors first became anatomically modern c. 200,000 years ago. (Venus figurines are much more recent: the earliest is dated to 35 KYA.)

The first known art: carved red ocher

Click the image for more information about Blombos Cave.

The term “anatomically modern humans” refers to ancestral humans whose remains fall within the range of variations exhibited by humans today. We refer to such humans as the subspecies Homo sapiens sapiens.

Note that as with all fossil classifications, “anatomically modern” is a judgment call. There was no instant transition: a beetle-browed, heavy-limbed, archaic Homo sapien did not suddenly gave birth to Salma Hayek, and there are indeed many transitional fossils with a mix of archaic and modern features, usually known as “Early Modern Humans”.

Furthermore, the behavior of the few remaining African hunter-gatherer tribes, such as the Hadza and the Ju/wasi, supports the interpretation that sexual selection simply reinforced the same selection pressures as natural selection:

Human Nature 15:364-375.
Mate Preferences Among Hadza Hunter-Gatherers
Frank W. Marlowe

“Women placed more value on men being good foragers (85% of those women said “good hunter”) than on any other trait.”

National Geographic, December 2009
“The Hadza”
Michael Finkel

“Onwas joked to me that a Hadza man cannot marry until he has killed five baboons. [...] Ngaola is quiet and introspective and a really poor hunter. He’s about 30 years old and still unmarried; bedeviled, perhaps, by the five-­baboon rule.

The Old Way: A Story Of The First People
Elizabeth Marshall Thomas

“A young man may not marry until he has killed a big game animal (preferably a large antelope, although a duiker or a steenbok will also suffice) and proved himself a hunter.”
“His [/Gunda's] victim had been only a duiker, but a duiker is plenty big enough to qualify a boy for marriage.
“He [≠Toma] had few living relatives and no close ones, and thus could offer her no in-laws who could help her if the need arose, but he was an excellent hunter. This would appeal to any girl. So !U nagged her parents until they consented to the marriage.

In conclusion: the evidence is that sexual selection, if it was an important force, was providing the same selection pressure as natural selection—and that the behaviors most attributed to sexual selection postdate our evolutionary transformation into anatomically modern humans. Furthermore, it seems prudent not to invoke a factor for which our evidence is entirely speculative when there are other factors sufficient to explain our ancestors’ transformation.

Therefore, while sexual selection is a fascinating subject worthy of discussion, I don’t see a need to invoke it as a separate force to explain the increase in hominid brain size and behavioral complexity from the beginning of the Paleolithic (2.6 MYA) to the time of anatomically modern humans (200-100 KYA).

Live in freedom, live in beauty.


Continue to Part III, in which we explain Optimal Foraging Theory and begin the story of our ancestors.

(Or, go back to Part I.)

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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”

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