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Big Brains Require An Explanation, Part III: Optimal Foraging Theory, And Our Story Begins On Two Legs

(This is Part III of a multi-part series. Go back to Part I, Part II.)

Our Story So Far

  • It is not enough to state that the availability of high-quality food allowed our ancestors’ brains to increase in volume from ~400cc to ~1500cc between 2.6-3 MYA and 100-200 KYA. We must explain the selection pressures that caused our brains to more than triple in size—instead of simply allowing us to increase our population, or to become faster or stronger.
  • To gloss over this explanation is a teleological error. It assumes that evolution has a purpose, which is to create modern humans.
  • Climate change is most likely a factor—but it is insufficient, by itself, to create this selection pressure.
  • The Paleolithic is an age defined by the use of stone tools (“industries”) to assist in hunting and gathering. It 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.
  • The Pleistocene began exactly 2.588 MYA and ended 11,700 BP, and is defined by the age of specific rock (or ice) formations.
  • Therefore, if we wish to locate an event precisely in time, we need to speak in terms of geological time—the Pliocene and Pleistocene epochs. If we wish to identify an event relative to human technological capability, we need to speak of cultural time—the Paleolithic age.
  • Sexual selection is a fascinating subject, but I see no need to invoke it to explain the increase in hominid brain size from the start of the Paleolithic to the rise of anatomically modern humans.

A Timeline Of Facts, A Narrative To Join Them

The factual knowledge we have about human behavior (including diet) during the Pleistocene is limited by the physical evidence we’ve discovered so far—which becomes thinner the farther back in time we go. Therefore, any narrative we construct from these facts must necessarily remain contingent on future discoveries.

However, the evidence we have strongly supports the currently accepted hypothesis for the evolution of human intelligence. I’ll do my best to compress several semesters of anthropology and evolutionary theory into a timeline that tells our ancestors’ story.

First, a key concept: in order to explain a more than tripling of brain size over nearly 3 million years, a single event is not sufficient. It’s not enough to say “Hunting is hard, so we had to get smarter.” We must postulate a sequence of events—one which creates the most parsimonious narrative from the physical evidence.

“Parsimonious” means “stingy” or “frugal”. It is frequently used by scientists as part of the phrase “the most parsimonious hypothesis/theory/explanation”, which means “the explanation which requires the least speculation and depends on the fewest unknowns.” (Also see: Occam’s razor.)

Before we start our narrative, we must define one more term: optimal foraging theory.

Optimal Foraging Theory

Optimal foraging theory (OFT) is a simple concept: “…Decisions are made such that the net rate of energy capture is maximized.” (Sheehan 2004)

This is because efficiency—obtaining more food for less effort—is rewarded by natural selection. Efficient foragers survive better during difficult times, and they spend less time exposed to the risks of foraging. This leaves them more likely to survive, and with more time to seek mates, raise offspring, or simply rest.

In the simplest case, herbivores select the most nutritious plants, and predators select the fattest, slowest herbivores. However, many complicated behaviors result from application of this simple rule. Two examples: for herbivores, leaving the herd costs energy and makes being eaten by a carnivore more likely; for predators, unsuccessful hunts cost energy and make starvation more likely.

Due to time and space constraints, we’re barely scratching the surface of OFT. This article provides a brief introduction, and Wikipedia goes into more detail—including many refinements to the basic model. For an in-depth exploration, including several interesting and complex behaviors resulting entirely from its real-world application, read this textbook chapter (PDF).

The result of OFT is, as one might hope, common sense: our ancestors would have eaten the richest, most accessible foods first.

Our Story Begins On Two Legs: Ardipithecus ramidus

Our story begins in an African forest during the Pliocene epoch, 4.4 million years ago. (Our ancestors have already parted ways with the ancestors of chimpanzees and bonobos. This occurred perhaps 6.5 MYA, in the late Miocene.)

The Miocene epoch lasted from 23 MYA to 5.3 MYA. The Pliocene epoch lasted from 5.33 to 2.59 MYA, and the Pleistocene lasted from 2.59 MYA to 11,700 BP.

It’s important to note that many different hominins existed throughout the Pliocene and Pleistocene. We aren’t absolutely certain which were directly ancestral to modern humans, and which represented stem species that subsequently died out…but the fossil record is complete enough that we’re unlikely to dig up anything which radically changes this narrative.

Though there are fascinating fossil finds which date even earlier (e.g. Orrorin), we’ll begin with Ardipithecus ramidus, a resident of what is now Ethiopia in the mid-Pliocene, 4.4 MYA. Today it’s the Afar desert—but in the Pliocene, its habitat was a lush woodland which occasionally flooded.

What Ardipithecus ramidus might have looked like. Click the picture for a BBC article.

“Ardi” was about four feet tall, with a brain the size of a modern chimpanzee (300-350cc). She was most likely what we call a facultative biped, meaning that she walked on four legs while in trees, and on two legs while on the ground: though her pelvis was adapted to walking upright, her big toe was still opposable and she had no arches, leaving her feet better adapted to gripping trees than to walking or running.

You can learn much more about Ardi at Discovery.com’s extensive and informative (though Flash-heavy and somewhat hyperbolic) website. For those with less patience or slow Internet connections, this NatGeo article contains a discussion of Ardi’s importance and possible means of locomotion. (Warning: both contain some highly speculative evolutionary psychology.)

From the evidence, we know that there must have been selection pressure to sacrifice tree-climbing ability in exchange for improved bipedal locomotion—most likely due to an increased ability to take advantage of ground-based foods. Though evidence is thin, its discoverers think (based on its teeth) that Ardi consumed a similar diet to its successor Australopithecus anamensis—nuts, root vegetables, insects, mushrooms, and some meat. (This supports the idea that Ardi ate more ground-based food, such as root vegetables and mushrooms, and less tree-based food, such as fruit.) And stable isotope analysis of its tooth enamel confirms that Ardipithecus was a forest species, lacking significant dietary input from grasses or animals that ate grasses.

Fruit Is For The Birds (And The Bats, And The Chimps): Australopithecus anamensis

Our next data point comes just a few hundred thousand years later.

“Early Humans Skipped Fruit, Went for Nuts”
Discovery News, November 9, 2009

Macho and colleague Daisuke Shimizu analyzed the teeth of Australopithecus anamensis, a hominid that lived in Africa 4.2 to 3.9 million years ago.

Based on actual tooth finds, Shimizu produced sophisticated computer models showing multiple external and internal details of the teeth. One determination was immediately clear: Unlike chimpanzees, which are fruit specialists, the hominid couldn’t have been much of a fruit-lover.

“Soft fleshy fruits tend to be acidic and do not require high bite forces to be broken down,” explained Macho. “The enamel microstructure of A. anamensis indicates that their teeth were not well equipped to cope with acid erosion, but were well adapted to masticate an abrasive and hard diet.”

The researchers therefore believe this early human ate nuts, root vegetables, insects—such as termites—and some meat. While they think certain flowering plants known as sedges might have been in the diet, Lucy and her relatives were not properly equipped for frequent leaf-chewing.

(Hat tip to Asclepius for the reference.)

Here’s the original paper:

Journal of Human Evolution Volume 57, Issue 3, September 2009, Pages 241–247
Dietary adaptations of South African australopiths: inference from enamel prism attitude
Gabriele A. Macho, Daisuke Shimizu

Unfortunately, as all we have yet found of Australopithecus anamensis are pieces of a jawbone and teeth, a fragment of a humerus, and a partial tibia (and those not even from the same individual!) we don’t know its cranial capacity. We do know that its range overlapped that of Ardipithecus—but since remains have also been found in transitional environments, it may have not been a pure forest-dweller.

Either way, it appears that our ancestors had been selected away from a fruit-based diet, and towards an omnivorous diet more compatible with savanna-dwelling, even before they left the forest.

Our Story Continues…With Footprints

This brings us to an unusual fossil find…the Laetoli footprints, left in volcanic ash 3.7 MYA, cemented by rainfall, and preserved by subsequent ashfalls. Their form and spacing shows that the hominins who made them were fully bipedal: their feet had arches and an adducted big toe, and they walked at or near human walking speed.

A footprint at LaetoliExcavating the Laetoli footprints

“Adducted” means “closer to the midline”. It means their big toe was close to their other toes, like a modern human—quite unlike the widely spaced, opposable big toe of Ardipithecus.

And though we’re not completely sure, it is generally accepted that the footprints were made by Australopithecus afarensis, the next player in our story. Here’s the original paper by Leakey and Hay, for those interested:

Nature Vol. 278, 22 March 1979, pp. 317-323
Pliocene footprints in the Laetolil Beds at Laetoli, northern Tanzania
Leakey, M. D. and Hay, R. L.

In summary, it’s clear from what we know of Ardipithecus, and Australopithecus anamensis, that bipedalism long preceded our ancestors’ move into savanna and grassland habitats. This makes sense: a clumsily-waddling knuckle-walker would stand no chance outside the safety of the forest, whereas a bipedal ape can survive in the forest so long as it retains some ability to climb trees—a talent even humans haven’t completely lost.

Furthermore, our dietary shift towards ground-based foods, and away from fruit, also preceded our ancestors’ move into savanna and grassland habitats.

Finally, and most importantly, both of these changes preceded the massive increase in our ancestors’ brain size.

Live in freedom, live in beauty.

JS

Continue to Part IV, “The Paleo Diet For Australopithecines”!

(Or, go back to Part I or Part II.)


(Yes, this series has expanded far beyond my original expectations! Frankly, the subject requires it…and you’re saving thousands of dollars in post-secondary tuition, so buck up and buy a T-shirt or something.)

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18 comments

Permalink: Big Brains Require An Explanation, Part III: Optimal Foraging Theory, And Our Story Begins On Two Legs
  • Bodhi

    Great series, my shirt has been ordered.

  • Fmgd

    I can see this easily going for another three parts, which is great. I notice lots of people tend to view the question of our brains (and really, most things) in too simplistic a light. “It was running”. “No, it was definitely fire”.

    It seems pretty obvious to me it could have been no single thing, so I appreciate how you’re taking your time with all the groundwork.

  • Bodhi:

    Great to see you around again!  Wear it with pride.

    Fmgd:

    It's easy to come up with a hypothesis and tell a long, involved story to justify it — especially at book length, when you can cherry-pick which evidence you present to your readers.  It's a lot harder to take all the evidence as it exists and figure out what story it tells.

    JS

  • Bodhi

    I have to ask this just for shits and grins. I know you’re not hip to the aquatic ape theory, but will you be including anything from “Them and Us” by Danny Vendramini or alternatives theories to the out-of-Africa theory?

  • Bodhi:

    It's not that I'm not hip to aquatic ape theory…it's that I don't see the evidence that it's true.  There's no evidence that Ardipithecus or any other transitional bipedal hominins were aquatic in any way, either in behavior or in diet — and the fact that the transition to bipedalism preceded the increase in hominin brain size is evidence against it.

    AAT is just one of many heterodox hypotheses that could be true…but given the evidence, they're far less parsimonious than the consensus narrative.

    Vendramini's hypothesis correctly anticipated the current science (i.e. “mostly out of Africa, with a few percent admixture from male Neanderthals”), and I suspect he's at least partially correct.  However, the changes he posits take place 100-50 KYA, after humans had become anatomically modern and reached our maximum brain size, and therefore won't be covered in this series.

    JS

  • brynnsweet

    Stoked to read more; so fascinating and you explain it so easily. More more more! :)

  • Asclepius

    Got my T-shirt today! :)

  • Vizeet

    It seems this will be a long series and I am hoping to have this in my favorite collections.
    Humans are extremely evolved species who could survive on virtually any junk produced in food factories (yeah he does get some nasty diseases :-( ), can run for several hours, can imitate many other animals sound, has very good ability of holding minute items and can pass huge knowledge through generations.
    All these requires an explanation and also –
    How nature ended up in a super creature which is a big trouble for most of the other species?

  • […] Big Brains Require An Explanation, Part III: Optimal Foraging Theory, And Our Story Begins On Two Le… […]

  • @Vizeet – “Humans are extremely evolved species who could survive on virtually any junk…”.

     

    This is quite literally true and it is an important distinction between ‘surviving’ and ‘thriving’.  The impoverished have been known to eat shoes and grass to survive.  (Arguably modern foods transit a line of nutrition between these two points, creating food that triggers our ‘thrive’ senses whilst at a metabolic/physiologic level barely satisfying a ‘survive’ criteria.)

     

    We might be ‘big trouble for most of the other species’ but we can’t get to anthropocentric about this.  Many species have had a dominant period before us and, in time, humans may well be superceded by other lifeforms – perhaps not in terms of intelligence, but better adapted.  Don’t forget we ourselves are a biome waiting to be exploited

  • Gayle

    When beginning to read #3 a question popped into my mind–I’m still at the beginning:
    Do you know if animals’ brain size increased during the same time period?
    Thanks!

  • Brynn:

    Thank you!  It's a gift I do my best to share.

    Vizeet:

    General-purpose intelligence and breadth of diet usually go together.  If you're adapted to eat one single thing, it generally doesn't require intelligence to get it, just instinct — whereas if you can eat just about anything, general-purpose intelligence helps you find and exploit it.  Example: ravens.

    Asclepius:

    The time of anatomically modern humans is vanishingly short by archaeological standards, perhaps 200,000 years so far.  The time of hominins is still very short, at perhaps six million years…and recall Part I, in which I compare the 65-million year reign of mammals to the 140 million year reign of dinosaurs. 

    Given our ability to alter our environment and our current trajectory, we could very easily kill ourselves off in another few hundred years, leaving humans as the biological equivalent of a neutron bomb…200 KYA from inception to self-destruction would be essentially instantaneous when viewed from ten million years hence.

    I'm glad your shirt arrived so quickly!  Usually overseas shipping takes longer…

    Gayle:

    The combination of Bergmann's rule and a cooling climate would imply that total body size might have increased during the Pliocene and Pleistocene, but I'm not aware of any data that shows brain size increasing.

    JS

  • Peter

    Why bigger brains? IMHO there are 3 reasons:
    1. Vision – better eyes and better interpretation of the world
    2. Hand – manipulating and carrying of tools
    3. Speech – development of communication

    While 1+2 are good reasons – especially for bipedals, i am sure that 3 had the most influence. And it made culture possible.

    See it this way: While 1+2 are simple feedback loops, 3 involves more than one individual! It is much more complex.

  • Peter:

    Unfortunately, we have no direct evidence for when speech might have developed…so it's difficult to say when or where it might have been selected for!

    JS

  • TOURICO VACATIONS

    Travel will widen your horizons

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