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Big Brains Require An Explanation, Part VI:
Why Learning Is Fundamental, Even For Australopithecines

Our Story So Far (abridged)

  • By 3.4 MYA, Australopithecus afarensis was most likely eating a paleo diet recognizable, edible, and nutritious to modern humans. (Yes, the “paleo diet” predates the Paleolithic age by at least 800,000 years!)
  • The only new item on the menu was large animal meat (including bone marrow), which was more calorie- and nutrient-dense than any other food available to A. afarensis—especially in the nutrients (e.g. animal fats, cholesterol) which make up the brain.
  • Therefore, the most parsimonious interpretation of the evidence is that the abilities to live outside the forest, and thereby to somehow procure meat from large animals, provided the selection pressure for larger brains during the middle and late Pliocene.
  • A. africanus was slightly larger-brained and more human-faced than A. afarensis, but the differences weren’t dramatic.

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

And here’s our timeline again, because it helps to stay oriented:

Timeline of hominin evolution

Click the image for more information about the chart. Yes, 'heidelbergensis' is misspelled, and 'Fire' is early by a few hundred KYA, but it's a solid resource overall.


It Doesn’t Take Much Selection Pressure To Change A Genome (Given Enough Time)

When we’re talking about the selection pressure exerted by the adaptations our ancestors made to different dietary choices, it’s important to remember that it only takes a very small selective advantage to make an adaptation stick.

The path to fixation

Remember, these are based on the most pessimistic assumptions possible.

The math is complicated, and I don’t want to drag my readers through it—but even under the most pessimistic initial assumptions (Haldane 1957), the following rules of thumb hold:

  • A mutation that confers a 10% selective advantage on a single individual takes, on average, a couple hundred generations to become fixed (present in 100% of the population).
  • Even a mutation that confers a tiny 0.1% selective advantage takes only a few thousand generations to become fixed.
  • Therefore, a 10% selective advantage would have become fixed in just a few thousand years—a fraction of an instant in geological time.
  • Even a 0.1% selective advantage would have taken perhaps 50,000 years to reach fixation—still an instant in geological time, and well beyond the precision of our ability to date fossils from millions of years ago.

I’m using approximate figures because they depend very strongly on initial assumptions and the modeling method used…not to mention the idea of a precisely calculated figure for “selective advantage” is silly.

Why is this important? First, because we need to remember that we are thinking about long, long spans of time. All of what we blithely call “human history” (i.e. the history of agriculture, from the Sumerians to the present) spans less than 10,000 years, versus the millions of years we’ve covered so far!

Second, and most critically, it’s important because we don’t need to posit that australopithecines ate lots of meat in order for the ability and inclination to be selected for—and to reach fixation. Even if rich, fatty, calorie-dense meat (including marrow and brains) only provided 5% of the australopith diet—and 4.9% of that advantage was lost due to the extra effort and danger of getting the meat (it doesn’t matter if you’re better-fed if a lion eats you)—the remaining 0.1% advantage still would have reached fixation in perhaps 50,000 years.

In other words: the ability and inclination to eat meat when available might have been a tiny advantage for an individual australopith…but given hundreds of thousands of years, that tiny advantage is more than sufficient to explain the existence and spread of meat-eating.

Most Mutations Are Lost: Why Learning Is Fundamental (Even For Australopithecines)

The flipside of the above calculations is that most mutations occurring from a single individual—even strongly beneficial ones—are lost.

Using the simple mathematical model, the probability that even a beneficial mutation will achieve fixation in the population, when starting from a single individual, is extremely low. J.B.S. Haldane calculated it at approximately 2 times the selective advantage—so even a 10% advantage is only 20% likely to reach fixation if it begins with a single individual! And for a 0.1% selective advantage, well, 0.2% doesn’t sound very encouraging, does it?

For those interested in the dirty mathematical details of simulating gene fixation, see (for instance) Kimura 1974 and Houchmandzadeh & Vallade 2011.

This low probability is because any gene carried by only one individual, or only a few individuals, is usually lost right away due to random chance while we’re on the initial part of the S-curve in the graph above. (As the number carrying the gene increases, the probability that everyone carrying it will die decreases.) So according to this naive model, we would expect individual australopithecines to have discovered meat-eating over and over again, hundreds if not thousands of times, before sheer luck finally allowed the behavior to spread throughout the population! Is that why it took millions of years to make progress?

Perhaps—but it seems doubtful. Meat-eating isn’t a single action: even if we assume that australopithecines were pure scavengers, it’s still a long, complicated sequence of behaviors involving finding suitable scraping/smashing rocks; looking for unattended carcasses; watching for their owners or other predators to return, which is probably a group behavior; grabbing any part that looked tasty; and using the rocks found earlier to help scrape off meat scraps, or to smash them open for marrow or brains. And hunting behavior is even more complex!

Of course, the naive mathematical model assumes that behavioral changes are purely under genetic control, and that individuals are not capable of learning. Since we know that the ability of humans to communicate knowledge by teaching and learning (known generally as “culture”) is greater than that of any other animal, it seems likely that the ability and inclination to learn from other australopiths was the primary mechanism by which our ancestors adapted a new mode of life that involved survival outside the forest—including meat-eating.

Note that chimpanzees can be taught all sorts of complicated skills, including how to make Oldowan stone tools—but they don’t seem to show any particular interest in teaching other chimps what they’ve learned.

Evidence That Increased Learning Ability Was The Key Hominin Adaptation During The Late Pliocene

We’ve just established that it’s very unlikely for a behavior discovered by one individual to spread throughout the population if it’s purely driven by a genetic mutation, even if it confers a substantial survival advantage—because the mathematics show that most individual mutations, even beneficial ones, are lost.

Here’s a summary of the physical evidence that our ancestors’ behavioral change was driven, at least in large part, by the ability to learn:

  • Body mass decreased by almost half between Ardipithecus ramidus (110#, 50kg) and Australopithecus africanus (65#, 30kg). Height also decreased slightly, from 4′ (122cm) to about 3’9″ (114cm). Clearly our ancestors’ adaptation to bipedal, ground-based living outside the forest didn’t depend on being big, strong, or physically imposing!
  • None of the physical changes appear to be a specific adaptation to anything but bipedalism, or to a larger brain case: faces became flatter and less prognathic, canines became shorter and less prominent, etc.
  • Despite a much smaller body, brain size increased from 300-350cc to 420-500cc. As brains are metabolically expensive (ranking behind only the heart and kidney by weight, and roughly equal to the GI tract—see Table 1 of Aiello 1997), this suggests that it was very important to conserve them.

Furthermore, it’s probably not a coincidence that bone marrow and brains are high in the same nutrients of which hominin brains are made—cholesterol and long-chain fats.

World Rev Nutr Diet 2001, 90:144-161.
Fatty acid composition and energy density of foods available to African hominids: evolutionary implications for human brain development.
Cordain L, Watkins BA, Mann NJ.

Scavenged ruminant brain tissue would have provided a moderate energy source and a rich source of DHA and AA. Fish would have provided a rich source of DHA and AA, but not energy, and the fossil evidence provides scant evidence for their consumption. Plant foods generally are of a low energetic density and contain virtually no DHA or AA. Because early hominids were likely not successful in hunting large ruminants, then scavenged skulls (containing brain) likely provided the greatest DHA and AA sources, and long bones (containing marrow) likely provided the concentrated energy source necessary for the evolution of a large, metabolically active brain in ancestral humans.

The learning-driven hypothesis fits with other facts we’ve already established. General-purpose intelligence is an inefficient way to solve problems:

…Intelligence is remarkably inefficient, because it devotes metabolic energy to the ability to solve all sorts of problems, of which the overwhelming majority will never arise. This is the specialist/generalist dichotomy. Specialists do best in times of no change or slow change, where they can be absolutely efficient at exploiting a specific ecological niche, and generalists do best in times of disruption and rapid change.” -Efficiency vs. Intelligence

Yet our hominin ancestors found success via greater intelligence rather than specific adaptations—most likely because of the cooling and rapidly oscillating climate previously discussed in Part I and Part IV. I’ll quote this paper again because it’s important:

PNAS August 17, 2004 vol. 101 no. 33 12125-12129
High-resolution vegetation and climate change associated with Pliocene Australopithecus afarensis
R. Bonnefille, R. Potts, F. Chalié, D. Jolly, and O. Peyron

Through high-resolution pollen data from Hadar, Ethiopia, we show that the hominin Australopithecus afarensis accommodated to substantial environmental variability between 3.4 and 2.9 million years ago. A large biome shift, up to 5°C cooling, and a 200- to 300-mm/yr rainfall increase occurred just before 3.3 million years ago, which is consistent with a global marine δ18O isotopic shift.

We hypothesize that A. afarensis was able to accommodate to periods of directional cooling, climate stability, and high variability.

The temperature graphs show that this situation continued. How did it affect our ancestors’ habitat and mode of life?

J Hum Evol. 2002 Apr;42(4):475-97.
Faunal change, environmental variability and late Pliocene hominin evolution.
Bobe R, Behrensmeyer AK, Chapman RE.

This study provides new evidence for shifts through time in the ecological dominance of suids, cercopithecids, and bovids, and for a trend from more forested to more open woodland habitats. Superimposed on these long-term trends are two episodes of faunal change, one involving a marked shift in the abundances of different taxa at about 2.8+/-0.1 Ma, and the second the transition at 2.5 Ma from a 200-ka interval of faunal stability to marked variability over intervals of about 100 ka. The first appearance of Homo, the earliest artefacts, and the extinction of non-robust Australopithecus in the Omo sequence coincide in time with the beginning of this period of high variability. We conclude that climate change caused significant shifts in vegetation in the Omo paleo-ecosystem and is a plausible explanation for the gradual ecological change from forest to open woodland between 3.4 and 2.0 Ma, the faunal shift at 2.8 +/-0.1 Ma, and the change in the tempo of faunal variability of 2.5 Ma.

In summary, 2.8 MYA is when things started to get exciting, climate-wise…and 2.6 MYA (the beginning of the Pleistocene) is when they started to get really exciting.

None of this is to say that the ability to learn was the only adaptation responsible for meat-eating: learning ability could easily have combined with other adaptations like inquisitiveness, aggressiveness, or a propensity to break things and see what happens.

Conclusion: A Tiny Difference Can Make All The Difference

  • Given the time-scale involved, a small selective advantage conferred by a small amount of meat-eating could easily have produced the selection pressure for meat-eating behavior to reach fixation in australopithecines.
  • Several lines of evidence—the mathematics of population genetics, the trends of australopithecine physical evolution, the ability of the nutrients in meat to build and nourish brains, and the increasingly colder, drier, and more variable climate—all point towards intelligence and the ability to learn (as opposed to physical power, or specific genetically-driven behavioral adaptations) being the primary source of the australopithecines’ ability to procure meat.

Don’t stop here! Continue to Part VII, “The Most Important Event In History”.

Live in freedom, live in beauty.

JS

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Big Brains Require An Explanation, Part V:
Re-Orienting Ourselves In Time, and Why Are There “Southern Apes” In Ethiopia?

In Part IV, we established the following:

  • Our ancestors’ dietary shift towards ground-based foods, and away from fruit, did not cause an increase in our ancestors’ brain size.
  • Bipedalism was necessary to allow an increase in our ancestors’ brain size, but did not cause the increase by itself.
  • Bipedalism allowed Australopithecus afarensis to spread beyond the forest, and freed its hands to carry tools. This coincided with a 20% increase in brain size from Ardipithecus, and a nearly 50% drop in body mass.
  • Therefore, the challenges of obtaining food in evolutionarily novel environments (outside the forest) most likely selected for intelligence, quickness, and tool use, and de-emphasized strength.
  • By 3.4 MYA, A. afarensis was most likely eating a paleo diet recognizable, edible, and nutritious to modern humans. (Yes, the “paleo diet” predates the Paleolithic age by at least 800,000 years!)
  • The only new item on the menu was large animal meat (including bone marrow), which was more calorie- and nutrient-dense than any other food available to A. afarensis—especially in the nutrients (e.g. animal fats, cholesterol) which make up the brain.
  • Therefore, the most parsimonious interpretation of the evidence is that the abilities to live outside the forest, and thereby to somehow procure meat from large animals, provided the selection pressure for larger brains during the middle and late Pliocene.

Keep in mind that, as always, I am presenting what I believe to be the current consensus interpretation—or, when no consensus exists, the most parsimonious interpretation.

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

Re-Orienting Ourselves In Time

Since we’re all returning to this series after a few weeks off, let’s take a minute to re-orient ourselves. Our narrative has just reached 3 MYA, between Australopithecus afarensis and Australopithecus africanus:

Timeline of hominin evolution

Click the image for more information about the chart. Yes, 'heidelbergensis' is misspelled, and 'Fire' is early by a few hundred KYA, but it's a solid resource overall.

And here’s an excellent reminder that while we’re making progress, there is much left to explain:

Graph of hominin brain size

With that in mind, let’s keep moving!

Australopithecus africanus: The Original Australopith

Back in 1924, the world still believed that the “Piltdown Man” was the “missing link” between apes and humans. Actually, Piltdown Man was a hoax, made from pieces of the skull of a modern human and the jaw of an orangutan—and though it was first publicized in 1912, it wasn’t universally acknowledged as a fraud until 1953. (Though several paleontologists of the time had immediately voiced their doubts, and its influence gradually declined as more and more African fossils were found. By 1953 its official repudiation was basically a formality.)

Strongly contributing to the acceptance of the Piltdown hoax was the early 20th-century belief that the ancestors of humans must have been European, and that brain enlargement must have preceded bipedalism.

You can read more about “Piltdown Man”, and other paleontological controversies, in Roger Lewin’s Bones of Contention.

Unsurprisingly, the Piltdown hoax sabotaged our understanding of human evolutionary history for decades. The first casualty was the Taung child, a skull (complete with teeth) and cranial endocast discovered by quarry workers in the Taung lime mine in South Africa, and officially announced by Raymond Dart in 1925—though not universally accepted as a hominin until two decades later.

Skull of Taung child

Note the short canine teeth.


Why Are There “Southern Apes” In Ethiopia?

The first person to publish the discovery of a new animal (or its fossil) gets to name it. Anyone who names a new genus runs the risk of “their” find being reclassified into an existing genus…but Dart’s classification has stood the test of time, and later finds (such as “Plesianthropus transvaalensis”, later reclassified as another A. africanus) have been absorbed into it.

Unfortunately, the context of a fossil often changes as more and more fossils are found, and the original name can easily turn out to be inappropriate. For instance, Australopithecus means “southern ape”, because the Taung child was found in South Africa…

…and now all australopithecines, even those found in Ethiopia and Kenya, are forever known as “southern apes”. (Even worse, “australo” is Latin, while “pithecus” is Greek.)

While his naming may have been clumsy, it’s important to note that Raymond Dart was correct in several important respects: subsequent fossil finds proved A. africanus was both a hominin and fully bipedal, as Dart had always asserted.

The Taung child dates to 2.5 MYA, and Mrs. Ples (which may actually be a Mr. Ples), discovered in 1947, dates to 2.05 MYA. In total, the time of fossils we classify as A. africanus spans nearly a million years, from 3.03 MYA to 2.05 MYA.

A. africanus vs. A. afarensis

Since we’re entering a time from which we have more fossils to study, the transitions from here on will be more gradual. A. africanus is a relatively short step away from A. afarensis, but the similarities and differences are instructive:

  • A. africanus is slightly shorter than A. afarensis: 3’9″/115cm for females, 4’6″/138cm for males. However, with so few fossils, this may simply be sampling error.
  • Body weight estimates are essentially identical: 66#/30kg for females, 90#/41kg for males. (Source for height and weight estimates.)
  • The africanus skull appears more human-like: the face is flatter and more vertical, the brow ridges are less pronounced, the cheekbones are narrower, and the forehead is more rounded.
  • Africanus teeth and jaws were more human-like than afarensis teeth and jaws: while the teeth and jaws were much larger than a modern human’s, the canines were shorter and less prominent (with no gaps between them and the incisors), and the jawline was more parabolic (human-shaped) and less prognathic. (Click here for a pictorial comparison.)
  • Most importantly, A. africanus adults had a brain volume of 420-500cc, meaningfully larger than the A. afarensis range of 380-430cc.

This implies that there was continuing selection pressure for larger brains—but not larger bodies. We’ve established in Part IV that the ability to somehow procure meat outside the forest most likely provided the necessary selection pressure up to that time…but what is the evidence during the time of A. africanus and beyond?

Continue reading! Big Brains Require An Explanation, Part VI: Why Learning Is Fundamental, Even For Australopithecines

Live in freedom, live in beauty.

JS

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


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The Paleo Diet For Australopithecines: Approaching The Meat Of The Matter (Big Brains Require An Explanation, Part IV)

In Part III, we established the following:

  • Bipedalism among human ancestors is associated with a dietary shift away from soft, sugar-rich fruit, and toward hard, fibrous, ground-based foods like nuts, root vegetables, insects, and mushrooms. (And perhaps some meat, though the evidence is inferential.)
  • Both bipedalism and this dietary shift occurred while our ancestors were still forest-dwellers—before we moved into savanna and grassland habitats.
  • Both bipedalism and this dietary shift preceded the massive increase in our ancestors’ brain size.
  • Therefore, neither fruit, nor potatoes, nor walking upright made us human.

Once again, I am giving what I believe to be the current consensus interpretation of the evidence…and where no consensus exists, I offer what I believe to be the most parsimonious interpretation.

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

A Quick Recap

4.4 million years ago, Ardipithecus ramidus still had a brain the size of a modern chimpanzee, but was a facultative biped partially adapted to a ground-based diet. By 4.1 MYA, Australopithecus anamensis had been selected for more complete dietary adaptation:

Science 2 October 2009: Vol. 326 no. 5949 pp. 69, 94-99
Paleobiological Implications of the Ardipithecus ramidus Dentition
Gen Suwa, Reiko T. Kono, Scott W. Simpson, Berhane Asfaw, C. Owen Lovejoy, Tim D. White

Ar. ramidus lacks the postcanine megadontia of Australopithecus. Its molars have thinner enamel and are functionally less durable than those of Australopithecus but lack the derived Pan pattern of thin occlusal enamel associated with ripe-fruit frugivory. The Ar. ramidus dental morphology and wear pattern are consistent with a partially terrestrial, omnivorous/frugivorous niche.”

And the Laetoli footprints show that hominins were fully bipedal by 3.7 MYA, though we have no evidence for brain size until…

Australopithecus afarensis: Upright Gait, Smaller Body, Bigger Brain

Australopithecus afarensis lived from approximately 3.9 to 2.9 MYA. (Once again, these are human-drawn distinctions between a continuum of hominin fossils.) It was slightly shorter than Ardipithecus (3’6″) and weighed much less: 65# versus 110#. The famous “Lucy” fossil is about 40% of an A. afarensis skeleton from 3.2 MYA.

One interpretation of Lucy

Lucy might have looked like this.

Additionally, its back had a similar double curve to modern humans; its arms were shorter than Ardipithecus; its knees support an upright gait, and its feet had arches like ours—meaning that it was fully bipedal, and that A. afarensis is very likely the hominin which made the Laetoli footprints.

This is a recent finding: only last year did its discoverers announce that they had found a foot bone from A. afarensis which appears to settle this long-simmering question.

Science 11 February 2011: Vol. 331 no. 6018 pp. 750-753
Complete Fourth Metatarsal and Arches in the Foot of Australopithecus afarensis
Carol V. Ward, William H. Kimbel, and Donald C. Johanson

“A complete fourth metatarsal of A. afarensis was recently discovered at Hadar, Ethiopia. It exhibits torsion of the head relative to the base, a direct correlate of a transverse arch in humans. The orientation of the proximal and distal ends of the bone reflects a longitudinal arch. Further, the deep, flat base and tarsal facets imply that its midfoot had no ape-like midtarsal break. These features show that the A. afarensis foot was functionally like that of modern humans and support the hypothesis that this species was a committed terrestrial biped.

Most importantly, A. afarensis’ brain was much larger than Ardipithecus: 380-430cc versus 300-350cc. This means that selection pressure was favoring bigger brains as early as 4 million years ago, while allowing our ancestors’ bodies to shrink dramatically.

Now we’re getting to the meat of the problem. What could have caused this selection pressure?

“Is It Just Me, Lucy, Or Is It Getting Colder?”

During the Pliocene (5.3-2.6 MYA), the Earth’s climate—though far warmer than today’s—become cooler, drier, and more seasonal (see the temperature graphs and detailed explanation in Part I), a multi-million-year trend which began with the Middle Miocene Disruption around 14.5 MYA. Consequently, African forests were shrinking, and savannas and grasslands were growing in their place.

With less forest available to live in, some number of our ancestors faced a stark choice: adapt to living outside the forest, or die out. Those that stayed in the trees became what we know today as chimpanzees and bonobos. Those that eventually left became our ancestors—the hominins.

PNAS August 17, 2004 vol. 101 no. 33 12125-12129
High-resolution vegetation and climate change associated with Pliocene Australopithecus afarensis
R. Bonnefille, R. Potts, F. Chalié, D. Jolly, and O. Peyron

Through high-resolution pollen data from Hadar, Ethiopia, we show that the hominin Australopithecus afarensis accommodated to substantial environmental variability between 3.4 and 2.9 million years ago. A large biome shift, up to 5°C cooling, and a 200- to 300-mm/yr rainfall increase occurred just before 3.3 million years ago, which is consistent with a global marine δ18O isotopic shift.

Our results show that a diversity of biomes was available to A. afarensis. Recovery of hominin fossils through the entire stratigraphic range suggests no marked preference by A. afarensis for any single biome, including forest. Significant cooling and biome change had no obvious effect on the presence of this species through the sequence, a pattern of persistence shared by other Pliocene mammal taxa at Hadar and elsewhere (6, 27, 32). We hypothesize that A. afarensis was able to accommodate to periods of directional cooling, climate stability, and high variability.

As we found in Part I, and as we’ve seen by the chimp-sized brains of Ardipithecus, shrinking habitat does not explain increased brain size by itself—but it does provide an incentive to find ways to live in marginal habitat, or entirely different biomes. And it’s clear that bipedalism would be an advantage in forest margins and open forests, where direct travel from tree to tree wasn’t possible. In addition, more light reaching the ground would mean more food available on the ground, versus up in the tree canopy—so bipedal ground-dwelling would have been a good survival strategy in forest habitat that was marginal for a tree-dweller.

My interpretation of the evidence is that bipedalism did not cause brain expansion, but it was a necessary precondition. It allowed our ancestors to expand beyond the forest margin—and it freed up our ancestors’ hands for other tasks, such as…

How Bipedalism Enables Tool Use, Re-Use, and Manufacture

Facultative bipeds, which cannot walk on two legs for very long, can’t carry tools around with them: they must make a tool out of whatever materials exist near the point of use, and discard it soon after. Therefore, the tools they make must remain relatively simple, since they can’t spend too much time making single-use items—and it greatly constrains the raw materials they can use. (Yes, I’m ignoring any hypothesis that gives Ardipithecus ramidus the ability to construct backpacks.)

In contrast, full bipeds can carry around their tools in anticipation of needing them, and can keep them for future use. Therefore, they can spend the time and effort to make complex, reusable tools—and they can use any raw materials they have access to, not just those near the point of use.

We know that modern chimpanzees make spears, termite sticks, and other wooden tools—but is there evidence for tool use previous to the Oldowan industry, 2.6 MYA?

Recall that the Oldowan industry marks the beginning of the Paleolithic age, and happens to coincide with the beginning of the Pleistocene epoch. (If these terms are confusing you, I explain them in Part II.)

Rocks, Meat, and Marrow in the Pliocene

Nature 466, 857–860 (12 August 2010) — doi:10.1038/nature09248
Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia
Shannon P. McPherron, Zeresenay Alemseged, Curtis W. Marean, Jonathan G. Wynn, Denné Reed, Denis Geraads, René Bobe, Hamdallah A. Béarat

“On the basis of low-power microscopic and environmental scanning electron microscope observations, these bones show unambiguous stone-tool cut marks for flesh removal and percussion marks for marrow access. … Established 40Ar–39Ar dates on the tuffs that bracket this member constrain the finds to between 3.42 and 3.24 Myr ago, and stratigraphic scaling between these units and other geological evidence indicate that they are older than 3.39 Myr ago.”

It’s fair to say that no one knows what to do with this particular piece of evidence, so it tends to simply get ignored or dismissed. What we know is that the researchers found several ungulate and bovid bones, dated to 3.4 MYA, which were scraped and struck by rocks. The scrapes are not natural, nor are they from the teeth of predators, and they appear to date from the same time as the bones.

A bone at Dikika

One of the bones at Dikika. The reality of paleontology is far less exciting than the hypotheses it generates.

Unfortunately, no stone tools or fossil hominins were found there, so we can’t say for sure who made them. But the simplest interpretation is that a hominid used a rock to scrape meat off of the bones of large prey animals, and to break them open for marrow.

It is likely that the reason this evidence isn’t more well-accepted is because the researchers make one huge assumption: that the scrape marks were made by deliberately fashioned stone tools, 800,000 years before the first evidence we have of stone tool manufacture—even though no such tools were found.

I believe the most parsimonious interpretation is that the scrape marks were indeed made by Australopithecus afarensisusing one of the naturally-occurring volcanic rocks found in abundance in the area. Given the slow pace of technological change (millions of years passed between major changes in stone tool manufacture, and that’s for later hominins with much larger brains than A. afarensis), it would be extremely surprising if naturally-occurring sharp rocks hadn’t been used for millions of years before any hominin thought to deliberately make them sharper—

It’s Not Just The Discovery…It’s The Teaching And The Learning

—and, more importantly, before their children were able to learn the trick, understand why it was important, and pass it on to their own children.

Those of you who were able to watch the documentary “Ape Genius”, to which I linked in Part I, understand that intelligence isn’t enough to create culture. In order for culture to develop, the next generation must learn behavior from their parents and conspecifics, not by discovering it themselves—and they must pass it on to their own children. Chimpanzees can learn quite a few impressive skills…but they have little propensity to teach others, and young chimps apparently don’t understand the fundamental concept that “when I point my finger, I want you to pay attention to what I’m pointing at, not to me.”

So: the developmental plasticity to learn is at least as important as the intelligence to discover. Otherwise, each generation has to make all the same discoveries all over again. It is theorized that this plasticity is related to our less-aggressive nature compared to chimpanzees…but that’s a whole another topic for another time.

In conclusion, the Dikika evidence pushes meat-eating and stone tool-using (though not stone tool-making) back to at least 3.4 MYA, well into the Pliocene. And though we’re not sure whether that meat was obtained by hunting, scavenging, or both, we can add it to the other foods that we’re reasonably sure formed its diet to produce the following menu:

The Paleo Diet For Australopithecus afarensis

Eat all you can find of:

  • Nuts
  • Root vegetables
  • Insects
  • Mushrooms
  • Meat (particularly bone marrow)

Eat sparingly:

  • Fruit (your tooth enamel won’t withstand the acids)
  • Foliage (your teeth aren’t shaped correctly for leaf-chewing)

In other words, A. afarensis was most likely eating a diet within the existing range of modern ancestral diets—3.4 million years ago.

The only major addition to this diet previous to the appearance of anatomically modern humans is the gathering of shellfish, known from middens dated to 140 KYA at Blombos Cave.

Our Takeaway (so far)

  • Our ancestors’ dietary shift towards ground-based foods, and away from fruit, did not cause an increase in our ancestors’ brain size.
  • Bipedalism was necessary to allow an increase in our ancestors’ brain size, but did not cause the increase by itself.
  • Bipedalism allowed A. afarensis to spread beyond the forest, and freed its hands to carry tools. This coincided with a 20% increase in brain size from Ardipithecus, and a nearly 50% drop in body mass.
  • Therefore, the challenges of obtaining food in evolutionarily novel environments (outside the forest) most likely selected for intelligence, quickness, and tool use, and de-emphasized strength.
  • By 3.4 MYA, A. afarensis was most likely eating a paleo diet recognizable, edible, and nutritious to modern humans.
  • The only new item was large animal meat (including bone marrow), which is more calorie- and nutrient-dense than any other food on the list—especially in the nutrients (e.g. animal fats, cholesterol) which make up the brain.
  • Therefore, the most parsimonious interpretation of the evidence is that the abilities to live outside the forest, and thereby to somehow procure meat from large animals, provided the selection pressure for larger brains during the middle and late Pliocene.

Live in freedom, live in beauty.

JS


We’re not done yet…in fact, we’re not even to the Paleolithic! Continue to Part V, “Why Are There Southern Apes In Ethiopia?”

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