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.

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.

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 afarensis…using 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?”

Are you enjoying this series, or is it too abstruse for you? Please leave a comment and let me know!