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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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