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:
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:
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.
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.)
I’m using a new “share” plugin: let me know if it isn’t working for you. And if anyone knows how to insert a Google +1 button that doesn’t have a counter (counters slow page loads tremendously), please let me know!
So glad you’re back to this series! Fascinating stuff.
Re: social sharing buttons: there are a few good plugins, one is ‘Digg Digg’ which lets you choose whether you want a counter or not (for most of the sites, anyway – have to double check if G+ lets you choose); another is Sexy Bookmarks, which seems pretty good but a tad flamboyant 🙂 I use Digg Digg a lot on client sites.
Love that you’re using Atahualpa, BTW – one of the best free themes out there.
Awww, JS, you left us at a cliffhanger!
Mean, but effective.
That graph of brain size vs. time is interesting–I didn’t realize the expansion between habilis and sapiens was such a large percentage of the total growth–although I would like to see it plotted as encephalization quotient instead of absolute size, since I believe body mass was also expanding significantly during much of that time, no? I suspect that correction would flatten out the rate of increase closer to a constant.
So in addition, I guess I should ask–does the increased body size begin about the same time as the inflection point on that graph, just before habilis?
Tracy:
Thank you! I'll look into Digg Digg. Counters are cute, but WOW do they slow down page loads, especially in Safari, which likes to put the spinner up instead of letting you do things while waiting for something to finish.
I like Atahualpa because it's clean and heavily customizable: most themes make your site look exactly like everyone else's with that theme. It takes a lot of experimentation and work to make Atahualpa do what you want — but I'm reasonably happy with the results.
That said, I'd like to have a more magazine-like front page, with the ability to have Featured Articles that stay in the Featured spot (and have a “Read More” for just that category) as well as smaller, more frequent squibs (again, using categories to tell which is which) but no one seems to make such a theme. Let me know if you have any leads.
Uncephalized:
Yes, this one was a bit short — but I realized that I needed a whole new article to do justice to all the issues that crash together at the beginning of the Paleolithic. Even then it's likely to be a long article!
Re: weight, modern Homo sapiens males are only perhaps 27% heavier than Homo habilis, but sapiens females are 70% heavier than habilis females. The sexual dimorphism is much smaller in sapiens. These figure aren't the whole story, however, because we're much smaller than our hunter-gatherer ancestors. Homo heidelbergensis, in particular, were huge and powerfully built: they averaged 6' tall, and males probably weighed well over 200#. Some populations regularly grew to over 7 feet tall (2.1m)!
JS
interesting. so it seems we lost mass overall as we developed larger brains, then regained mass over time to get back to where we were.
(or this seems to be the process now generations seem to have a pattern of taller, shorter, taller than previous tall generation, shorter but not as short as previous short generation etc etc)?
eddie:
I'm not sure which way the causality runs — but my guess is that the selection pressure of living more on the ground vs. in the trees pushed Ardipithecus towards being lighter in weight and more nimble (Australopithecus). After that the picture becomes more complex because it involves technology.
JS
I jokingly say that heidelbergensis is my favourite ancestor and the high point of hominid evolution! There was a kill site down the road from me at Swanscombe Where they were killing adult male rhinos!!!
One of my other interests is painting. Interestingly (to me, anyway) I’ve relied on the exhaustive information about pigment and color at Handprint.com for more than a decade. It gave me quite a chuckle to find here a completely different like to Bruce’s site.
Neal:
I agree…the evidence is that heidelbergensis was the most capable and fearsome of our direct ancestors. Perhaps too capable…but I'm getting ahead of myself!
David:
There are hundreds of different graphical timelines of human evolution, but the handprint.com chart is the best I've found. It's an eclectic site, and I confess I haven't explored it in depth yet.
JS
Great post again JS your research is fascinating. Your previous on climate ties in nicely with this one, but you did not mention that rapid climate change was also a driver to select for intelligence, there were many evolutionary factors going on, one being that the trees were retreating from a cooling climate and fruit was not available year round…as grass took over ruminent game was the best source of food but man had to compete with every other predator on the plain. The quick, smart ( tooled ) were more likely to survive, and with such a diet, big then became an advantage.
I'm really enjoying this series! Along the lines of the comment above by BPT, one idea that has struck me as I’ve read these articles is that it seems that humans have been the only animal whose evolution has depended on and been shaped by tools. Our use of tools, in conjunction with hunting in packs, allowed us to hunt large ruminants, which in turn, gave us the nutritional support for the continued development of our brains.
I am not a historian or scientist, and I could be wrong in my assumptions, but I find this idea very exciting. It seems to point to many explanations, from providing a response to the silly vegan argument that we aren't meant to eat meat because we can't kill prey (without tools) to humanity's general fascination with weapons. Even our desire for possessions–we need to keep our tools in order to kill our food and process it.
How long have we been using tools? And has the use of tools, even simple ones, been an overwhelmingly significant factor in our evolutionary development? I have a new appreciation for the cave paintings of people and their spears–maybe the two have been linked longer than I had imagined.
—
Tom
BPT:
You're getting a bit ahead of me. I need to lay the groundwork of evidence before jumping to any conclusions…and you'll note that I already mentioned that meat enters the picture in Part IV.
The problem with the size argument, of course, that no matter how big our ancestors got, it wouldn't have helped them against the much larger carnivorous predators of the time: ancestral lions, giant Pachycrocuta hyenas, and several species of saber-toothed felid. So there must have been another explanation for increased body size.
mirrormtn:
You'll note that Part IV presents evidence that australopithecines were using naturally-occuring sharp rocks to deflesh bones 3.4 MYA. As the series continues, I'll present the evidence for purposely manufactured stone tools in their time.
Keep in mind that composite weapons (stone-tipped spears, etc.) are a comparatively recent development, postdating anatomical modernity. Wooden spears are older — at least 400 KYA — but because wood tends not to fossilize, we may never know exactly how old. (Though chimpanzees make small wooden spears to kill sleeping monkeys in their dens, so for all we know, spears may predate stone tools!)
JS
JS:
One benefit I see of being tall is that our ancestors can see a predator from longer distance but it could be seen other way round also (Predator can see him from longer distance). So I think size might have started increasing when becoming big gave real advantage. I think that would have happened when we were able to kill them so tools and intelligent may have played an important role. And of-course food availability to support increasing size and population.
Vizeet:
I'm also quite skeptical of the “being taller let you see farther” hypothesis. Your second thought, I believe, is the key: “size might have started increasing when becoming big gave real advantage.” I suspect the advantage was more against our fellow apes of genus Homo than against our predators or prey…but that will enter the picture in future installments.
JS
Re the slow +1 button, Google implemented an asynchronous version of the button which at least theoretically won’t slow page loads.
I say theoretically because I couldn’t get it to work when I tried it.
You may have better luck.
http://googlewebmastercentral.blogspot.com/2011/07/1-button-now-faster.html
[…] Big Brains Require An Explanation, Part V: Re-Orienting Ourselves In Time, and Why Are There “Sout… […]
Tom:
Thanks! I'll take a look after I'm done with this week's article.
It's annoying that page load speed is one of the metrics they use to rank results — while their own social media button slows them down so dramatically!
JS