Bigger Butts … Bigger Brains

The Amazing Journey of Human Evolution

I absolutely loved a recently aired episode of RadioLab titled: Man Against Horse. It originally aired December 28, 2019, but I heard it May 23, 2021. I had been working on my story trying to getting straight in my head man’s long line of evolutionary changes that ultimately lead to us, the living beings who stare at screens and do everything to extremes.

Man & His Ancestors

There was Australopithecus afarensis who emerged 3.67 to 2 million years ago in the Middle Pliocene to Early Pleistocene of South Africa, an extinct species of australopithecine. Spread: Southern Africa (Lucy’s species). I love them. Look at those eyes!

Source: Natural History Museum Smithsonian

There was Homo habilis who emerged 2.4 to 1.5 mya inhabiting parts of sub-Saharan Africa from roughly 2.4 to 1.5 million years ago (mya). In 1959 and 1960 the first fossils were discovered at Olduvai Gorge in northern Tanzania – roamed Eastern edge of Africa, moving from the Horn of Africa to the tip. Spread: Western to Southern African


There was Homo ergaster who emerged (“working man”) is an extinct hominid species (or subspecies, according to some authorities) which lived throughout eastern and southern Africa between 1.9 to 1.4 million years ago with the advent of the lower Pleistocene and the cooling of the global climate: 1.9 to 1.4 mya (although some classifications include additional individuals that extends their range to between about 700,000 and 2 million years ago). Spread: Africa: 1.9 to 1.4 million years ago. Considered an early, exclusively African form of Homo erectus. Started making stone tools 1.6 million years ago.


And of course, there was Homo erectus who emerged 2 mya, evolving from either a late form of australopith or one of the more primitive forms of Homo, and went on to spread into many parts of Asia. Spread: Western African, Europe, Arabian Peninsula, Southern Asia, Indonesia, Philippines, New Zealand, Australia, Eastern coast of Asia to Bering Strait

There are many more early hominoid species that evolved, lived for thousands (and some more than a million) years, and then died out and disappeared. This is where I was getting lost, and this is when I took a break and tuned into RadioLab and heard this episode that straighten everything out in my mind. It all came down to the nuchal ligament and the human butt.


It’s All About the Butt

I was skeptical at first because this episode started out with Matt who began saying:

Okay, so this story comes to us from Heather, who is a fantastic writer who brought us this story that, if I were to boil it down, is about a horse, a lone man running through the desert, and what it fundamentally means to be a human being. And weirdly, butts. I didn't see this coming, but it's about butts. Just butts. Your butt. It's about your butt.

Heather is writing a book about the cultural history of the female butt. She said:

I thought I'd save that one for on tape. It started as an essay that I was just working on because I have a big butt, and I grew up in, you know, the suburbs of mid-Michigan. That was -- it was pretty white. And in high school in the '90s, it was very much like, not good to have a big butt. Like, I got made fun of, et cetera, et cetera. But then sometime in the mid-aughts, all of a sudden this body that had sort of been bringing me all this shame became attractive in sort of a mainstream way.

As Heather started taking apart and looking into issues such as race, appropriation, beauty, her essay about the butt ended up becoming a book about the butt. She asked herself:  what does the butt mean? Like, what does it symbolize and why does it symbolize that? Then, she realized she had to answer a more fundamental question: Why do we even have a butt at all?

Butts | Animation by Genolve

Gluteus Maximus & Evolution of Man

Daniel Lieberman is an evolutionary biologist at Harvard University who is interested in the evolution of the human body and the effects of physical activity for a long time. He wanted to understand how and why the human body evolved the way it did. Back around 1992, he was a post-doc doing research on pigs…miniature pigs running on treadmills!

Treadmill Pigs | Animation by Genolve

Lieberman was looking at how different parts of the skeleton respond to the effect of the loads caused by exercise. Lieberman says, “Sounds like an exciting thing, but believe me it eventually gets kind of — kind of dull.” This is until the day a fellow called Dennis Bramble, a professor at the University of Utah, came to Harvard to do his own research next door to Lieberman.

Dennis Bramble recalls turning to his co-researher saying, “What the hell’s that sound? Is somebody doing something there?” And they said, “Yeah, and this guy Dan Lieberman is running pigs over there.” I said, “Oh, I gotta — I’ve gotta see this!”

Pig on a Treadmill | 32,031 views•Oct 13, 2007 | Petunia the pig trains for glory.

Lieberman recounts Bramble popped his head in and watched the pig, then cocked his head to the side and said, ““You know Dan, that pig can’t hold its head still when it’s running.” Lieberman said, “It’s funny I’d spent hours watching pigs run on treadmills, but I never really thought about it.

Bramble said: “You know Dan, I bet that pig’s head is flopping all around because it doesn’t have this thing called the nuchal ligament.” This ligament provides support for the head and neck. It is like a rubber band attached to the back of the animal’s skull and runs down the spine to keep the head straight as it runs. Bramble pointed out that all mammals that have specialized as runners have this nuchal ligament–everything from cheetahs to leopards to antelopes to horses, to jackrabbits and dogs. Animals who are bad runners don’t have this ligament–like pigs.

This is where my attention perked up: humans have a nuchal ligament.

But, our closest hominoid cousins do not have a nuchal ligament. This includes apes, chimps, gorillas.

It’s All About the Nuchal Ligament | Animation made by Genolve

Humans Evolved to Run

Way back, our closest hominoid relatives split off into the genus Pan, while humans split off into the genus Homo. The first hominoid in the genus Homo to have this ligament was Homo erectus. Paleontologist can tell this by a sharp ridge on the back of the skull that this ligament leaves behind as a trace.

Daniel Lieberman says, “It doesn’t have a snout, it has smaller teeth. It’s — it’s the first species that’s really very much like you and me from the neck down.

Around the time that Homo erectus emerged, spectacular changes were occurring with its foot (e.g., toes were shortening, arch was forming, Achilles tendon), hips (i.e., taller, narrower, twisty that helps us stay stable on two feet), arms (shorter), legs (longer), inner ears (semicirucular canals got larger to balance), joints (got bigger to bear the load of running), and butts!

Butts evolved for running. Lieberman explains that when humans run, the gluteus maximus muscles fires twice with every stride to prevent the trunk from pitching forward and falling face first.

Let’s Go! Gluteus maximus! | Animation by Genolve

Lieberman explains:

"Running is a controlled fall. Very different from walking. And so your gluteus maximus fires just before your body's about to -- your trunk is about to pitch forward and make you hit your nose on the ground, and it helps pull your trunk backward. And the other time the gluteus maximus fires is when your leg is swinging forward when you're in the air, and it helps decelerate the leg so that you bring your leg down onto the ground. So the gluteus maximus plays a very important role when you're -- when you're running, and turns out to barely be active when you're walking. And, you know, you don't need the fancy equipment in my lab to figure this out. You can just do this yourself at home. Just walk around the room and hold your butt and, you know, clench your kind of butt. And -- and when you're walking your butt will just stay kind of normal, right? It'll stay kind of, you know ..."

But Why Did Homo Erectus Evolve Bigger Butts?

Climate change! That’s what happened about two million years ago. The tree filled jungles were disappearing and being replaced by open grasslands. This was triggered by an ice age that was drying out Africa. These vast open spaces were quickly filling up large grass-eating animals such as the kudu and antelope. Carnivores were rapidly evolving to catch and eat these big food sources such as lions, tigers, and cheetahs.

Compared to these apex predators, Homo erectus was puny and not a good runner. But, Homo erectus could do something they could not do. Homo erectus could sweat! This meant Homo erectus could chase his prey over long distances. He didn’t have to be fast; he simply had to have endurance, pay attention to tracks, and be patience.

Daniel Lieberman explains:

"The trick is you find that animal before it's cooled down, because of course the animal would have run away, and when it runs away it gets hot. Like, when you -- running generates a lot of heat. And these animals aren't very good at dumping heat."
Sweat Fitness | Animation by Genolve

There is a lively, fascinating argument on this episode of RadioLab as to whether Homo erectus tracked and followed its prey to exhaustion or if he simply looked for vultures and other scavengers that an apex predator killed and banded together to scare them away. We don’t know. Probably a little of both. But, the extra protein, fat, and nutrients he got this way helped his brain grow bigger and other evolutionary changes to occur. So, the evolution of a bigger butt and nuchal ligament were pretty important to get to modern human beings.


Man vs Horse

The last half of this episode you just have to listen to… really, you should listen to all of it… I skipped a lot of good stuff. But it is all about a crazy race that takes place in Prescott, AZ every year. It is a high desert long distance race (50 miles) between a group of human runners and a group of horses with riders.

The story goes like this:

HEATHER: So in 1983, a city councilman in Prescott comes into this bar in Whiskey Row, like super-old west America.

MATT: And he gets there, he sits down, and he has a beer. And down at the end of the bar …

HEATHER: There’s a couple of cowboys. The city councilman’s just run a marathon.

MATT: And at some point …

HEATHER: The city council guy says, “I just ran this crazy race.”

MATT: And one of the cowboys says …

HEATHER: “My horse could run that far easily.”

MATT: “You’re not that fast.”

HEATHER: “My horse could do that in an afternoon. Wouldn’t even break a sweat.” And then the city councilman’s like, “You know, I’m not sure he can.”

MATT: “Actually, in fact, I bet I can outrun your horse.”

HEATHER: And for 30-plus years, they have been sort of seeing who’s right.

Matt and Heather follow the racers and it is fantastic, fun story. Who do you think wins? Listen and see!

Man vs Horse | Animation by Genolve | Listen to the blow by blow race as covered by RadioLab here
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