By Studying Skinny Apes, Science Figured Out Why Humans Are So Fat

It involves our evolutionary ancestors learning to sweat and having weak arms.

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Jun 5 2015, 3:00pm

Photo by Flickr user Hrishikesh Premkumar

A new study published in the Proceedings of the National Academy of Sciences explains how humans became, as Science magazine puts it, "the fat primate." Over the course of about 30 years, two scientists studied the composition of apes and humans, a feat that required 49 dead humans—which are easy to get—and 13 dead bonobos—which are almost impossible to get.

The idea behind the research was to come to a plausible hypothesis about how humans became human. What few fossils we have can tell us a little about what's sometimes called "chimp-human divergence" in evolutionary history, but fossils lack something very helpful in understanding biology: flesh.

It occurred to Dr. Adrienne Zihlman, anthropologist at University of California Santa Cruz, that non-extinct apes do have flesh, so she cut a whole lot of the dead ones up in order to figure out the differences between them and us. From there she was able to make some educated guesses about what happened in that unknown period.

One of those differences is that humans got fat. Chimps and bonobos are 13 percent skin, and we're only 6 percent skin, but we compensate for that by being up to 36 percent body fat on the high end of average, while bonobos average 4 percent. That's a wildly disproportional fatness differential.

In the paper, Zihlman and her partner, Debra Bolter hypothesize that our ancestors wandered around diverse ecosystems, sometimes in intense heat while looking for food. We needed fat for lean times, while chimps just hung out in forests. I called Dr. Zihlman to find out more, and just to hear about dissecting 13 dead bonobos.

VICE: Hi, Dr. Zihlman. How'd you get all the dead bonobos?
Dr. Zihlman: That's a long story.

I have time.
I got the first one through Sue Savage-Rumbaugh in 1975. She was interested in the behavior, and I was interested in the anatomy. Eventually I started collecting them. Three of our best animals came from [the Milwaukee Zoo] a year ago January. Three adults, beautifully preserved. One of them had died more than two years ago. They were kinda saving them for us.

Were they preserved in formaldehyde, or frozen?
Frozen. At my lab, we don't do pickled animals. You get a truer measure of the tissues [from frozen animals].

How do you study their body mass?
So we started just seeing what we could find, taking them apart, doing a "quantitative anatomy."

What's that?
You take the last body weight in life. Then you take the animal apart and you weigh everything. What it gives us is a real database for looking at the evolution of the rest of the body.

What did taking apart apes teach you?
This gives us kind of a representative possibility for something that could have been like our common ancestor. Before the common ancestor to ourselves and apes.

So what happened on the path from common ancestor to Homo sapiens?
One of the things is, you've gotta shift the body around and change the muscle from the forelimbs if you're a quadrupedal ape. Our ancestors—and most apes—can venture into open areas, but they live in forests. They're really tied to having tree cover available, because they get hot.

So we developed fat so we could get away from forests?
Compared to the apes, we have less muscle, which is an energy savings, because it's such an expensive tissue. Two important things about the way we store fat: We store it around our buttocks and thighs, but you want to make sure that you're storing fat so it doesn't interfere with locomotion. You don't want it on your feet, for instance. So you concentrate it around the center of gravity. And you also don't want it to interfere with being able to get rid of heat.

What was the benefit of having fat down low and weak arms?
If you're moving away from the forest and tree cover, you want to be able to exploit food in a more mosaic habitat that has areas of bush and a few forests around rivers. You want to be able to move into a lot of different areas. So you've gotta get rid of your hair, and really ramp up those sweat glands. Our skin has really been reorganized for a lot of different functions.

Do chimps and bonobos not have sweat glands?
They have sweat glands. They're not really functioning. All primates have eccrine sweat glands in their hands and feet. Monkeys have them on their chests. [But] they're not stimulated by heat.

What else happened to human fat?
What happens in humans is, without hair cover, you can really see the form of the body more. You can really see more prominently the breasts, the buttocks, and so forth.

That's different from chimps?
When chimpanzees and gorillas are nursing, they develop fat deposits in their breasts. Usually when they're no longer lactating that fat disappears.

I have to point this out: some apes are super fat.
Orangutans have much more of a problem in captivity. What they do is, in the wild, it's like feast or famine. When there's a lot of food they put on a lot of body fat. What happens in captivity is that they're really prone to putting on fat, and they can get diabetes.

A fat orangutan face, via Flickr user Jim Bowen

The other thing you notice, if you're looking at male orangutans, is how much fat they carry in their cheek pads. We weighed those. That's several pounds of fat on your face.

But chimps and bonobos don't normally carry fat like orangutans?
Males don't have much fat at all. They are solid muscle.

So if you had a starvation contest, between a male chimp and a human, the human would last longer?
It's a solid working hypothesis.

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