What Made Human Beings the Fat Ape

We may bemoan the human predisposition toward pudginess, but it may also be the key to our specialness

FILE Photo: A chimpanzee eats its lunch using a spoon at Villa Lorena animal refugee center in Cali, Colombia.

People are fatter than any other primate, meaning we have more fat reserves at our basal level. Other primates have less than 9 percent body fat, while the slimmest humans have 14 percent and the bigger ones more than 30 percent. How did that happen? Is it all fried chicken and TV? 

Those don’t help, but we are genetically programmed to put on the pounds and keep them too, scientists have concluded. Now, a new paper from  Duke University, published in Genome Biology and Evolution, has identified the mechanism by which evolution created our relative propensity to store fat — and argues it may be key to our specialness.

Not everything humans evolved into has turned out well for us. Some argue that our very braininess spells our doom, for example. The ability to store fat was surely useful at earlier stages, as long as we subsisted on hunter-gathering. Now, though, over half of all adults in the developed world are defined as overweight or obese, a condition the Organization for Economic Cooperation and Development defines as an “epidemic.” 

“We’re the fat primates,” says Devi Swain-Lenz, a postdoc associate at Duke University. 

And how did this come about? Some time after the Homo line split from the Pan line of chimps 6 to 7 million years ago, our fat storage mechanisms mutated. We got better at storing fat and worse at burning it, say Swain-Lenz and her Duke colleagues.

Apes can be lazy and never get fat. We can’t," the Irish Times wailed in January, reporting on a Scientific American review of how apes got so lucky, which underscored that the sedentary lifestyle is "corrosive" to our health.

However, that ability to store more and burn less fat is possibly key to our very “success” as a species (some would argue that we’re like bacteria in an overcrowded petri dish). While the other great apes don’t move much unless they have to, archaic humans were extremely active.

That could help explain why they wandered out of Africa in the first place. 
Human and chimp DNA is very close. Comparing the genomes, and that of the rhesus macaque, the Duke team sought differences in the genes governing fat metabolism. They found differences in how their DNA is packaged in the adipocytes — i.e., fat cells.

FILE Photo: Chimpanzees in the Taï National Park in the Ivory Coast vocalize with another group nearby.
Liran Samuni / Taï Chimpanzee P

Adipocytes look like fried eggs with outsized yolks. We all have them, and need them. Like all our other cells other than erythrocytes, adipocytes have DNA.

DNA is always condensed into coils and loops, and tightly wound around proteins. Differentiation between cells involves differential expression of the DNA: Some genes are expressed in skin cells, others in kidney cells, others in fat cells. A specific sequence of DNA can be expressed when its packing is loosened, rendering it accessible to the cellular machinery that turns genes on and off, and translates them.

So, the researchers identified roughly 780 DNA regions that were accessible in chimps and macaques but that had become more bunched up in humans.

Awesome. And what do these 780 DNA regions expressed in apes, but not (or less) in us, do? Part of it is involved in converting fat cells from one type to another.

That tire around your waist and those ripples on your tush consist of white fat, storing calories. White fat is the bane of exercisers everywhere. You also have specialized fat cells with so-called beige or brown fat, where calories are burned.

During human evolution, our genes that convert “stored” white fat into “burnable” brown fat were turned down, if not entirely off. Obviously we can still convert some white fat into brown fat — for instance, when we are very cold or by strenuous exercise. But this is how the multibillion dollar industry of futile dieting was born.

The SciAm review suggests that general health in the industrialized world would “vastly improve if we all simply took regular aerobic exercise.” True. But evolution turning down our white-fat converters may be the reason you can read this edifying article. 

Since our split from chimps, our brains grew threefold while theirs did nothing, the Duke scientists point out. We also know that our brains are burning enormous amounts of calories even while we sleep. Storing calories not only for a rainy day but in order to think better may have conferred a survival advantage on our ancestral apes, they postulate. 

That’s all, folks. No diet advice in this. Eat better, exercise more and reflect that while the chimp and gorilla at zoos may be able to lounge all day without gaining an ounce, they’re the ones in cages, not us.