[♪ INTRO] Even though we share more than 98% of our DNA with chimpanzees and bonobos, we still,  ya know, look pretty different.

There’s the obvious stuff: our hands and  feet, the shape of our faces, the body hair. But have you ever noticed  like how jacked a chimp is?

It can’t be just me noticing this. Well, it turns out, the difference in our  muscle mass is only half of the story. Humans and our closest great ape  relatives also store fat differently.

And that’s probably good  news for us in the long term. Now, the average bonobo is definitely  stronger than the average human, especially in their upper body, where they have  much more muscle mass than we do. But studies have shown that  their physical superiority over humans with similarly sized  muscles is actually pretty modest.

When comparing the actual force and velocity  of a chimpanzee’s muscle fibers to a human’s, scientists observed that the  chimp’s muscles were a little less than one and a half times stronger than the human’s. So even though chimps are  definitely stronger than us, they’re not like “Incredible  Hulk” strong by comparison. But if chimps don't actually  possess superhero strength, why do they look so much brawnier?

Well, it turns out, compared  to our primate cousins, we keep a little more body fat around. Now you might think that  it makes sense that humans store more fat, just based on our lifestyles. But it actually is hard to  compare humans and apes, because human cultures have a huge  range of nutritional consumption.

Meanwhile, zoo apes have different  diets than wild apes, though the diet of zoo animals is carefully formulated  to make sure it meets their needs. Still, one study has compared human  body fat to our ape counterparts. Researchers performed necropsies, which  is the veterinary term for autopsies of non-human animals, on 13 bonobos  who lived in zoos and research institutions, and died of natural causes.

Despite living in captivity, their  body fat percentage was very low compared to their total weight. Male and female bonobos had,  on average, less body fat than average adult humans of any gender. Bonobos clocked in from less than one  to a bit over eight percent body fat.

For humans, a healthy range can be anywhere from around fourteen to thirty percent, on average. Now, these apes were sedentary,  but they were also on that specialized diet we talked about. So while it does give us  some information about how much body fat they stored,  it’s not the whole picture.

For that, we can turn to something much deeper: the way our DNA is packaged. You see, mammals actually have a  couple of different kinds of fat cells. The ones we’re going to talk about  here are white adipose tissue, or white fat, and brown  adipose tissue, or brown fat.

Both white and brown fat cells  function by storing fat, but it’s much easier for brown fat cells to  tap into fat as an energy source. This is because they have a lot more  mitochondria than white fat cells, and mitochondria are the cellular  structure responsible for converting things like fat into chemical energy. So, the more white fat cells an animal has, the greater their ability to store fat,  and the more brown fat cells it has, the more rapidly it can turn  that fat into a fuel source.

Now the cool thing is, white fat  cells can actually become brown-ish, turning into a type of cell that is  literally called beige fat cells. That makes it easier for the animal to  convert all this stored fat into energy. And the instructions for how to  change from a white fat cell into a beige fat cell is stored in our DNA.

But there is a catch. There’s so much DNA inside your cells that  there’s no way it could exist as these long, unruly strands. It has to be folded just  right to fit inside the nucleus of the cell.

The long strands of DNA are wrapped and  folded around proteins called histones. Scientists call this complex of DNA  folded up with proteins chromatin. Exactly how tightly packed  a particular stretch of DNA is affects how easily the cell can get to it.

Densely packed regions mean  that bit of genetic code is hard for the cell to get to and use. While  more loosely packed areas are more active. And according to a 2019 study,  it’s the overall accessibility of the chromatin in our fat cells  that may be at least partially behind the difference in chimp  and human body fat percentage.

In chimps, the region of DNA that holds the  instructions for white fat cells to convert to brown fat is pretty easy for the  cellular machinery that reads DNA to get to. But in human fat cells, the  chromatin is folded and packaged in such a way that these  instructions are kind of buried. The end result is that humans  end up with more white fat cells than chimps, thereby storing way more fat.

And that fat, turns out, is good news for humans. Some scientists hypothesize  that our enhanced ability to store fat might be one of the factors that  allowed us to develop such large brains. Gram for gram, our brains require more  energy than almost any other tissue.

So it’s easy to imagine how our  ancestors, roughing it out on the savanna, had a survival advantage with  a little extra fat around to feed those energy-hungry thinking machines. Which means that we may have  our enhanced fat storage to thank for our big, beautiful  brains. And that’s pretty neat.

Thank you for watching this episode of SciShow, and thank you to our patrons  for helping to make it possible. If you would like to be one of  those people that I just thanked, you can check it out at patreon.com/scishow. There’s a bunch of cool stuff you can get there.

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