Carrots. Or French Fries... How are you feeling about this choice - Be honest. Which would you rather eat? That's what I thought. 

Humans love fats and carbohydrates. Why? Well, because they have a lot of energy in them and energy's what we need to do all of the stuff that we humans do. But, we're omnivores. We can get our energy from meat, grains, vegetables, nuts, tubers, fruit - we're extremely versatile animals, compared to like a cow, which is pretty much stuck eating grass, or a lion which has no business eating anything other than raw meat. 

So if we can get calories from a bunch of different kinds of food, why, at a very basic level, do we really just wanna eat fats and carbs? Well, it has a lot to do, apparently, with our evolutionary history. Let's start with a member of my extended family, grandma Australopithecus. This pretty lady was a direct ancestor of the first member of the genus homo.  She was different from us in a lot of ways: her brain was 35% the size of ours, her stomach was a lot bigger, and her teeth were larger, flatter, and duller. All of this leads us to the conclusion that Australopithecus ate mostly fruits, vegetables, and tubers. But raw fruits and vegetables, especially before the agricultural revolution, weren't very high in calories, and if that's all you're eating, it takes a big, patient digestive system to process that. 

So if your body's working really hard all day to squeeze every last calorie out of your not-very-energy-rich-food, something's gotta give, and for Australopithecus, it meant having, ya know, kind of a tiny brain, because brains require lots of calories. 

It wasn't until Australopithecus' direct descendant homo habilis came along two million years ago (about the time Australopithecus went extinct actually) that we start seeing smaller, more efficient guts, smaller teeth, and brain about 50% bigger than Australopithecus. These clues, coupled with the fact that this is around when habilis began to control fire, suggests that they were able to cook. And that means they were suddenly able to eat meat without worrying about dying of nasty festering meat disease, and they were suddenly able to unlock all sorts of hidden delicious energy from the vegetables that are pretty stingy when they're raw. 

So since then every species in the genus homo from habilis to homo sapiens have had larger and larger brains, thanks in part to increasingly high-energy diets. These huge brains of ours are expensive, and we've needed a bunch of fat and calories for them to evolve, not to mention just to maintain them. 

So because our brains are not stupid, they've evolved to love the crap out of high-energy food. And now look at us, we can eat a donut-hamburger whenever we want to because we're the kings of the freakin' earth. And our brains, they're like, "yeah!" In fact, some studies have revealed that eating high-fat food not only activates our brain's pleasure centers, it also activates the touch centers. So our brains respond to eating fatty, high-calorie foods the same way we respond to hugs from our moms, literally comfort food. 

But even though our brains are totally pumped that grocery stores are now packed full of sweet, sweet snickers bars, the rest of our bodies are like, "I'm so tired of dealing with all of this nougat, help us!"Which is why, even though the carrot doesn't look as good, you should eat it. Your body will thank you. 

Thank you for watching this episode of SciShow, if you have any questions or ideas or comments or thoughts for us, please leave them down below, or on Facebook or twitter and if you want to continue getting smarter with us, go to youtube.com/scishow and subscribe. This episode brought to you by nutritious things... not by fries... just to be clear.