Junk food aisle
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Q: Why do we eat this junk?
A: Because we're supposed to be eating animal fat, but we won't let ourselves!
It is an indisputable fact that humans crave fat.

"Why Can't I Stop Eating Fatty Foods?"

French fries, onion rings, donuts, and everything else that comes out of a deep-fryer. Corn chips, potato chips, Cheetos, Fritos, Doritos, Tostitos, and all the other oil-soaked, salt-coated starches in the snack aisle. Oreos, buttered toast, salad dressing. Cheese, mayonnaise, and Alfredo sauce. The list goes on, and on.

Decades of diet propaganda, telling us over and over again that fat will kill us, have been unable to break us of our 'fat tooth'. Why do we crave fat so much?

It's because animal fat is the primary constituent of the evolutionary human diet. "Low-fat" diets just make us crave fat more keenly - and anti-animal-fat propaganda makes us binge on unsatisfying substitutes.

Fruit Isn't Enough: Leaving The Equatorial Forests

Humans are (mostly) fruit-eating chimpanzees who have become meat-eating, predatory omnivores, most likely due to the pressures of massive and continual climate change throughout the Pleistocene. Our continually shifting environment strongly selected for physical adaptations and behavior that let us survive outside the equatorial tropical forests of Africa.

How did this happen?

Well, first we had to adapt to eating something besides fruit, because fruit is only available year-round in tropical forests. We needed to eat something available year-round on the savanna and plains, in wet and dry seasons, in cold and warm seasons.

We needed to eat meat.

Fortunately we had a head-start: chimpanzees already eat meat.
The Predatory Behavior and Ecology of Wild Chimpanzees, by Dr. Craig B. Stanford

"I estimate that in some years, the 45 chimpanzees of the main study community at Gombe kill and consume more than 1500 pounds of prey animals of all species. [...] In fact, during the peak dry season months, the estimated per capita meat intake is about 65 grams of meat per day for each adult chimpanzee. This approaches the meat intake by the members of some human foraging societies in the lean months of the year. Chimpanzee dietary strategies may thus approximate those of human hunter-gatherers to a greater degree than we had imagined."
"When we ask the question 'When did meat become an important part of the human diet?'," we must therefore look well before the evolutionary split between apes and humans in our own family tree."

(Further reading: Dr. Stanford's magisterial Meat-Eating And Human Evolution.)

Kleiber's Law and the Expensive-Tissue Hypothesis

Kleiber's Law states that all animals of similar body mass have similar metabolic rates, and that this rate scales at only the 3/4 power of size:

Kleiber's law meat eating paleo
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Click here for an informative discussion of Kleiber's Law.

What this means is that to spend more energy to grow and maintain one body part, an animal has to spend less energy on another. And what this means for human evolution is that in order for our brains to grow, something else had to shrink.
Brains are expensive to own and maintain. At rest, our brains use roughly 20% of the energy required by our entire body!
So what did we lose in order to gain our big, smart brains?

Our guts.

It takes a much larger gut, and much more energy, to digest plant matter and turn it into an animal than it does to eat an animal and turn it into an animal. This is why herbivores have large, complicated guts with extra chambers (e.g. the rumen and abomasum), and carnivores have smaller, shorter, less complicated guts.

The caloric and nutritional density of meat allowed our mostly-frugivorous guts to shrink so that our brains could expand - and our larger brains allowed us to become better at hunting, scavenging, and making tools to help us hunt and scavenge. This positive feedback loop allowed our brains to grow from perhaps 350cc ("Lucy") to over 1500cc (late Pleistocene hunters)!

In further support of this theory, the brains of modern humans, eating a grain-based agricultural diet, have shrunk by 10% or more as compared to late Pleistocene hunters and fishers.
For a longer explanation, read this seminal paper:

The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution
Leslie C. Aiello and Peter Wheeler
Current Anthropology Vol. 36, No. 2 (Apr., 1995), pp. 199-221
Most importantly, fruit is only available year-round in tropical forests, and even then the supply ebbs and flows seasonally. Meat, in contrast, is available everywhere year-round. If we hadn't become meat-eaters, we'd still be living in tropical forests with the rest of the chimps and bonobos.
You can demonstrate the necessity of meat and root starches by looking at the calorie density of vegetables: an average asparagus spear has four calories. You'd need to eat 500 asparagus spears just to survive a relatively sedentary day...and even if you could somehow choke them down, you'd have to eat one every two minutes!

That doesn't leave much time for anything else...and it's why herbivores graze constantly. Even with a ruminant's stomach, there's just not very much energy in grasses and foliage.

The few calories in most vegetables are rounding error to whatever you sautee them in, and the calories in salad greens all come from the dressing you put on them. In other words, when you eat 'vegetables', you're really eating fat - plus a lot of indigestible fiber and perhaps some nutrients.
Why You Crave Fat: The Protein Problem

Animal flesh contains protein and fat, but no significant amount of carbohydrates (sugars). Most animal tissues can oxidize either sugar or fat for energy, and ketones can replace some of our need for glucose - but brains, red blood cells, and some kidney cells absolutely require glucose. Therefore, all animal bodies, including ours, need to maintain a certain level of glucose in the bloodstream ("blood sugar") or cells start dying, starting with the brain.
Just as 'carbohydrates' are just chains of simple sugars, 'protein' is just chains of amino acids.
Furthermore, unlike fat and carbohydrate, there is no way to store excess dietary protein: it must be used immediately, or converted to something else. So when an animal ingests protein in excess of its need to repair and grow its body, it must convert the protein into glucose. Humans do this primarily in the liver, by a process known as gluconeogenesis.

It turns out that the liver of an obligate carnivore, like a lion, wolf, tiger, or hyena, is great at gluconeogenesis. A 130# spotted hyena can eat nearly a third of its body weight at one sitting...and over the next several days, convert all the glucose it needs from that 40 pounds of meat.

Human livers, however, aren't quite as good at gluconeogenesis. Sources aren't consistent...but they seem to indicate that we can only metabolize somewhere between 200 and 300 grams of protein per day. Furthermore, some of that is used directly for cellular growth and repair, and isn't available for energy.

Unfortunately, 250g of protein is only 1000 calories! That's not nearly enough to sustain a sedentary adult, let alone an active hunter. People who eat too much lean protein and not enough fat end up in a situation called "rabbit starvation" or "mal de caribou".

Therefore, in order to survive on hunted meat, Paleolithic humans had to get the rest of their calories from something besides protein. Dead animals don't contain significant amounts of carbohydrate...

...which leaves us with fat.

Simple math tells us that a sedentary adult surviving on hunted meat would require half their calories from fat, and an active hunter would require 3/4 or more of their calories from fat!

And that's why humans crave fat -

- because we require a meat-based diet in order to feed our big brains, but our livers haven't yet caught up.

Humans aren't mostly frugivores, like chimps, true carnivores, like lions and hyenas, or true omnivores, like pigs.

We're fativores.
Unlike the canids and felids who have been carnivores for perhaps 40 million years, our evolutionary transition from mostly-frugivores to mostly-carnivores is both recent and incomplete. It began perhaps 2.6 million years ago, and it's been interrupted by the transition to a Neolithic lifestyle based on farming and eating grains - a transition that is shrinking our brains and stunting our growth. (An argument neatly summarized here, by Jared Diamond.)

Further Reading: "Evidence of Human Adaptation To Increased Carnivory", and Peter Dobromylskyj's Hyperlipid.
Live in freedom, live in beauty.

JS

Postscript: You'll notice that you stop craving fatty junk food once you start eating a high-fat paleo diet and stop eating birdseed, "low-fat" milk and yogurt, and boneless/skinless/tasteless chicken breasts...but that's another article for another time.