I'm going to make the claim that eating meat, and specifically DHA-rich meat—fish and most importantly: the brains of land animals—is core to who and what we are as human beings. And because we have mostly stopped doing this in the amounts that are necessary, we are now, long-term, totally screwed.

Or, in other words: before civilization there was meat, and once civilization came, enough-meat ended. Since then, the biological machinery that made civilization possible in the first place—our absurdly large brains—has been slowly eroding, while cultural and social technologies have allowed civilization to keep running on inherited hardware, it's on a timer.

Simply put. We scaled our brains by eating brains

We have big brains.

Really big brains. The adult human brain is only a few percent of body mass, but it burns roughly a fifth of our resting energy. At the same time, compared with other primates, we have relatively small digestive systems. We have, comfortably, the largest brain-to-gut ratio among primates.

You do not get to massively increase the metabolic cost of one organ without paying for it somewhere else. If you want a brain that consumes an insane amount of energy without increasing total basal metabolism by the same amount, something else has to become cheaper. In our case that something was the gut. But a smaller gut creates another problem: you can no longer live as efficiently on enormous quantities of bulky, difficult-to-digest food. So you need better food. Food that gives you large amounts of energy and nutrients without requiring a gigantic fermentation chamber to extract them. So like, animal flesh.

We really, really liked animal tissue.

How much?

Nitrogen isotopes give us a rough way of reconstructing trophic position. As you move upward through a food chain, the heavier isotope nitrogen-15 becomes progressively enriched. Plants sit low. Herbivores eating those plants sit higher. Carnivores eating those herbivores sit higher still.

So if you take human bone collagen and compare it with the bones of animals from the same ecosystem, you can get a rough idea of where the humans were eating in the food chain. Neanderthals repeatedly show nitrogen-isotope values as high as, and sometimes higher than, the large carnivores living beside them. Analyses from places like Vindija, Marillac and Scladina put them right at the top of their ecosystems, with most of the protein represented in their collagen apparently coming from animal sources.

And it was not just Neanderthals. Upper Paleolithic Homo sapiens in Europe show the same basic pattern: very high trophic-level signatures, consistent with humans eating enormous amounts of animal food. Some mid-Upper Paleolithic modern humans had even higher trophic values because substantial amounts of their protein came from freshwater animals.

So for a significant part of our evolutionary history, we were the apex predator, and we ate like the apex predator.

Then civilization happened and civilization discovered one of the greatest ecological hacks in human history: stop eating the herbivore and eat the herbivore's food directly: wheat, rice, maize, tubers.

This is, from the perspective of producing as many humans as possible from a fixed amount of land absolute genius because every trophic step wastes energy. Plants capture sunlight. Herbivores eat plants and burn most of that energy staying alive. Humans eat the herbivores and lose another enormous fraction along the way.

And as a result: Our brains have been shrinking.

We took an animal that had spent hundreds of thousands of years occupying an unusually high trophic position, with an unusually large brain, an unusually small gut and access to extremely nutrient-dense animal tissues, and we reorganized its entire food system around cheap plant calories. In this way our collective intelligence can rise while the biological hardware running it deteriorates.

Before civilization there was meat.

Civilization replaced meat with calories and culture. But the computational unit it runs on is now steadily deteriorating. If we keep eating just like any other primate, we will eventually (tens of thousands of years I guess) go back to having the brain size of any other primate.