So looking it up, all I can find is that the brain imposes relatively high caloric needs with respect to other organs, but not that those calories need to come from carbohydrates. Do you have any sources on the latter claim? The last time I got exposed to this claim was when a doctor (not a nutrition expert) told me that we must eat fruit for breakfast because our brains need the sugar. To me, both anthropology and personal experience seem to contradict this idea. Maybe I can change my mind.

We need sugar to power our brains, but we need protein and fat to build our brains. That requires more than incidental amounts of protein.

For that to be a good counter-argument, you would need to demonstrate that vegetable protein was “merely incidental” in the case you are arguing against.

Vegetable protein was and still is less abundant than vegetable sugars, if I'm not mistaken? Agriculture eventually made that less of an issue but this was not the case until not that long ago (tens of thousands rather than hundreds of thousands of years). It's easier to eat a bug or any small prey than spend a morning gathering spinach and watercress. Humans also ate leftovers from predator kills, like the partially digested contents of herbivore stomachs that had been killed by let's say wolves.

Vegetable protein was part of our diet but not by any means the main source of protein. Humans did not eat pulses until quite recently (10-15 thousand years ago?)

> Vegetable protein was and still is less abundant than vegetable sugars, if I'm not mistaken?

I'm no expert here, but humans do tend to need far more carbs than protein to be healthy - or at least can operate on something like 10% protein per joule of food consumed. And that's with a decent safety buffer. I think the predictions (based on natural experiments - i.e. famines) were that you could push as low as 4% before a human body can't handle it. Though I'd guess that if you get anywhere close to that your body is going to start making some significant trade-offs.

Also: Don't forget about nuts and seeds.

> Vegetable protein was part of our diet but not by any means the main source of protein.

I think a citation (or three) is needed here.

People can be perfectly healthy on a wide range of carbohydrate intake. Native peoples in the Arctic lived almost entirely on animals, for example, whereas there were tropical island societies that lived almost entirely on plant sources.

> People can be perfectly healthy on a wide range of carbohydrate intake

Again, citation needed there. The recommendation of all major national and international dietetics associations (based on currently available peer-reviewed studies) is that a healthy diet should consist mostly of carbohydrates.

There certainly are "extreme" diets, but these seem to require at least a bit of genetic adaptation:

> "a new study[0] on Inuit in Greenland suggests that Arctic peoples evolved certain genetic adaptations that allow them to consume much higher amounts of fat than most other people around the world"

https://www.npr.org/sections/thesalt/2015/09/17/441169188/th...

[0] https://science.sciencemag.org/content/349/6254/1343

No, merely that we can't get enough fat and protein to build brains from fruit.

You can definitely do that today.

I have no idea if our pre-human ancestors could or couldn’t do that in the rift valley while they were inventing cooking without fire — I don’t even know if it is possible to find out now if they could have done it then — but you can do it today.