Healthy cultures around the world tend to consume roughly 10 to 20% of calories from protein:
Masai - 19%
Tokelau - 12%
The balance comes from fat and carbohydrate. Ask a traditional Inuit. If there's no fat on your meat, you may as well starve. Literally. "Rabbit starvation" was a term coined by American explorers who quickly realized that living on lean game is somewhere between unhealthy and fatal.
In the early 1900s, anthropologist and explorer Vilhjalmur Stefansson lived for several years among completely isolated Canadian Inuit (Eskimo) who had never seen a white person before. They were literally a stone-age culture, completely uninfluenced by the modern world. They are representative of how some of our paleolithic ancestors would have lived. Here's Stefansson, quoted from My Life With the Eskimo (1913):
In certain places and in certain years, rabbits are an important article of diet, but even when there is an abundance of this animal, the Indians consider themselves starving if they get nothing else, - and fairly enough, as my own party can testify, for any one who is compelled in winter to live for a period of several weeks on lean meat will actually starve, in this sense: that there are lacking from his diet certain necessary elements, notably fat, and it makes no difference how much he eats, he will be hungry at the end of each meal, and eventually he will lose strength or become actually ill. The Eskimo who have provided themselves in summer with bags of seal oil can carry them into a rabbit country and can live on rabbits satisfactorily for months.
This high reliance on animal-based foods coupled with the relatively low carbohydrate content of wild plant foods produces universally characteristic macronutrient composition ratios in which protein is elevated (19- 35% of energy) at the expense of carbohydrates (22- 40% of energy).Later, he states that the most plausible range of fat intakes is 28- 58%. I agree with his assertion that hunter-gatherer diets tended to be relatively high in protein compared with contemporary diets, but I think his protein numbers are a bit high. Why? Because he calculates macronutrient composition based on the whole-carcass fat content of "representative" animals such as deer.
It's clear from the anthropological literature that hunter-gatherers did not go after representative animals. They went after the fattest animals they could find. They knew exactly which animals were fattest in which seasons, which individuals were likely to be fattest within a herd, and which bodyparts were fattest on an individual animal. For example, Stefansson describes how the Inuit relied on (extremely fat) seal in the spring, wolf in the summer, and caribou and bear in the fall and early winter. If necessary, they would discard lean meat in favor of tongue, marrow, internal organs, back fat and other fat-rich bodyparts. This was in order to obtain a minimum of 65% of calories from fat.
Hunter-gatherers would sometimes even provision themselves with enough fat in advance to last a lean season or two. This was true for dozens of tribes along the Northwest coast of North America that relied chiefly on animal foods. Here's another excerpt from My Life With the Eskimo:
...[spring] is the season which the Eskimo give up to the accumulation of blubber for the coming year. Fresh oil is not nearly so palatable or digestible as oil that has been allowed to ferment in a sealskin bag through the summer, and besides that it is difficult often to get seals in the fall... Each family will in the spring be able to lay away from three to seven bags of oil. Such a bag consists of the whole skin of the common seal... This makes a bag which will hold about three hundred pounds of blubber, so that a single family's store of oil for the fall will run from nine hundred to two thousand pounds.That's a lot of oil! Some of it would have been used to light oil lamps, but much of it would have been eaten. I think Cordain's estimate of the protein intake of hunter-gatherers is a bit high due to his underestimating fat intake. His paper shows that if you break historical hunter-gatherer cultures into 10 groups based on their reliance on animal foods, the most numerous group (46 out of 229) obtained 85- 100% of their food from animal sources. In other words, approximately 20% of historical hunter-gatherers were carnivorous or nearly so. If the human protein ceiling is 35% of calories, that means roughly one fifth of hunter-gatherers ate 65% or more of their calories as fat. It also meanscarnivory and high-fat diets are not just anomalies, they are part of the human ecological niche. Zero out of 229 groups obtained less than 16% of calories from animal foods. Vegetarianism is not part of our niche.
Further, although the human body can theoretically tolerate up to 35% protein by calories, even that amount is probably not optimal in the long term. I think that's suggested by the fact that diverse cultures tend to find a source of fat and/or carbohydrate that keeps their protein intake roughly between 10 and 20%. I think it's fine to eat plenty of protein, and there's no need to deliberately restrict it, because your tastes will tell you if you're eating too much. However, "high-protein diet" as a euphemism for low-carbohydrate diet is a misnomer. Low-carbohydrate diets are, and have always been, high-fat diets.