Well, no, we aren't carnivores. We're omnivores.
Well okay, my intention is not to split hairs. Humans were never
obligate carnivores like lions, eagles, seals, sharks, alligators and sperm whales, unable to extract nutrients from plant tissue. But we're
omnivores only to the extent that bears, crows and coyotes are omnivores: we can extract the calories from the sugar and starch in fruits, nuts and seeds, but we
cannot extract the calories from raw cellulose, the most abundant plant tissue, because we don't have the enzymes or acids to break it down. And we
cannot use plant tissue as a source of protein because we do not have the long intestine loaded with bacterial culture to convert starch into amino acids.
Some omnivores get their protein from animal tissue by scavenging the leftovers in the kill of other predators; dogs, jackals, vultures, hyenas and raccoons make a lifestyle of it. Others get it by spending a lot of time digging up insects, licking up grubs and maggots, and various other unsavory activites; bears do a lot of that.
But humans don't have the constitution for that. Dogs have such a highly acidic digestive tract that their saliva was used as an antibiotic by a few primitive tribes. They can thrive on bugs, the garbage the bugs were eating, and three-day old road-kill. We can't.
Humans have to get their
protein from meat--or at least we did until around 8000BCE when we finally had access to the amino acids in cooked, cultivated grains.
As for sheer
calories, well sure, when fruit is in season and the size of your tribe is pretty small, you can get a lot of your calories from wild-growing fruit. But the rest of the year there isn't an adequate natural source of digestible sugar and starch, so you have to get a significant portion of your calories by eating the meat of the herbivores whose bacterial cultures have conveniently turned the cellulose in grass and leaves into protein.
I feel like this would be relatively easy to resolve with data from the calorie sources of modern stone age tribes. I haven't found any with a preliminary google search.
Modern Stone Age tribes are a little too "modern." They have the technology of fire so they can cook roots and tubers, turning them into nice sources of digestible starch. They also raid the farms on the outskirts of their territory so they can add cooked grains to the vegetarian portion of their diet. Cooked grains combined with nuts or seeds provide a perfect balance of amino acids and are the reason today's vegans can survive on their milkless, eggless and meatless diet long enough to reach the age of reproduction. (Legumes like soybeans
don't do it; beans are seeds and have a similar amino acid profile.)