Turns out the model of the primarily meat eating ancestors may be wrong.
https://journals.plos.org/plosone/article?id=10.1371/journal...
Which sort of makes sense when you consider our closest relatives are primarily plant eaters.
https://journals.plos.org/plosone/article?id=10.1371/journal...
Which sort of makes sense when you consider our closest relatives are primarily plant eaters.
Our ancestors have shifted to a diet with a large percentage of animal fat and meat, which has lead to a simplification of our digestive system, for more than a million years.
Only during a little more than the last ten thousand years there has been a reverse shift towards a lower percentage of animal food, most likely after we had exterminated most suitable wild animals, which forced us to become creative and discover agriculture, otherwise we might have become extinct, like our prey.
Now, with our reduced digestive system, we could not live eating the kind of plants eaten by a gorilla or even by a chimpanzee. We need much more nutritious food, which can be provided only by plant seeds, the only parts of plants that are rich in proteins.
What they're actually doing is measuring the isotopes in the carbon of the bodies of the human remains. You don't get any carbon in your body from the water or from the fiber in plants.
So your estimate correction needs to go the other way - the study says that > 80% of carbon in the bodies of early human foragers comes from plants. That means that also > 80% of the caloric intake comes from plants. Or alternatively, it would mean that > 95% of what they ate (by weight) was plants.