Ahh so the progressive wind also blows over you :). Another MD with an opinion, wether you are vegan, vegitation, pro or anti seed, carnivore. There is always an MD who knows and fully understands everything.
Kind of strange you are going all the way back to primates. The last 200.000 years we've evolved to eat 60-90% of our kcals diet of animal based food.
The last 10.000 - 15.000 years we moved more to agriculture, mainly grains and rice. Which is not that long but allowed us to be better adapted to grains and other plants.
Overall the body seems to be very good at processing different kinds of food. From meat, dairy, eggs to plants. In these debates all sides invest too much in their moral positiion and have to resort to very complicated logic that doesn't have any backing in research but makes sense for them with a few graphs and 1 or 2 principles that they think define are the base of human health.
Meat & dairy come with its issues, but so do plants. They contain lots of antrinutrients blocking nutrient intake (which is main reason why iron is spinach is not taken up well). And plants are much less nutrient dense and are harder to digest. It's the vegans & vegetarians that need to supplement, which says something.