Most of our Evolution we ate mostly plant based diet, and meat was actually not always available. There are paleontologic studies made about the subject, which show that people ate mostly high starch foods. And this is really long time in the timeline of our evolution. This means that we are not able to lean on only hunter-gatherer era of our evolution, which really is not the majority of our metabolic and physiological developmental stretched through timeline. Meat is not really needed. Even our body produces DHA and EPA from ALA in sufficient matter for brains to be highly developed. We can't even compare ourselves to hunter-gatherers in that respect, because they were masters of their environment. This meas that they used hundreds of diffent plants for medicine, nutrition etc. We have always thought that they lived in scarcity, but in many areas this is not true. We also know the effect of our environment on eating habits. Meat is not always available and neither are plants. If its winter or you live on areas like northpole, your main source of nutrition will be fatty fish and meat. There was study made about eskimo, where they found that even when they ate lot of seal meat, they were able to survive, but they died younger, and cardiovascular system was thoroughly calcified.
Good books about these subject to read:
- Dr. Michael Greger - How not to die. (going through current scientific journals about Nutritional science.)
- Dr. Michael Greger - How not to diet. (same as before.)
- Sapiens - Yuval Noah Harari.
There is TED talk about paleontology study if I remember correct.