> And eating meat's fine (from a health perspective, if not ethical necessarily). We've been eating it for a couple of millions of years.

The current theory is that we developed sweating and bipedal walking to be endurance hunters, i.e. running after a prey animal until it is too exhausted to run any further. Tool use and development were also driven in part by hunting (spears, atlatl, bow and arrow, skinning knives etc.). Overall humans are omnivores, as evidenced by tooth layout. Some populations still manage to live on primarily meat-based diets (Inuit), albeit with some difficulties.

The main issue is the amount of meat: Hunting was difficult and dangerous, limiting how much meat we got. Being able to eat meat every day for almost every meal is a recent development and is a major difference to prehistorical diets. Plus, factory-farmed meat is significantly different from free-range animals w.r.t fat, hormone and antibiotics content.