Evolution is a process of survival after mutation.
Hang on ‘cus this is going to get deep. It’s magical that I am alive. In order for me to exist every one of my ancestors had to survive to sexual maturity. If you think about it my family tree, and yours, runs all the way back to the first single celled creature. If it didn’t we couldn’t exist.
The mathematical probability of existing is so astronomical that it is incredulous, though we plainly do exist. Evolution works by a specific mutation increasing the likelihood of surviving to adulthood. This mutated gene is then passed on to offspring.
Man evolved a large brain because each mutation in brain size helped increase his odds of survival, What he ate was important because malnutrition effects the brain growth. In order to sustain a large working brain you need a high energy food source. The only practical way for mankind to maintain a large brain ‘mutation’ was to eat meat to attain adulthood and thus pass on his mutation.
Chimpanzees survive because they eat meat, insects, etc. They are in effect viable, but prototype humans (this is not meant in a derogatory way). Who knows if our specific genetic line is wiped out from eating Big Macs and food additives, maybe they will evolve into a better race.
The ratio of brain weight to body weight for fish is 1:5000; for reptiles it is about 1:1500; for birds, 1:220; for most mammals, 1:180, and for humans, 1:50.
Roughly 2% of our body weight is our brain. That's a serious overhead for a herbivore.