humans wouldn't be what they are if they didn't start eating animal protein at some stage - it's how our brain size developed. however, it's not _animal_ protein per se that enabled this - it was the extra calories. there are theories that it was seafood, in particular shellfish that kickstarted human evolution.
i went vego 10 years ago, when i was 19 (and still likely growing) with disasterous consequences. then i ate meat again for the last 2 years, before realising that i couldn't do it for the ethical reasons alone. so now i eat lots of organic eggs and fish.

pure vegans that eat mainly things like lentils, beans and grains are not doing themselves any favours with all the anti-nutrients in those things, and we didn't evolve to eat them.