Early humans derived most of their protein from plants. We were stunted in stature and not particularly successful as a species until we developed the social skills to hunt in groups to take down large game. This ability and the high quality protein diet that it allowed is arguably what saved our species.
Generally speaking, most mass starvation events are the result of a missing essential amino acid (protein) in the diet. You can have all the carbs and protein you can eat but if you're missing even a single essential amino acid in your diet then it will eventually kill you. One of the chief benefits of an animal based protein is that it contains these essential amino acids in abundance. Not necessarily true with a plant based diet. There's a reason herbivores spend every waking moment eating. They have to in order to derive enough nutrition to survive.
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