Your premise is wrong, our diet has never consisted pretty much only of meat. We were hunter-gatherers, gatherer being a very important part of that phrase.

The diet of the earliest hominins was probably somewhat similar to the diet of modern chimpanzees: omnivorous, including large quantities of fruit, leaves, flowers, bark, insects and meat.

https://www.nature.com/scitable/knowledge/library/evidence-for-meat-eating-by-early-humans-103874273/