Very few apes actually have a plant based diet. Most apes are opportunistic omnivores: the majority of what they eat is plants, but they will eat meat if they can catch it.

Fossilized dental evidence indicates that omnivorism is a conserved trait, stretching back to the proto-monkey ancestors of apes and humans. Australopithecus dentition indicates they ate plants but could also eat meat, as does early Homo specimens.

There was an Australopithecus branch that specilized in a plant only diet, as indicated by their very heavy molars, suited for grinding tough grasses. They went extinct long before anything that could be considered human came along. If you saw one you would think tall chimpanzee instead of odd human. They lived alongside* the branch we are descended from, and it appears that they overspecilized and were not able to survive changing environmental conditions.

Early human finds (H. habilis*, erectus, neanderthalis, and sapien) all have evidence of meat eating, shown via dentation, tools, butchered bones, and cave art.

In summary, humans have been eating meat for longer than we have been human. There are cultures who eat more/less meat, and there are implied questions about quality of life and how diet impacts it, and comparing western, high meat diets to historical diets, but I'll let an expert in those fields address those questions.

*Alongside is an iffy term here. Fossil specimens are rare, and scattered, and it isn't clear if the various branches of Australopithecus shared habitats.

**Early human terminology and the dividing lines between species is a constant argument in the field. There is a tendency to say that this fossil, found by my team/my university/in my country, is the oldest human, and everything older isn't human.