Most carnivores are technically omnivorous, as are most herbivores. Lions occasionally eat grass. Deer occasionally eat carrion. Many plant-eaters also eat some insects.

However most omnivores heavily specialize in some area(s) of food availability, with areas of specialization differentiating between insects, verterbrate meat, fruit, leaves/grasses, tubers, seeds, gums, and fungus.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4100522/

As to human evolution in particular, there are a variety of lines of evidence supporting human carnivory such as stomach pH (and many other modern anatomical features), nitrogen isotope evidence of paleolithic trophic level, and direct artefacts such as bone with butcher marks and cave paintings of animal hunts.