I think of humans as facultative carnivores rather than omnivores, as there are nutrients (B12 being most well known) that either cannot be obtained at all from plants or not in very absorbable or directly useful form, such as DHA and EPA omega 3's (ALA from plants not being very readily convertible to the former two) or real vitamin A, which plant beta carotene doesn't easily convert into, and so forth. So in nature (absence of manufactured supplements) humans must consume animal foods to survive and thrive. If I understand the technical definition of omnivore correctly it refers to an animal that can eat plants or animals, doesn't matter which or either, it's all good, whereas the facultative carnivore must have some animal foods but in addition can also eat some plants as well. Since coming to that realization I've put a lot more emphasis on animal and less on plant foods in my personal diet and seem to be doing quite well with it.