There actually is some truth to this.

Of the millions of animal species on earth (granted that counts microscopic organisms), every last one provides a complete protein to humans--aardvarks to zebra sharks.

Of the 391,000 plant species? Well, unless they wanted to eat garbage bags full of vegetation every day, those cave men had better have lived near soy, lentils or maybe peas. Or combined plants (rice and beans) when agriculture came along around 10k years ago.

Grab your spears and fishing rods. We're not meant to be vegan.

Many animals are able to manufacture all the amino acids they need without outside sources (we can't), so they can live on vegetation (and can also digest cellulose, which is the basic building block of plants--we can't digest it, that's why grass has no calories for a human but can support a horse). As for carnivores . . . Sure, we're food and many would eat us if they could.

We are omnivores -- that's why we get scurvy if we don't include vegetation (or fish eyes, it turns out).

Funny story -- when the Vikings first landed in Canada, they got into a beef with the local Indians. An author I read believes it's because the Vikings offered them cheese and they got sick!