[jjtheblunt]

It's primarily about what livers produce, which determines which chemicals are vitamins (which varies across mammals, for example) and which amino acids (the lego blocks that form proteins) are "essential amino acids" for a species.

It's possible as a human to ingest from non animal sources all the essential nutrients, but there's HUGE misdirection in at least american commerce about how to do so. For instance, labels talk about grams of protein, which is inadequate: you could be ingesting a ton of a subset of amino acids in protein, and entirely deficient in an essential amino acid.

It seems that eating animals was a heuristic that supplies essential amino acids and vitamins.