This question was originally published on Quora. Answer by Matan Shelomi, Zoologist.
We are definitely omnivores.
The best evidence is our teeth: We have biting/tearing/ripping incisors and canines (like carnivores) and chewing molars (like herbivores). Animals with such diverse teeth tend to be omnivores.
Chemically, we lack cellulases or cellulosic symbionts that many herbivores have, and have lots of proteases that carnivores do. But we do have sucrases that let us digest fruits. Humans require vitamin B12 to thrive, which can only come from animal sources or certain bacteria (vegans must supplement their diet). We also require vitamin C, which is present in citrus fruits and organ meat, the latter probably being our evolutionary ancestor’s main source.
Interestingly, we have very powerful livers (the detoxification organ) and a very strong ability to smell rot/decay/decomposition relative to other animals. This suggests we may have evolved as scavengers, eating dead (but not too decayed) carcasses killed by other animals.
Lastly, our closest evolutionary relatives, the chimpanzees, are omnivores. The leading theory as to how humans evolved is that we became long-distance runners and hunted food by running it down until it tired, and that our access to meat and protein enabled our brains to evolve further than otherwise. So meat-eating is in our history as well as our DNA and physiology.
Vegetarians need not despair: now that we’ve evolved beyond having to hunt, we don’t need to eat meat to survive or thrive at all. A vegan diet is possible and healthy for humans (with care to get enough vitamin B12 and a diverse set of amino acids… see Anne Agard’s answer to this question). A meat-only diet is also possible and even healthy (with care to get a diverse set of vitamins and minerals by eating organs, not just muscle and fat). Omnivores can pick and choose their foods, as opposed to, say, carnivorous cats, that would die on a vegan diet. Still, if someone tries to push a vegan diet on you by saying that we were originally herbivores, that’s simply not true.
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