It is a very debated issue, which has been discussed a lot, if humans are omnivores or carnivores or herbivores. Sometimes appeals to our status as omnivores - or of course carnivores - to try to justify the fact that we eat animals. In the same way, other times we appeal to our supposed condition of herbivores to argue that we should not eat animals.
Here we will try to analyze briefly if that argument really justifies that we eat animals.
We are omnivores
I think it is important to be clear that the condition of herbivore or omnivore or carnivore is first and foremost a physiological condition.
However, humans are not physiologically carnivorous nor are we herbivores. It is scientifically proven that human beings are omnivores.
The human being is omnivorous because he has in fact the ability to digest and assimilate nutritionally animal substances.
If we were not omnivores we simply could not live by eating animals, but it is a fact that humans have been doing this for many thousands of years.
Appealing to nature is not a moral criterion
There is an argument that sustains that eating animals is "natural" for human beings. That is, they argue that we have evolved to eat animals and that eating animals is what nature "wants" for us. They say that not eating animals is acting against what we were designed to do and, therefore, the idea that we should not use animals as food cannot be correct. If we have evolved to be humans who have eyes, then to say that we have a moral obligation to permanently cover our eyes, and not to use our sense of sight, would be properly considered a mistake.
Now, try to justify some type of action or behavior - for our part or for other people responsible for their actions - simply claiming that it is "natural", is an argument that if it were right would be worth morally justifying anything. Point out that if we have fangs, or if we are omnivores, justifies eating animals, since it is something "natural" would be equivalent to suppose that if men have penises then there would be no moral problem in raping women, since having sex -also like eating animals- is also something "natural".
The fact that we can eat animals does not support the conclusion that doing so is morally justified, just as our ability to use violence does not support the conclusion that rape - or any other form of violence - is morally justified. The mere fact that we can do something is not relevant to deciding whether it is morally acceptable or whether we should do it.
From an ethical perspective, the relevant fact is not that we are omnivores but that we are moral agents. This is the ethically relevant characteristic. This means that we can reason morally. The moral agents can discern between good and evil; and we can act according to moral standards.
Other animals cannot be moral agents since they have no ability to understand the moral dimension of their behavior. We do have that capacity. Therefore, we have the duty to live in accordance with ethical norms, based on logic, that regulate our behavior.
Eating humans is not biologically distinct from eating animals
If the fact that we are omnivores justifies us using food animals then it would also justify using food for other humans - which are biologically also animals. From humans we can also obtain meat and milk and other products that we obtain from the use and killing of animals.
Therefore, when trying to justify the consumption of animals we would be justifying while there would be no moral problem in which other humans would use us for food. The argument that "we are omnivores" supports cannibalism.
The prejudice of the speciesism that governs our mentality does not allow us to realize the absurdity and self-destructiveness of the perspective of pretending to justify the consumption of animals claiming that we are omnivores.
We are not obligated or conditioned to eat animals
Humans have evolved as omnivorous beings. We can eat animals and vegetables. But this only means that we have evolved as beings that can choose what to eat and that we have the option of living exclusively on plant products.
The reasonable thing is to conclude that the fact of whether we are omnivores, or anything else, has no moral relevance. The important thing is that it is a proven fact that we can feed ourselves, and live in general, without exploiting other animals. No physiological trait prevents us from eating a healthy and vegan diet.
Thus, the fact that we are omnivores does not imply in any case that we are physiologically obliged to eat animals or anything that comes from them. It only implies that we can do it - we can digest substances of animal origin - but it does not mean that we should do it or that we are biologically bound to do so.
Even if we were carnivores and we could live based on some type of food prepared without substances of animal origin, this is what we should do morally.
Therefore, the fact that we are omnivores does not imply in any way that our behavior should be omnivorous. We can, and should, feed ourselves through a diet that does not involve using anyone as food. Precisely because we are omnivores we can live without eating other animals.