The carnist here claims that our bodies, teeth, or some other body part are evolved for eating meat, or that evolution is somehow prescriptive by some necessity.
Carnists will claim this on the one hand, but then turn around and claim that atherosclerosis as a result of eating meat means nothing and that we should ignore that meat eating is a contributing factor to many of the top causes of death in the developed world, such as heart attacks, strokes, type 2 diabetes, and cancer. So then, which is it, are we adapted towards eating meat, or does the fact that saturated fat coats our arteries a clear sign that we need to steer away?
Also, this contradicts the argument that animals are bred for consumption, because if evolution is prescriptive in this manner, then one would have no reason to breed animals for consumption because they would already be designed for it.
Furthermore, this also would contradict the argument that eating meat was necessary for the development of certain important human traits (such as intelligence) because you run into a chicken-and-egg problem. If it's only ethical because it helped us develop, then it wouldn't have been ethical before the development. This would require you to lean on some other justification, in which case, that would be your actual justification and therefore this argument would be a moot point at best.
Also, your brain evolved to make you know it's unethical, which is just as valid as any other body part. This is especially highlighted in conversation by carnists' long sighs followed by some statement about the helplessness of the situation, that they tried and failed to go vegan because it's too hard. So if evolution made your brain - and your brain made you think it was unethical as those other claims show - then shouldn't this argument make you go vegan?
Plenty of objectionable things that are made possible by evolution are not moral, "whatever evolution made possible must be moral" for humans. This is just "might makes right" with the additional step of "evolution made might". Consider some parasites evolved to eat their hosts from the least-to-most important parts to keep them alive for as long as possible and thereby inflict the maximum possible suffering. Most humans regard this with some level of horror, and thus it seems unlikely that this is the argument that carnists truly rely on for moral consistency.
If you want to argue that we developed to eat meat and therefore it must be healthier, it's worth pointing out that longevity is not favored by evolution. Once you pass the age of peak reproduction (teens to early twenties), evolution doesn't really care about you and your longevity, as it would not impact how much your genes reenter the gene pool. Therefore, you would need to make an actual epidemiological point to defend the health aspects, as evolution is a rather tepid defense of healthfulness. Given all the major health problems that are linked to eating meat (heart disease, stroke, diabetes), this is a poor defense.
What was ethical or necessary in the past isn't ethical or necessary now. If at one point it was necessary to do anything, that justifies nothing once that condition is no longer met. For example, the first killing in self-defense can't now justify all murders once the self-defense is removed from the scenario; that situation is in the past and what is in front of us needs to be objectively evaluated independently of it. Similarly, whatever evolved was just for survival advantage at the time, but these pressures largely no longer exist.
You can't respond to the claim that "carnism hurts animals (in addition to other humans, the environment, and yourself)" with "yeah, well evolution makes it easy!". That is a complete non-sequitur and does not counter the argument. Carnism remains unethical for those reasons, the latter is an irrelevant fact.