I disagree with you, but not completely. I don't think that humans aren't meant to eat meat. But I also don't think they are meant to eat meat. In other words, it isn't necessary or sufficient, but it won't hurt either and can be useful in some situations (e.g. starving in a wilderness or something).
Let me predicate this by saying that I am a vegan, but only because I don't agree with how animals are treated before/during death. I think if they're going to die for us, we should respect them highly. I don't believe that eating animal products in and of itself is wrong. I do believe that treating animals horribly for it is.
1) Humans are animals. They want to survive.
-- As I mentioned, eating meat can be useful (higher calorie count than lettuce) when it is needed for survival. Not essential, however. No one ever died from not eating meat (you can get all your nutrients without it, even B12, which is from a bacteria, not meat itself).
2) Humans are predominantly carnivorous.
--No, we're not. We're omnivorous and sometimes nearly scavengers. It's just that in survival situations (like most of our history) meat has higher calories than veggies. More calories = longer survival. Especially in areas where plants aren't abundant (wintery climates, etc.). We're predominantly carnivorous in the USA, but that's not natural. Really, humans eat what they can to survive, meat or no.
3) Humans surviving on vegetarian diet
s are evolutionarily novel.
--Again, back to my point about meat = more calories = survive longer. Especially in areas with sparse vegetation. We also have no precedent for using iphones or getting laser eye surgery or using new drugs, etc. etc.. And yet we do it and don't consider it wrong. Again, my point about meat = calories, more calories = survival.
4) Carnivores tend to be smart.
--Maybe. Or do smart creatures tend to eat meat? For example, the smart ones are smart enought to figure out how to kill other things? I sincerely doubt we are smart because we eat meat. That is a correlation and does not mean any causation at all. There are also other intelligent species that don't eat mostly meat (cows, primate families, and so on). We are the most intelligent, but once again, correlation and not causation. We may eat meat because we can think up smarter ways to kill and eat it, you see?
--Also, there are totally carnivorous animals and they are CERTAINLY not smarter than we are, not by a long shot.
5) Our primate cousins are also omnivorously adapted, though not as carnivorous.
--I repeat my point about smart things being smart enough to kill other creatures when needed for food. We are smarter than other primates, so we can think up smarter ways to kill and eat things.
--Again, there are totally carnivorous animals that are not more evolved or smarter than we are. "Eat more meat" does not equal "more intelligence."
A) Losing a great deal of their intelligence.
--Meat consumption may be partially correlated with intelligence, but we do not know the nature of that relationship. And it does not in any way mean there is a cause between eating meat and being smart (I encourage you to look up the difference between correlations and causations). And again, there are animals that eat way more meat than we do and who are not smarter, more evolved, more capable.
B) Returning to meat-eating.
--This is predicated on your former argument A, which is not based on a causation but rather a partial correlation. Not enough support for this. And once again (again again), eating meat isn't a must - it's a convenience.
C) Being very violent in how they procure their meat.
--Um, killing things is always violent (have you seen how we factory farm our meat now? It's even more violent than those apes you used in an example). We might just do it smarter (go for areas that kill things quickest) because we are smarter and know those things (once again, see my previous points).
Overall, not a great argument. Fun to think about, but there's really no foundation here.