This is just easier than going through and rewriting the whole thing. So here goes:
We hear some expert say that humans are so smart because we learned to hunt and eat meat and, as vegans, we collectively cringe.
After further study, it seems the emphasis is on cooking the meat, or any food for that matter, for early humans that somehow jump-started them into a more "civilized" manner. You will still, however, find people that will say that specifically meat protein is what supposedly makes us smart. Omega-3 fatty acids have an effect on brain function and DHA, mainly found in fish oil, is said to be the most potent. It is not yet known precisely how effective plant sources of Omega-3 such as ALA and EPA are at helping brain function, so it could be just as good as the DHA, but we do know that the body can also convert the other two into DHA. Furthermore, the reason there is such a high concentration of DHA in fish is because there is a lot of it in the micro-algae in the ocean, which the fish consume. Therefore, it is possible to just skip the middleman (or middle-fish, as it were) and gain the DHA directly from the source, which for vegetarians means not having to worry about how efficiently your body is converting the other ALA and EPA into DHA. For everyone in general, this would mean not having to worry about how much mercury and other pollutants you're consuming with your fatty acids.
We see tribes of chimpanzees in Africa fashioning "spears" to stab other animals to death and then eat them, and another expert compares it to (and holds it up as proof of) our own early history.
Anyone who points to another animal, however close to us they are on the evolutionary line, to "prove" that humans are supposed to do this, first off, is obviously just picking and choosing which primates he/she wants to represent us and, secondly, obviously has no understanding of evolution. Pointing out that chimpanzees eat meat and then suggesting that this is evidence that humans have a good excuse to as well is like saying that polar bears are carnivores so all bears must be carnivores. (Note: most bears are omnivores.) It also completely disregards the fact that a lot of apes, such as gorillas, and primates in general, are herbivorous.