J. Stanton said:

That being said, we have reasonably solid evidence that the majority of their calories came from hunted meat.  The reason we needed to be adaptive is because hunting smaller, faster, and more dangerous prey as we drove megafauna to extinction is difficult -- and so is hunting new prey in new environments.  Warthogs are perfectly capable of digging up roots and tubers.

JS

Our adaptability has allowed us to maintain our dominance as a species post-agriculture though.  The origin is irrelevant.  Similarly, that Warthogs do something isn't sufficient to prove that it's bad.  There are plenty of apex predators on the verge of extinction today, and plenty of herbivores and omnivores that are thriving.  So there's a genetic fallacy here, that adaptability is only good in reference to traditional apex predator behavior because that was its origin, and there's an association fallacy here, that warthogs are bad and we don't want to be like warthogs.  Maybe this was just poor word choice.

Well, past a certain point rationality just structures things.  There's no such thing as a rational (or irrational) motive, only a rational set of actions given motivation and context.

To your second point: there are some immutable facts about what it means to be human, but human beings also make decisions and are thus self-determining to a large degree.  So they aren't just "what they are" in the sense that an inanimate object or a machine would be.  Human beings have potential, which means that at any given future point they could be almost anything, and as a consequence of the decisions they make in the present.  You can probably establish a narrow range of facts about humans by evolutionary theory, but you need to be careful not to exceed your scope.