For example, are humans greedy all the time?
If not then you can't say that greed defines a fixed human nature. Some human are greedy some of the time. Other humans are never greedy. Some are always greedy. It depends on the environment and their experiences. Certainly all humans have the capacity for greed. The potential for this behavior exists in all humans precisely because it is an adaption to certain kinds of stress.
That doesn't mean the tactic is employed unconsciously or involuntarily, or that is the only tactic available, or even if the original adaption is even related to the current behavior. There are vestigial behaviors just as there vestigial organs in the human body.