How can the devil be real?

We Christians say the devil is responsible for our sins and temptations. However, I don't see how this can coexist with scientific knowledge. Specifically, we know that life has adapted over time to surviving and reproducing as much as possible (after all, that is what causes more similar lifeforms to arise - it's called natural selection). This includes hormonal systems in the brain that create a positive response to things like reproduction (sex) or eating (gluttony), along with other behaviors like the challenging of authority, or the seeking of (evil) novelties to carry out, which positively impact the species' prosperity. However, this also extends to "rational" (non-chemical) sins as well. In the end, every lie, every insult and even every murder comes from an underlying reasoning (in the latter case, it comes from a reasoning which has been heavily corrupted by the conditions in which one has developed, but the point still holds). Bearing in mind all this, how can we say that there is an external influence "creating" these sins and temptations when it is the internal systems of the brain that are? Saying so would be like saying that demons are responsible for our survival, or like saying that they created these systems in the first place (which, inevitable as they are due to the conditions of the Earth, would imply that they would have created the Earth itself!) Where does this reasoning go wrong?