This really isn’t hard. Morality is just goal-based behavior. Paths that lead to a goal are called good; paths that lead away from a goal are called bad. Whether or not a goal is itself good or bad depends on where it lies on a larger path.
So, what are the ultimate natural goals? Easy. One is life, which is the prequisite for being able to make choices; the other is death, where no more choices are possible. Certainly, you can switch between these goals, but the key point is that nature gives no clue which you should choose. Therefore, all natural ethics is based on personal taste.
N.B. In Christianity, something outside of nature steps in and says, “choose life.” That’s why atheists can act morally (search heuristics can end up being similar).