Good point and the whole idea of what is natural tends be anchored at some point in time, from some point of view. All too often, people use natural as a (unimpressive) rhetorical technique to justify some moral claim. Such claims fall apart when you (a) define nature at some other time or point of view; or (b) reject the notion that behavior we see in nature (in all its wonder, complexity, ruthlessness, and cruelty) should be the fundamental basis for human morality.