AbbyNormative Ethics

Can there be a system of ethics without God? I've been curious about this question since I had a recent conversation with a professor who was a self-described secular humanist, and I couldn't understand what his view on right and wrong were. It really seemed as though he believed morals arose more or less out of cultural forces, much like any other natural phenomenon. But my view of things is simply different: I can't help but think, Well, are the naturally arising ethical systems right? I realize this question doesn't make sense if "right" only has to do with those evolving ethics. So, how do secular humanists arrive at ethics? There must be some goal, or central ideal (in my mind) to build upon, like love, or truth, perfection, technology, the ubermann,... something.