Part of morality comes innately from our biological evolution. That evolution, however, produced a brain that is complex enough both for symbolic contemplation and for participating in unique human forms of social life. Through the process of early childhood development, highly localized modifications of our innate tendencies may emerge. Moreover, since we are hardwired for flexibility, moral rules will vary from culture to culture (or subculture to subculture).
Added to these evolutionary and socio-developmental dynamics, we each have the opportunity to contemplate our moral principles and modify them, or even reject them. The process is entirely natural, but very dynamic. Above all, it is up to us as communities to think through and discuss and determine what is right, and where necessary, make policies to implement what is right, all with high but not absolute regard for individual differences.