Tell that to some poor soul living under a dictatorship. Rights are indeed 'discovered in nature'. It promotes human flourishing. All legal and political 'contracts' should be based on personal moral conclusions and principles. Shermer bases the morality of abortion on the legal concept of 'murder'.
Murder is taken as a moral starting point - a moral absolute.
He misses the point that the concept 'murder' actually implies and is based on human rights - each individual's right to his or her life. Failure to derive the unlawfulness of murder from morality would leave us open to arbitrarily define it to, say, exclude certain races, women or children - as has frequently been done at various times, in various parts of the world. This line of reasoning could lead to the unfortunate conclusion that late-term abortions, that threaten the mother's life, are immoral. Abortion ultimately comes down to the reality that the mother does not want the child. From a social, legal point of view child neglect and adoption policies are important factors.