If the bible is the unchanging "word of God", how do you explain the evolution of morals?

Through out history we have seen morals evolve. Slavery was considered just fine by devout christians 200 years ago. Children worked in factories as recently as the early 20th century in the US. The people who beat the "Freedom Riders" were church going people. Women were discriminated against very badly during my life time. I would hope that every christian in RS would be repulsed by at least some of these things. Why do we consider these things wrong, when our ancestors did not?