Darwin sailed on the Beagle in 1831. He subsequently would organize his findings into the Evolutionary Theory in 1838. Publication of The Origin of Species in 1858 was immediately sold out, indicating that there was enormous interest and discussion around it. Plus, it was condemned by the Christian church, both Protestant and Catholic.
The vaunted (and overstated) closeness between man and ape in Darwin’s theory was immediately used to “prove” the “scientific basis” for slavery.
It was argued that because everything—including humans—continued to evolve, it was logical that the three races (oriental, occidental, and negroid) occupied different points on the evolutionary journey.
It was further argued that by simple observation, it was obvious that the order of evolution was occidental (white) as the most evolved, then the oriental race. The negroid race was clearly the least developed and in need of direction and supervision until it was ready to “join the human race.”
Realize that this was the belief of the great majority of Americans, north and south. The blacks were not yet quite human. They were closer to apes than to white people.
Thus, science was added to the Bible, whose note that “Ham shall serve his brothers” was interpreted as a command instead of a statement of fact, to build a case for slavery of the black population.