Not in 1859, but 150 years earlier, race and slavery were not tightly linked, and the idea of "equality" in terms of natural rights was in it's infancy. Only when the Enlightenment and the idea of universal human rights caused intractable intellectual conflicts with the economic institution of slavery did it become necessary to construct justifications for slavery based on racial inferiority.
Before that, slavery was pretty much an equal opportunity employer. By 1859, the racial justification for slavery was firmly embedded, based on the bastardization of Biblical sources, not the bastardization of scientific sources.
Don't kid yourself. The Bible and Evolutionary theory have both been abused to justify racial bigotry.
You are correct. The post I was responding to claimed that evolutionary theory was the basis of racism. Reading Darwin or Huxley or any of the early writers would show that they very anti-racist, at least for their time. Huxley was against slavery.