Skin color is one of our most important biological traits. Its many shades evolved as humans moved far and wide into regions with different intensities of sunlight. Skin color is also a trait that has come to have deep cultural meanings -- developed over many centuries -- that influence our social interactions and societies in profound and complex ways.
We notice one another's skin because we are visually oriented animals, but there is no genetically programmed bias to favor one color over another. Over time, however, we have developed beliefs and biases about skin color that have been transmitted over decades and centuries and across vast oceans and continents.
The first scientific classification of humans, published by Carl Linnaeus in 1735, was simple and separated people into four varieties by skin color and continent. Later, Linnaeus not only added more physical traits to his descriptions but also changed them to include information that he had surmised about temperament. Europeans were White and "sanguine," Asians were brown and "melancholic," Native Americans were red and "choleric" and Africans were Black and "phlegmatic."
This analysis was the first authoritative classification that combined physical traits with folk beliefs about dispositions and character. The folk beliefs had little to do with fact or observation but were mostly just fables -- racist pronouncements that were personal and emotional expressions of, at best, discomfort and, mostly, prejudice. From this point on, debasing associations of physical appearance with temperament and culture became commonplace and were considered scientific. Racism had found its intellectual foundation.
The first person to formally define "races" was the noted philosopher Immanuel Kant, who in 1785 classified people into four fixed races, which were arrayed in a hierarchy according to color and talent. Kant had scant personal knowledge of human diversity but opined freely about the tastes and finer feelings of groups about which he knew nothing. For Kant and his many followers, the rank-ordering of races by skin color and character created a self-evident order of nature that implied that light-colored races were superior and destined to be served by the innately inferior, darker-colored ones.