I was looking at Bosch's Garden of Earthly Delights. There's both black and white people in the painting, and it also includes a depiction of Adam and Eve, who are white. As the two were considered humanity's forebears in medieval and renaissance Europe, I was struck by the question of how black and brown people were thought to originate. Did God make them that way later, or did they just become different over the ages through some concept of quasi-evolution?
Specifically looking for answers from Christian (and Muslim) perspectives.
A common European Christian explanation for much of the Middle Ages was that darker-skinned people are darker-skinned as a direct consequence of their climate. Africa is hot; heat burns things; therefore, an individual from the hottest parts of Africa will be burned by that heat into having a dark complexion. An individual's complexion is both a token of their overall constitution -- how God made them, their parentage and national qualities -- and a sign of inner qualities that govern their composition of humors. (Humoral theory accounted for more than just different races -- it also had to account for why some European white people were ruddy and others pale, for instance.) This is where you start to get European depictions of all people from the continent of Africa as generally but not exclusively "black", as well as the developing division between "tawny Moors", lighter-skinned-to-borderline-white North Africans and Middle Easterners, and dark-complexioned Africans, who might be designated "Ethiops". There was also a struggle in this era for Europeans who might never have met a person they understood to be mixed-race to understand what a mixed-race union would look like, or what children of one dark-skinned African parent and one light-skinned European parent might look like. This sounds really naive today, but this is where you get fanciful medieval depictions of children of interracial marriages as "piebald", spotted dark and white rather than a single uniform skintone averaging out that of their parents. (Not that RL biracial people necessarily look like an average of their parents' features, genetics is much more complex than just "white + black = light brown".) In short, people of different races look different because God made them different, but their difference isn't necessarily in terms of different extractions or strains of the human species, except inasmuch as ruddy cholerics and pale phlegmatics also occupy different strains but share the same common forebears. Depictions of familiar figures like Adam and Eve, Jesus Christ, most allegorical figures, etc. in Europe are still by and large fair-skinned; darker skin is reserved for depictions of individuals, both good and bad, who are designated as different.
In the Early Modern era, as humoral theory persists but exploration and colonization puts Europeans in contact with other races outside of Africa, this black-and-white-and-tawny model of race falls apart a little -- what about indigenous people of the New World? what about natives of the Indian subcontinent? what about natives of China and Japan? -- and the rise in enslavement of Africans prompted a new development in European racial theory. Climate and constitution-related accounts of why people have different skin tones persisted, but they took on all the baggage of the European understanding of the relationships between races. Dark skin is still an outward descriptor of the dark-skinned individual's origin, composition, and character, but beginning in the 16th century it situates the dark-skinned person on a rough scale of refinement and physical development -- dark-skinned Africans are inherently base and suited only to be slaves, darker-skinned indigenous people are similar, but lighter-skinned non-European races are closest to European and thus the next best thing. This derives a lot from European encounters with non-European regions, and it wasn't the same as modern quasi-scientific divisions of race -- a native of Cambodia and a native of Japan are both "Asian" in modern understandings of race, but aristocratic Japanese were described as "white" by 16th and 17th century European commentators because of their perceived cultural development and superior natural constitutions. Cambodians were not "white", or only conditionally. "White" became a descriptor that stood in for a lot of other positive qualities -- refinement, intellect, independence, and so on -- and their ostensibly natural and inherent opposition to the qualities of blackness -- cultural coarseness, lowness, lack of intelligence, suitability to slavery. This is a consequence of racial hostilities in the Early Modern world as well as a justification of them.
There's multiple competing and overlapping accounts of the origin of racial difference by the later Early Modern era. James Boswell gives Samuel Johnson's understanding of possible models for racial difference as follows:
Johnson and an Irish gentleman got into a dispute concerning the cause of some part of mankind being black. “Why, Sir, (said Johnson,) it has been accounted for in three ways: either by supposing that they are the posterity of Ham, who was cursed; or that God at first created two kinds of men, one black and another white; or that by the heat of the sun the skin is scorched, and so acquires a sooty hue. This matter has been much canvassed among naturalists, but has never been brought to any certain issue.”
Religious accounts of the nature of non-whites had begun to emerge parallel to 16th century European Christian missionary activities which sought to expand Christian influence to non-white populations. (Though these missions, led by white Europeans, often did determine non-white individuals as unsuited for any form of religious leadership that would position them as equals to white people.) Missionary activities were predicated on the idea that non-European non-whites were at least suited to receive Christianity and to adopt civilized norms of life, not barred from this by their inherent dispositions. This still treats European-ness, or being European-like, as synonymous with being civilized, and European culture as the inheritance of humanity's forebears (Adam, Moses, Abraham, Jesus) even if those forebears were situated in the Middle East.
But what about the posterity-of-Ham thing? The "curse on [the posterity of] Ham" being the origin of African blackness is notably a 17th, 18th, and 19th century phenomenon, drawn from the Genesis account of a curse on the children of Canaan and descriptions of the posterity of Canaan as doomed to servitude. (Scripture itself does not specify blackness as an attribute of Canaan, and neither does earlier Jewish scholarship.) It coincides with the rise of European and American slavery, rather than medieval accounts of African Christians, and functions as a scriptural justification for enslavement of Africans and other dark-skinned peoples. So, three possible models by the 18th century -- black people are the posterity of Canaan cursed by God with blackness as an outward token of their intended servile status, black people have been scorched by the heat of their native sun, or God simply made some people black and other people white because it suited Him. The last one still allowed for European (quasi-)scientific inquiries into the nature of blackness and its differences within a Christian framework, but on its surface it's relatively less loaded than the God's-curse theory.
Some sources: