it's not the US alone, this concept originated from the UK. Charles Darwin's theory of evolution was used by the ruling class (think, east India company directors and such) to divide humans into sub-species with different levels of evolution. This would allow them the moral justification they needed to continue their conquest. There is a reason "all people are created equal" is the phrase used to abolish things like slavery. people focus on the "created" part, but the equal part is just as relevant.

This concept of race is designed so that one race can claim better evolution than the other, as a whole that is. People with specific ancestry might be better at specific things (provided they pursue those things to their potential), but associating that with an entire race was only useful at the time of this social constructs' creation because Europe needed to conquer the world and what do Europeans have in common the rest of the world doesn't have? Skin color. Which due to the latitude of Europe as a continent, people whose ancestors are from there have less melanin in their skin to account for lesser sunlight (You can see the same effect with north-east Asians). If you think about it, the western classification of "race" has more to do with geography than genealogy.