First, historically, race was only firmly established as a legacy of the colonial era. For example, in the colonial era, how was it decided in Louisiana if one man was black or not? The answer is, less than 1/32 of his ancestry must be of "black blood"[0]. This sort of thinking and arbitrary ruling is how race was established as a social construct.
Science at the time, based on the physiology of skull shapes and other body features, was shaped in order to draw a line between the "superior" race and the "inferior" race; this is a belief we sometimes still carry. In modern science we know that there is enough variation in people that it is basically impossible to concretely associate certain genes with certain races[1]. So in fact, race IS a social construct; race is the generalizations we unfairly make.
[As a bonus, any two humans average 99.5% the same DNA]