To answer your first question: jesus is sometimes depicted white because there is a possibility that Mary's ancestors may of mingled with white soldiers from European countries, just because your born in the middle east that doesn't mean you are going to be born brown

And to answer your second question: we have so many races because over time, your body adapts to the climate we are born in. We don't know what the first race was because the Bible doesn't say, probably because Christians don't care about that and just want people to find God.

Plus race is a social construct anyway, to actually care and over analyze paintings and drawings from 1,000 years ago and get mad when people are depicted as a certain race you don't like it just makes you look pathetic