Races - When it comes to races, why is it that we do not see on a whole that we are NOT pure races?

There are some people that would agree that we are from one race. From the beginning of time, do we actually know what the first humans were (racially). No? We only go on hear say. Just like you lie and have lied, trust that some how someone may have lied to you. I am a "black" American with European, Irish, Choctaw Indian, and Scottish heritage. Why is it that I can only choose one from the bunch. Any and all opinions are welcome. Thanks!