The definitions of what is a race, or whether someone is one race or another, have varied tremendously throughout history

That's because people started using the scientifical term 'race' to base their bullshit beliefs, so to just make it easier they narrowed and narrowed the meaning down to just skin color, because it is a more noticeable and constant trait of race. Not only that, they added in many bullshit social and economical traits and somehow said that they have something to do with race. This dumbing down and mixing is what makes the common "race" expression a social construct, which is a shame, because in real science, the expression serves a good, non-malicious purpose.

But in reality, skin color is just one of many defining traits of race (I'm speaking biology here, don't kill me), others being hair growth and form (both head hair, armpits, and ding-dong), facial shape (like how asians tend to have more stuffed cheeks, and africans tend to have more pronounced noses), among others, but still, this is in constant debate.

The problem is that, way back when, I'm talking from the birth of man up untill the end of the medieval era, people didn't really move a lot, I mean intercontinentally, well, not nearly as much as they did/do in the modern era. Because of that, generations and generations and generations of people have fucked and lived in generally the same continent they were born in, because of that, genetics came into play, and usually the people with the bodies best adpated to live in their respective area tended to leave more offspring, yadda yadda yadda, well it wasn't exactly this that happened to races and human beings, but you get the gist.

Now, people have began moving everywhere, and races are starting to get mixed together more and more, now there are people who literally don't really fit into any of the defined races, so the categorization is getting less and less accurate, and will probably slowly fade out. The thing is, race as a social categorization is utterly wrong, and race as a scientific categorization of human biological traits is a real, valid thing, that seems to be getting more useless and inaccurate with time, but we'll see happens.