"Romans in Africa vs. black skin" is actually a different point. Differences existed between people, but the notion of "race" did not. I'm trying to explain what the history of the notion of "race" was, because we take it for granted now. If you would read the history of race, especially the colonial time around the 17th to 19th century[0], you can see that that is when the notion of "race" as we know it really started to develop as a result of anthropological studies and colonial goals (but NOT genetic, modern scientific studies).
>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.
This is an example of how arbitrary and socially constructed race is.
Not sure if you read my second paragraph, but I wrote this: "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]."
You seemed to be convinced that "race is a social construct" has no scientific basis, but my argument is that there is no strong basis for race in science. The only alternative choice then seems to be social.