I don't understand how the first Europeans had a dark complexion and yet they virtually do not exist, or at least the gene for a dark complexion in Europe and Asia was such a disadvantage it got weeded out, except for areas where there's more sun.

Looking at skin tone compared to the climate/Sun UV rays makes complete sense to me.

I understand that a gene can be selected or dissappear in a population within 7000 years, that makes sense, it's a lot of time, but I have trouble putting the two together.

Let's go back 7000 years ago. I assume the people of the world are brown. Maybe different colors of brown? Whiter skin appears few times in either Africa, Asia or Europe, or each at once. The people who appear whiter in those areas survive and mate, and they have whiter complexion babies at a higher rare than a darker shade so it eventually iso successful that there's no trace of darker people from those areas after 7000 years? That means every single clan within the lines where the UV lines hit less would have selected those genes.

If that gene was selected so quickly shouldn't it always be selected that quickly? The odds of it spreading so fast but not continually being selected for is surprising to me. Although we cant predict human evolution, but if this trait was selected so strongly in the past then if there wasn't a lot of human migration in the future then couldn't we predict something similar along the same uv lines on a map?

Why aren't Inuits/Native Canadians as white as Europeans if the gene spread so quickly? Why don't we see populations today that have a mix of traits being selected or at least both being selected for along a longitudinal line? Like Spainards are very white, Italians are black and Japanese are tan? Or why doesn't Japan have multiple native black, white, and brown ethnic groups?

It makes sense to me too that as people moved out of Africa and in South Africa into hotter climates then darker skin would be selected for. So why don't we see a baseline medium skin tone "seeping" in all populations?

Maybe this is all naive but I'm curious. Apologize for the ignorance.

Edit: reviewing all of these wonderful comments thank you.