Everyone was dark skinned 10k years ago. Ligth skin is an adaptation to the convination of a vitamin poor diet and low uv radiation. If your population lives way up north but have a good diet like eskimos or mongols or siberians you are not as white. If your population has a shity diet but live further south you are not as white.
I think the general consensus is that light skin evolved a few times and some of those times were at least 20-30k years ago. There were folks with different amounts of melanin running around for a long time.
Oviously the first instances of those genes are cery old. Maybe even neanderthal old. But selection preassure for them to become dominant in a population is due to vitamin d deficieny in pregnancy. And that is mostly an agriculturalist thing. But if there is enough sunligth its not as bad. That explains the north south gradient in skintones as well as darker skinned huntergatherers in high latitudes. And that diserent version of the gene were selectd in west and east asia.
Those are definitely reasonable explanations, especially since you do get more vitamin D w/ less sun the lighter your skin is - and there are trade-offs when you get a lot of sun with light skin. Though the story could also be more complex or even seemingly random (or just historically contingent), also having to do with how people migrated, how big their populations got, etc etc.
