An ancient human lineage roamed Europe's frozen tundra for nearly 80 generations. Then they died out.

[H2O Man]

11. The time sequence

for human beings in Europe becoming "white" is interesting, and has become fairly well documented in recent times. All of the human of "modern" humans were dark-skinned when leaving Africa. Those who traveled into Europe some 40,000+ years ago were black, and the series of genetic changes -- described correctly as "patchwork" in the below article -- were the result of the amounts of sun light and some dietary factors.

All non-African humans did indeed go back to the great rift valley ...... indeed, a while back -- maybe a couple decades -- all modern humans outside of Africa could be traced to connections to one female in the valley. This mistakenly was considered "proof" of "Eve" by some at the time. Since then, of course, it's known that other fully modern humans rose in western Africa.

The beginnings of skin turning lighter appears to be a process that began around 10,000 years ago.

https://www.science.org/content/article/how-europeans-evolved-white-skin