There is a significant amount of genetic variation between different etnic groups of humans. Blonde hair and blue eyes were and are a favorable mutation for northern European populations. Why? The paler complexion is more favorable to the absorption of UV light, needed for Vitamin D production.
However, for populations living in tropical regions, a dark skin is a favorable genetic variation. Why? For protection against much higher levels of UV light.
When the gene for sicle cell anemia is heterozygous, the gene offers some protection against malaria. However, when the individual is homozygous for the trait, the person develops sicle cell anemia, a potentially fatal disease.
In general, populations from Northern Europe, cary the gene variation for digesting lactose. The European populations had a greater need for milk, to supply the needed nutrients for producting vitamin D. However, populations from tropical regions had less need for the ability to consume milk, because they receive a lot more sunlight.
Cows have a Northernmost limit. The gene for lactose metabolism would have limited benefit to people living North of that limit. Instead, they developed an immense appetite for fish and seamammals.