How do we know? Every time a population adopts a "Western" diet high in processed foods, they get sick.
It happens within a few years. Their genes don't change, their food does.Real vs Processed Food
The word "processed" often causes some confusion, so let me clarify what I mean.
Obviously, most foods we eat are processed in some way. Apples are cut from trees, ground beef has been ground in a machine and butter is cream that has been separated from the milk and churned.
But there is a difference between mechanical processing and chemical processing. If it's a single ingredient food with no added chemicals, then it doesn't matter if it's been ground or put into a jar. It's still real food.