Most adult mammals lose the ability to digest lactose after weaning, yet a substantial share of adult humans in populations with a long history of dairy farming retain this ability throughout life. Genetic studies show that the mutation enabling adult lactose digestion is far more common in populations descended from early cattle-herding societies than in populations without such a pastoral history, suggesting the mutation spread because it conferred a real nutritional advantage where dairy was a reliable food source.

Which one of the following is the most logical assumption underlying the passage?