I think Chinese experienced colder climate and higher altitudes in the past. Singapore may have been inhabited by a darker "islander" race and the light Chinese took over. If racial evolution exists, it would take too long for us to notice the change. For us to notice a change would require a black or darker population to come to China and interracial marry en masse, and dominate the lighter races and over time , or like in USA where we either ran off or killed the Native Americans, the color would change.