Why can it be argued that from a biological point of view, all races are equal?

All races belong to the same species. This is already a proven fact. The differences arose from various environmental factors. People have lived in certain areas for a long time, therefore they are different. Narrow eyes help save the eyes from sandstorms and wind. Dark skin saves from the negative effects of sunlight. Moreover, in different parts of the African continent, people have different skin tones and physiques. But at the same time, all people do not differ genetically and healthy offspring are born in representatives of different races.