Why do black people have big lips?
The organs in the human body or organs of any animal body are evolved gradually from the ancestors of that evolutionary line. And the evolution of life can be influenced by many things that include the living necessities, the climate of the region, the basic programmed genetic makeup in the organism that get changed due to many reasons, the food habits. We have tropical and temperate countries all over the globe distributed based on the extent of summer duration existing over the period of 12 months. In the tropical country on an average summer spans for more than 6 months in a full year. Especially in the case of African countries it goes without saying by seeing the pictures of African people that summer months are more for them. So, tropical country people will have the advantage of melanin pigment growth sufficiently taken place in their skin. This pigment can absorb the ultraviolet rays from the sunlight and prevent the body damage due to them.
The synthesis of melanin will not be much in temperate country people as their skin does not find the necessity to filter UV rays that much as sunny days are very less over there. Similarly, black people known to be originated from Africans might have developed big lips according to their environment conditions few thousand years ago. And that would have been carried over in their genes till now. As there are many diverse ethical groups within Africa and most of them get married to their own people within their community, the genes get concentrated within their region. So, all the common characteristics of Africa people like big lips, curly hair and black body are retained in them. But all Africans are not with broad nose, big lips and curly hair. The central Africa people have long nose, small lips and straight hair. This was found as not because of mixing with non-Africans but due to diverse ethnic groups existing in Africans also.