Lips are a very sensuous part of the body and look attractive. The disadvantage wouldn't necessarily have to be large to change the genetic makeup of a population over many, many years. I think this is less likely. As an artist, though, I am able to see and appreciate the differences and beauty in everyone I run across. It's a good question.
Blacks come from a hot climate.
Our behinds are larger to store fat in a place where we can stay cool rather than on legs or arms. We tend to be taller because taller, lean bodies dissipate heat better. Straight hair warms the head and neck. There is a biological reason why our lips are larger. I'd love to know the answer.