以進化/演化的角度,其實之前的連結也有說:
A leg hair, for example, generally lasts just two months before falling out; an armpit hair might hang in there for six months. Head hair, though, may grow nonstop for six years or more. It's not clear why this is, though some researchers have suggested that human head hair somehow became insensitive to the growth inhibition factors that constrain other animals' fur. http://www.signonsandiego.com/ne ... -9999-1c13hair.html