The study of phylogenetics is the examination of genetic relatedness among groups of animals so as to better understand their evolutionary history.
The family Hylobatidae is estimated to be about 20 million years old by molecular clock radiation analyses, with the modern gibbon being around 10.4 million years old (Chatterjee 2005). As a basis for comparison, archaic Homo sapiens (who looked quite different) evolved around 500,000 years ago, and anatomically modern humans have only been around since 200,000 years ago! (source) There are at least 12 species of gibbons, of which 9 perform duets (Chatterjee 2005). Some species look very similar and can only be distinguished by the male's call, which suggests that the songs played an important role as a selective force in the isolation of different Hylobatidae species. The fact that it is the male's call that varies among these species strongly implies that female choice was an important factor in the sexual selection of gibbons (Marshall 1976).