Thus, females of species tend to be more selective, males tend to be more promiscuous. There is pressure upon animals for them to be attractive in various ways to members of the opposite sex in order to secure mating opportunities. Among the male animals this usually has to do with size, strength and fighting abilities. Often among birds it has to do with colourful plumage. The female birds choose the male bird which has either built the better nest or has the most colourful feathers.
Colourful feathers are often a sign of good health and robust genetic stock.
Humans and chimpanzees shared a common ancestor from which we diverged about 6,000,000 years ago. It is important to understand that our ancestors were hunter-gatherers for hundreds of thousands of years and our bodies evolved in adaptation to that lifestyle and those environmental conditions in which the availability of food varied. Bodies which could not cope with periods of relative scarcity and abundance did not survive to pass on their genes. Women, in particular, need to have bodies which are capable of storing up food reserves as fat in order to be able to feed their children. Our ideas and preferences for what is beautiful slowly evolved during the long course of human evolution.