The theory I've heard (on Nature, PBS) is that many primates (from whom we evolved) used to scream and hoot in unison so that predators would be confused as to how large or how many animals they were following. If the predator became convinced that the group was actually one large animal, or that there were too many in the group, it eventually went away. As we showed up and began getting organized, we also organized our hooting and shouting at predators (for the same reasons), and it eventually became singing. We came to enjoy it because it had saved our lives. And as we became better at it, it sounded better. Makes sense to me. Dancing might have started the same way. If we moved in unison and stomped to raise up dust, a predator might have thought we were to large an animal to hunt and went away.
Singing is an innate ability that every human being is born with. Everyone can sing in their own unique way though there are people who are exceptionally good singers. Some world renowned singers include people like Kenny Rogers, Celine Dion and Whitney Houston.