First the chicken's ancestor(probably something that would, were it still alive, be part of the family phasianindae) existed. Said ancestor continued to mutate repeatedly and through natural selection eventually became a new species(so different from the original species that it can no longer interbreed with it's ancestral species. This new species is called the red junglefowl and still exists today in asia. Humans then got involved in the picture roughly 5,000 years ago and selectively bred the red junglefowl to be more and more docile and have a larger breast. Eventually, after many generations of selective breeding, we ended up with the modern chicken, a new species. This process is called domestication and is essentially human-fueled evolution. Every single species that has ever been birthed from an egg started as an egg.