The egg.
Evolution. Adaptation. Natural selection.
First there was this whatever the hell it was animal previous to the chicken as we know it today, natural selection, adaptation, changes in its body and more stuff. After that, a new specie came up. It continued to evolve and adapt to the environment until a new specie came up, and so on. Then, the "last specie" layed and egg, and from that egg came up the chicken as we know it today.
Saw this on Discovery Channel.