And again until around 178 generations (the reason for difference is very complicated) it was asexual so there was no need to inbreed. When inbreeding was 'safe' they evolved (pff evolution...) and began reproducing normally (to human standars). According to some, this all began much earlier than the "Chicken theory" and so these chickens had to go through a semi-ice age (they burrowed into the ground to hibernate; some died, some lived).
Other theorizers believe that both the Chicken Theory and the Egg Theory are co-existent, that all the chickens died in the ice age and The Chicken God made his/her first appearance some time after. This hypothesis isn't very popular though, because neither side wants to have anything to do with the other.