I will say....the egg came first.
The bird that laid the egg didn't have to be a chicken. The first chick that hatched could have been a sport - a mutation from some other species. Birds have no problem raising babies of other species or breed - how many of you have broodies who have hatched out ducklings, guinea fowl, turkeys, and even goslings? The Cuckoo bird lays its egg in the nests of other species, and those mothers raise the usurper baby at the expense of their own progeny.
Of course, I do not understand the mechanism by which so great a mutation occurs, to change one species into another. A change of feather color or shape, yes, a shift of body carriage or beak shape, yes. But given that most mutations are deleterious to the creature, and lead to it being eaten or otherwise vanishing from the gene pool in a hurry....
I'm just saying. What laid the egg didn't have to be a chicken. Only what hatched out of it had to be a chicken.