>>> What comes first, a chicken or an egg?
The egg. The two parents were not sufficiently chicken like to be called chickens, but their combined genetic material produce a mutation which was now sufficiently chicken like to be called a chicken. Thus, there was not a chicken prior to the production of the first chicken egg.
An esoteric point, but the fertilised egg cell was already destined to be a chicken before it was encased in the hard shelled "egg".