My answer to this puzzle would be a logical one, someone, somewhere must have painstakingly designed or created the first master copy of that hen, once the design was accomplished, the designer incorporated a mechanism whereby it would then just be a matter of reproducing another hen from the master hen, through mating with its opposite sex, the master hen would lay the egg and transfer the blue print through its egg, and the designer made it more interesting by including a diversity to create other hens with slightly different features by allowing half the chromosomes from a male and half from a female.
It is a bit like a master copy and from which you could burn millions of other copies, except, the designer added the diversification feature, which simply means instead of listening to Simon & Garfunkel on strings, I may be given a choice when burning to replace the strings with guitar, or mixture of both sounds, then someone uses my copy as a master and reproduces another with guitar replaced with drums, that way we have S&G with a wide range of effects.
Imagine if this rather interesting feature was eliminated from the master plan, we would all look the same, and how boring would that be!