An age-old question, though we have not find any satisfying answer yet. This explanation is collected form a Science magazine, "Chickens, as a species, became chickens through a long, slow process of evolution. At some point, a chicken-like bird produced an offspring that, due to some mutation in its DNA, crossed the threshold from mere chicken likeness into chicken actuality. That is to say, a proto-chicken gave birth to a real-life official chicken. And since that real-life official chicken came out of its own egg, we can say that the egg came first.".
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