Chicken or the egg

The chicken or the egg causality dilemma is commonly stated as "which came first: the chicken or the egg?". The dilemma stems from the observation that all chickens hatch from eggs and all chicken eggs are laid by chickens. "Chicken-and-egg" is a metaphoric adjective describing situations where it is not clear which of two events should be considered the cause and which should be considered the effect.

Although the question is typically used metaphorically, literal answers have been formulated for whether the chicken or egg came first.

If the question refers to eggs in general, the egg came first, as the first egg-laying animals evolved millions of years prior to birds. If the question refers to chicken eggs specifically, the answer is again the egg, but the explanation is more complicated. An animal nearly identical to the modern chicken (i.e., a -chicken) laid a fertilized egg that had DNA identical to the modern chicken (due to mutations in the mother's ovum, the father's sperm, or the fertilised zygote). Put more simply by Neil deGrasse Tyson: "Which came first: the chicken or the egg? The egg – laid by a bird that was not a chicken."

Ancient philosophers were not aware of biological evolution. Aristotle (384–322 BC) was reportedly puzzled by the idea that there could be a first bird or egg and concluded that both the bird and egg must have always existed:

If there has been a first man he must have been born without father or mother – which is repugnant to nature. For there could not have been a first egg to give a beginning to birds, or there should have been a first bird which gave a beginning to eggs; for a bird comes from an egg.

...
Wikipedia