Chicken or the egg
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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 still the egg, but the explanation is more complicated. An animal nearly identical to the modern chicken (i.e., a proto-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." For Richard Dawkins, "The chicken is only an egg's way of making another egg."
Christian theologians, however, insist that the chicken came first and often citing the book of Genesis as evidence. According to Genesis 1:21 NIV on the fifth day of creation:
... God created the great creatures of the sea and every living thing with which the water teems and that moves about in it, according to their kinds, and every winged bird according to its kind. And God saw that it was good." 
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:
- Engber, Daniel (2013-03-20). "FYI: Which Came First, The Chicken Or The Egg?". Popular Science. Bonnier Corporation. 282 (3): 78. Retrieved 2017-07-11.
- Breyer, Melissa (2013-02-11). "Finally answered! Which came first, the chicken or the egg?". Mother Nature Network. Retrieved 2017-07-11.
- Fabry, Merrill (2016-09-21). "Now You Know: Which Came First, the Chicken or the Egg?". Time Magazine. Retrieved 2017-07-11.
- Neil deGrasse Tyson (2013-01-28). "Just to settle it once and for all: Which came first the Chicken or the Egg? The Egg -- laid by a bird that was not a Chicken". Twitter. Retrieved 2017-07-11.
- "Genesis 1:21 NIV - So God created the great creatures of - Bible Gateway". Bible Gateway. Retrieved 2017-10-11.
- François Fénelon (1726). Abrégé des vies des anciens philosophes: avec un recueil de leurs plus belles maximes. Paris. p. 314.
- François Fénelon (1825). Lives of the ancient philosophers, with a life of the author. London. p. 202.