Chicken or the egg

Thechicken or the eggcausalitydilemmais commonly stated as the question, "which came first: thechickenor theegg?" 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 thecauseand which should be considered theeffect, to express a scenario ofinfinite regress, or to express the difficulty of sequencing actions where each seems to depend on others being done first.Plutarchposed the question as a philosophical matter in his essay "The Symposiacs", written in the 1st century CE.
The question represents an ancient folk paradox addressing the problem of origins andfirst cause.Aristotle, writing in the fourth century BCE, concluded that this was an infinite sequence, with no true origin.Plutarch, writing four centuries later, specifically highlighted this question as bearing on a "great and weighty problem (whether the world had a beginning)".In the fifth century CE,Macrobiuswrote that while the question seemed trivial, it "should be regarded as one of importance".
By the end of the 16th century, the well-known question seemed to have been regarded as settled in the Christian world, based on the origin story of theBible. In describing the creation of animals, it allows for a first chicken that did not come from an egg. However, laterEnlightenmentphilosophers began to question this solution.Carlo Datiin the mid 17th-century published an erudite satire on the subject.
Although the question is typically used metaphorically,evolutionary biologyprovides literal answers, made possible by the Darwinian principle that speciesevolveover time, and thus that chickens had ancestors that were not chickens,similar to a view expressed by the Greek philosopherAnaximanderwhen addressing the paradox.
If the question refers to eggs in general, the egg came first. The firstamnioteegg – that is, a hard-shelled egg that could be laid on land, rather than remaining in water like the eggs of fish or amphibians – appeared around 312 million years ago.In contrast, chickens are domesticated descendants ofred junglefowland probably arose little more than eight thousand years ago, at most.
If the question refers tochickeneggs specifically, the answer is still the egg, but the explanation is more complicated.The process by which the chicken arose through the interbreeding and domestication of multiple species of wild jungle fowl is poorly understood, and the point at which this evolving organism became a chicken is a somewhat arbitrary distinction. Whatever criteria one chooses, an animal nearly identical to the modern chicken (i.e., aproto-chicken) laid a fertilized egg that had DNA making it a modern chicken due to mutations in the mother's ovum, the father's sperm, or the fertilisedzygote.
It has been suggested that the actions of aproteinfound in modern chicken eggs may make the answer different.In the uterus, chickens produce ovocleidin-17 (OC-17), which causes the formation of the thickenedcalcium carbonateshell around their eggs. Because OC-17 is expressed by the hen and not the egg, the bird in which the protein first arose, though having hatched from a non-reinforced egg, would then have laid the first egg having such a reinforced shell: the chicken would have preceded this first 'modern' chicken egg.However, the presence of OC-17 or a homolog in other species, such as turkeysand finchessuggests that such eggshell-reinforcing proteins are common to all birds,and thus long predate the first chickens.