Chicken or the egg

This article is about the philosophical dilemma. For the dish combining both chicken meat and eggs, see oyakodon.

A chick hatching from an 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, or to express a scenario of infinite regress. Plutarch posed the question as a philosophical matter in his essay "The Symposiacs", written in the 1st century CE.

Ancient paradox

The question represents an ancient folk paradox addressing the problem of origins and first cause. Aristotle, writing in the fourth century B.C., would describe the problem. He 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, Macrobius wrote 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 the Bible. In describing the creation of animals, it allows for a first chicken that did not come from an egg. However, later enlightenment philosophers began to question this solution.

Scientific resolutions

Although the question is typically used metaphorically, evolutionary biology provides literal answers, made possible by the Darwinian principle that species evolve over time, and thus that chickens had ancestors that were not chickens, similar to a view expressed by the Greek philosopher Anaximander when addressing the paradox.

If the question refers to eggs in general, the egg came first. The first amniote egg — 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 of red junglefowl and probably arose little more than eight thousand years ago, at most.

If the question refers to chicken eggs 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., 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."

Alternatively, if the question refers specifically to the chicken egg as it exists today, the answer may be different. Chickens produce a protein, ovocleidin-17 (OC-17), that is expressed in the uterus and causes the formation of the thickened calcium carbonate shell around modern chicken 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. This is only the case, however, if OC-17 arose after the domestication of their wild-fowl ancestors gave rise to chickens.

See also

• Catch-22

Further reading

• Experts apply new technique to crack egg shell problem 12 July 2010 doi:10.1002/anie.201000679 10 June 2010