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 making it a modern chicken due to mutations in the mother's ovum, the father's sperm, or the fertilised zygote. It has been suggested that the actions of a protein found in modern chicken eggs may make the answer different. In the uterus, chickens produce ovocleidin-17 (OC-17), which causes the formation of the thickened calcium carbonate shell 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 turkeys and finches suggests that such eggshell-reinforcing proteins are common to all birds, and thus long predate the first chickens. See also Bootstrapping (compilers), the solution to an analogous problem in computer science Catch-22 Sorites paradox References Further reading Experts apply new technique to crack egg shell problem 12 July 2010 Chickens Eggs in culture Metaphors referring to birds Paradoxes Quotations from philosophy