<P> 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 wild of multiple species of 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 ." </P> <P> 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 . </P>

Who came first in world hen or egg