Science does actually have an reply to this oft-repeated conundrum, What came first, the chicken or the egg? This naturally means that you should never let anyone offer this as a riddle ever again without adjusting them.
In nature, organisms evolve through modest changes in their DNA. This is because DNA replicating during reproduction is never 100% precise, and these slight mistakes in copying are the fuel for natural choice. Beneficial mutations that enable an organism to better pass on its own DNA are naturally chosen by several environmental pressures (sexual, predatory, etc.), and can eventually become dominant in a population of organisms, leading to new species.
What came first, the chicken or the egg?
In a creature such as a chicken, DNA from a male sperm cell and a female ovum (egg) match and unite to create a zygote: the first cell of a new baby chicken. This first cell divides innumerable times to form all of the cells of the complete creature. In any animal, every cell contains precisely the same DNA, and that DNA comes from the zygote.
From zygote into hatchling, the first chicken (or what we’d consider a chicken) would need to be generated from mutations resulting from non-chicken parents. All new species arise this way, through small mutations in the DNA which are eventually realized in the new offspring. But two entirely different animals didn’t suddenly plop a chicken out; the parents of the very first true chicken could be genetically very similar to the newborn, yet not quite modern chickens. The first poultry could pass on its genetic material and, through the pressures of natural selection, the mutations that gave rise to the first poultry would permeate a community, developing a population of “true” chickens.
Chickens evolved from non-chickens through little fluctuations caused by the mixing of both male and female DNA or by mutations into the DNA. These changes and mutations only have an impact at the point where a new zygote is made. That’s, two non-chickens mated and the DNA in their new zygote contained the necessary mutation(s) to the embryonic body plan to generate the first true chicken. This one zygote cell then split and shaped a biologically modern chicken.
Before this first authentic chicken zygote, there were just non-chickens. The zygote cell is the only place where DNA mutations could generate a fresh creature, and the zygote cell is housed in the egg. Therefore, the egg must have come first.