First, let’s get the scientific answer out of the way. Eggs, generally speaking, existed before chickens did. The oldest fossils of dinosaur eggs and embryos are about 190 million years old. Archaeopteryx fossils, which are the oldest generally accepted as birds, are around 150 million years old, which means that birds in general came after eggs in general. That answer is also true — the egg comes first — when you narrow it down to chickens and the specific eggs from which they emerge. At some point, some "almost-chicken" creature produced an egg containing a bird whose genetic makeup, due to some small mutation, was fully chicken. Given the incremental nature of genetic changes, locating that precise dividing line is pretty much impossible. However, chickens were domesticated, diverging from their wild counterparts, sometime in the range of 7,000 years ago. So, in a nutshell, two birds that weren’t really chickens created a chicken egg, and that means that the egg came first, and then it hatched a chicken.
