When referring to a situation in which the original cause out of two likely explanations is unknown, many people like to use the old chestnut:
"Which came first, the chicken or the egg?"
This is an example of metacircularity and is designed to be unanswerable. But the biological answer in terms of evolution is: the chicken.
The reason for this is because the egg is part of the chicken's reproductive strategy. If you traced the chicken's ancestors back in time throughout evolutionary history, you'd find that the eggs were more watery, from the amphibian lineage that we all share. If you went back further still, to simpler and primordial ancestors of the chicken, which were aquatic plankton, you'd find that eventually there was a point where sexual reproduction began. Before this time, the modus operandi of all reproductivity was binary fission ie. one cell dividing into two - in which case, there were no eggs to speak of. Eggs arose after the first organisms developed meiosis, the ability to divide chromosomes between gamete cells.
So the answer to this question is that the chicken came first, although the ancient 'chickens' we are referring to are quite different to the chickens of today.
Lee's Korea Blog: Helping you lose friends since 2006™
(...by equipping you with wildly inappropriate responses to well-meaning questions)