This may be reducing any kind of metaphorical meaning it might have had to zero, but here goes. We know that, if we subscribe to the theory of evolution, all wildlife evolved from something else, which in turn evolved from something else, until eventually we come back to the primordial soup which begat us all. At one point in the evolution of a chicken, one of whatever came before the chicken suffered a mutation which made it into what we now know as a proper chicken. It was almost certainly hatched from an egg. Therefore, if we define a chicken as being the present form of a chicken, the egg must have come first. We can’t really call what came before it a chicken, because then where do we draw the line? The sequence of gradual changes will reach back to the primordial soup, you see, and single-celled organisms certainly aren’t chickens.

However, if we do decide to call what came directly before the chicken a chicken, we can’t know (for now) what came first. This is because the laying of the egg was probably a mutation somewhere along the line, and we don’t know whether that mutation happened to what we define as a chicken (the broad term), or an ancestor of the chicken that is unrecognisable as such. Of course, it may not even be that simple, because the laying of the egg is probably the result of a series of mutations, and therefore we also have to define what we mean by egg—that is, whether it is an egg as we know it now, or a general egg that might have a number of different shapes and sizes.

Which is all very confusing. So it is much better (read, easier) to define the chicken as being the modern chicken, in which case the egg definitely came first. If we define it more vaguely, then we don’t know, and we might never know.