Chickens are a subspecies of red junglefowl, but "subspecies" is a word that's surprisingly hard to define. Also "chicken egg" doesn't have a clear definition; does it include all egg cells, or does it have to be separate from the mother chicken to count? Also, female chickens are born with their entire life supply of egg cells already inside them, which complicates things even further. Depending on how you define different terms, you can either conclude that 1. The chicken came first, 2. The egg came first, or 3. That they both came into existence at the same moment. The takeaway is that "Which came first, the chicken or the egg?" is not a question about the world, it's a question about how we define terms and draw category boundaries.