Chicken or the egg
technically, in order for the first chicken to have ever existed, we must first consider what counts as a chicken. If you look at modern day chickens, you can make comparisons to chicken ancestors. So technically, the chicken came first, because in order for the first chicken egg to come, there needs to have been a first properly evolved chicken
but then again, if the first egg ever had a chicken inside of it, the world's first chicken, then technically the egg came first, but which developed first?
Look to how chickens reproduce, if chickens develop first before an egg covers them, then you have your answer