So, was it the chicken or the egg?
The vast majority of the world’s Biologists believe that species evolved from other species and the chicken/egg question is a great springboard to launch our sorry asses to this subject. How does evolution work?
The first thing you need to know is that evolution happens to populations of species not to individuals.
But what happens to individuals?
Queue in zombie apocalypse or X-men at this point and, of course, I will implore you wipe such images to the side for the time being. A mutation is a change in a gene. Most of the time, they don’t change anything, sometimes they are lethal, sometimes, they change the way something on the organism works such as eye color or clearing a substance out of your body, etc, etc.
When there are two or more ways that trait can be expressed, there will usually be one trait that works better than the other in a given environment. The individuals lucky enough to get the more successful trait survives at higher rates and more of that kind is seen in the next generation. When two groups of the same species become isolated from each other and a series of mutations or some other events happen such that they can’t breed with one another, you have a new species.
Since the new mutation must be passed to the next generation, it happens when the mom birdie’s eggs were forming or when the dad’s sperm were forming so the very first of these started with the fertilized egg, not the chicken.
If you are a six day creationist, the chicken was made on day five and who knows if it was ready-made with eggs making the question silly like asking if Adam had a belly button.