When I was in high school, I volunteered as a guest science teacher at a local middle school. One day a student raised his hand and asked me, “Which came first, the chicken or the egg?” I looked him in the eye and answered, “The egg.” The student who asked the question was trying to be funny, but my answer was completely serious. People think of the chicken-egg question as an unsolvable paradox: chickens all come from eggs, and those eggs are all laid by chickens. But this is not really how it is. There are a couple of different ways to look at this question.
First is that the egg obviously came before the chicken. The egg evolved as an encapsulation and protection for a developing embryo long before chickens or other reptiles did (remember, chickens are reptiles). Amphibians, which are older than reptiles, lay eggs. Fish, which are older than amphibians, lay eggs. Even most invertebrates, which predate fish, lay eggs. Eggs were around during the Cambrian explosion half a billion years ago, whereas chickens have only been here for a few thousand years.
But this is not really the question he was asking. He was asking whether the chicken or the chicken egg came first. To answer this question, we have to go back in time a little bit but not as far back as the Cambrian. Chickens are descended from the Red Jungle Fowl, which is still a wild bird in Asia.
The process of domestication, which eventually resulted in the modern chicken, was in full swing at least 8,000 years ago.
Domestication is just a type of evolution called “artificial selection”: every generation the individuals with the most favorable characteristics are chosen for breeding. Thus, every generation during the domestication process brought the red jungle fowl closer to what we now call chickens, by however small an increment.
The point at which chickens became chickens is one that can be difficult to fully understand. Some people believe the following about the evolution of a new species: Species A continues for many generations until suddenly one individual of species A gives birth to the first individual of species B. A chimpanzee might give birth to a human, or a crocodile might give birth to a duck. This is not remotely what happens, or what any scientist thinks happens. During the evolution of modern chickens from their parent species (the red jungle fowl), there was a lot of grey area. During this time, scientists might have argued over whether what they were seeing was a new subspecies of jungle fowl, or it was just a population of the same subspecies of jungle fowl with slightly different characteristics, the same way dogs are all the same subspecies of wolf (Canis lupus familiaris) but different breeds have different characteristics. But just about everyone agrees that the chickens we have today are sufficiently different from their parent population of red jungle fowl to be called their own subspecies — the red jungle fowl are Gallus gallus, and the modern chicken is Gallus gallus domesticus.
So when did the chicken egg evolve?
As domestication moved forward, generation by generation, early chickens became more and more chicken-like. And so did their eggs. There may not be a whole lot of differences between the eggs of the modern chicken and the eggs of the ancestral jungle fowl, but they were subject to some modifications — the size, for example. There was not a bright line that separated the chicken from their ancestor, but we can consider each generation of chicken to be more chicken-like than the last, and each generation of egg to be more chicken egg-like than the last. And each generation of more-chicken-like-than-the-last hatched from an egg that was more-chicken-egg-like-than-the-last. Thus, the egg always precedes the chicken, no matter how chicken-like it is at the time.
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