What came first, the chicken or the egg?
The gradual adaption to life on land was a milestone in evolution, and can be largely attributed to the evolution of eggs in reptiles and birds.
Firstly, let’s look back to one step before reproduction became fully independent of water. Amphibians, although able to live on land once adults, are limited to areas containing water for reproduction. This is due to the spawning of their eggs, with external fertilisation, making the eggs very susceptible to desiccation.
Reptiles, however, have eggs contained within an eggshell, reducing water loss. The egg also shows a number of other advances, such as a substantial food supply (the yolk), and internal fertilisation. This enabled the domination of the terrestrial environment, with few other competitors.
The evolution of the egg, and how its advances shaped the development of life on land, still begs the question: what came first, the chicken or the egg?
We must first define an ‘egg’. Egg laying animals obviously existed long before the chicken so technically the egg came first but in this context, we are referring to a chicken egg. But secondly, we must define a chicken egg. Is a chicken egg an egg produced from a chicken or an egg containing a chicken?
Eggs produced by a chicken require a protein, known as OV-17, which is only found in chicken ovaries; therefore you can’t produce a chicken egg without a chicken. However, new species are formed from small genetic mutations over thousands of generations, and these must occur in the zygote. A creature (let’s call it a proto-chicken) very similar to a chicken produced an egg which, due to a small genetic mutation, developed into the first chicken. Therefore the first chicken hatched from the egg of the proto-chicken, and the egg came first. Although you could argue that this egg was not a chicken egg, no one mutation could constitute a new species.
Despite this question still being an on-going debate, it does provide some food for thought over breakfast.