Carla points to this article on cnn.com as a possible definitive answer to this age old question.
Now a team made up of a geneticist, philosopher and chicken farmer claim to have found an answer. It was the egg.
Put simply, the reason is down to the fact that genetic material does not change during an animal’s life.
Therefore the first bird that evolved into what we would call a chicken, probably in prehistoric times, must have first existed as an embryo inside an egg.
I’d never thought about it too hard, but it all makes sense. Now we’ll need to come up with a term to replace chegg!