A team of scientists has concluded that it was the chicken that came first and not the egg. Egg shells can only be produced in the presence of a protein found in the chicken's ovaries. An egg cannot exist on its own without having first existed within the chicken.
Ovocleidin-17, the protein found inside the chicken's ovaries, commonly referred to as OC-17, kick-starts the formation of the shell. OC-17 turns calcium carbonate into calcite crystals, which join together to make up the hard shell. The shell is necessary to protect the liquid yolk and the developing chick inside the egg.