The tiny crescent-shaped pink corner of your eye is known as the “plica semilunaris”. It has been long discussed among evolutionists and creationists, because they thought it was a remnant from our evolutionary history.
We don’t have spare third eyelid that can slide horizontally across our eyes, such as reptiles, amphibians, fish, birds and some mammals has. They have translucent structure which is known as “nictitating membrane”, and is used to clean and protect their eyes.
Some of the experts consider the pink corner of our eye as a proof that we evolved from creatures and once we had functional nictitating membrane.