22. Plica Semilunaris use in humans today
Isn’t it weird to see that little bit of pink flesh-like stuff in the corner of your eye? What the heck is that for anyway? Today, it seems to be one of the coolest ways evolution is proven. In humans, this particular part of our eyes remains with us from something we used to fully develop called “a nictitating membrane.” Technically, it was like a third eyelid. Today, you see this eyelid develop in other mammals as well as birds, amphibians, and fish.
It essentially covers the eye horizontally, and it’s often clear or translucent. This allowed us to see underwater and even manage to see well in strong winds. Our eyes were covered with this membrane and it’s quite obvious that it would be of use to us today, so why don’t we form it like before? It still develops but the role merely changed. It stops growing completely around the eye and remains in the corner to prevent anything from getting inside the eye itself.