You’ve probably noticed that while waking up, a cat looks at you through a white membrane, which disappears after a few blinks. Human beings also have a third eyelid, but it’s not well-developed. When humans had started walking on 2 legs, they stopped bending down to the ground while chasing their prey. Third eyelids work as wipers: they clean the eyeball from dust more effectively than the eyelids we have today.