You know that little pink thing nestled in the corner of your eye?
It's actually the remnant of a third eyelid, known as the "plica semilunaris."
It's much more prominent in birds and a few mammals, and functions like a windshield wiper to keep dust and debris out of their eyes.
But in humans, it doesn't work.
It's vestigial, meaning it no longer serves its original purpose.
There are several other vestigial structures like the plica semilunaris in the human body.
Most of these became vestigial long before homo sapiens existed, quietly riding along from one of our ancestor species to the next.
But why have they stuck around for so long?
To answer this question, it helps to understand natural selection.
Natural selection simply means that traits which help an organism survive and reproduce in a given environment are more likely to make it to the next generation.
