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Why do humans have a third eyelid?

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You know that little pink thing nestled in the corner of your eye? It's actually the remnant of a third eyelid. In humans, it's vestigial, meaning it no longer serves its original purpose. There are several other vestigial structures in the human body, quietly riding along from one of our ancestor species to the next. But why have they stuck around for so long? Dorsa Amir investigates. [Directed by Avi Ofer, narrated by Addison Anderson].