TED-Ed: Why do humans have a third eyelid? | Dorsa Amir

The video explores vestigial structures in humans, such as the plica semilunaris, tailbone, palmaris longus muscle, and appendix. It explains how these structures, once useful to our ancestors, have persisted due to natural selection. While some vestigial traits remain because they are not harmful, others like the appendix can pose risks. The video highlights the imperfect nature of evolution and how it has shaped human anatomy over millions of years.

What is the plica semilunaris, and why is it considered vestigial in humans?

• A bone in the arm that is no longer needed

• A remnant of a third eyelid that no longer serves its original purpose

• A part of the ear that no longer functions

• A muscle in the leg that has no current function

Why did the tailbone persist in humans despite the loss of external tails?

• It is harmful and selected against

• It serves a minor function and is not harmful

• It is essential for walking

• It helps in climbing trees

How can you identify the presence of the palmaris longus muscle?

• By checking for a band in the wrist when touching the thumb to the pinky

• By observing the length of the fingers

• By measuring the width of the palm

• By feeling a bump on the elbow

What is one possible reason the appendix has not disappeared from humans?

• It is crucial for the immune system

• It might serve as a reservoir for beneficial bacteria

• It is essential for digesting meat

• It helps in the absorption of nutrients

What does the presence of vestigial structures in humans suggest about evolution?

• Evolution is a perfect and precise process

• Humans have no evolutionary history

• Evolution involves trial, error, and random chance

• All human traits are currently useful