Why do humans have a third eyelid? - Dorsa Amin
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Modern organisms are the result of millions of years of evolutionary change. This process is slow and imperfect, but powerful. And sometimes we get to see evidence of this process on our bodies in the form of vestigial structures.
There are several more structures in the human body that are thought to be vestigial. Can you wiggle your ears? If so, you are able to move some of the vestigial muscles that were once much more important for moving our ears, kind of like satellite dishes to capture sound. Vestigiality can extend to reflexes, as well. One possible explanation for the goosebumps that you get when you’re cold or scared is that it used to be a reflex that would push up your body hair to help you trap heat or make you look bigger. Since we lost our body hair, that reflex isn’t quite as useful anymore.
And of course, humans are just one of the millions of organisms that have evolved over time. And just like us, many other animals have vestigial features. For instance, the wings of an ostrich are technically vestigial structures, inherited from ancestors that could once fly. That doesn’t mean they’re function-less — ostriches use their wings all the time for things like mating displays — but it does mean that they no longer serve their original function. Some snakes, like the African rock python, also display small vestigial legs passed down from a time when their ancestors used to walk around on limbs. Similarly, as whales were once terrestrial mammals that re-entered the sea, some still have bones for hindlimbs inside their bodies that once attached to limbs.
If you’d like to read more about these processes, Jerry Coyne’s book “Why Evolution is True” is an accessible account of the overwhelming evidence for evolution. Further, Nathan Lents’ book “Human Errors: A Panorama of Our Glitches, from Pointless Bones to Broken Genes” is entirely focused on the imperfect but wonderful mosaic of traits that make up humans.
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