Original photo by ljubaphoto/ iStock
If you look closely in the mirror at the inside corner of either of your eyes, you’ll notice a pinkish protuberance. This thin, curved membrane sits directly adjacent to the eyeball and is called the plica semilunaris, which is an evolutionary remnant of the nictitating membrane, known colloquially as the “third eyelid.” (This is not to be confused with the lacrimal caruncle, a tiny bump at the very edge of the eye that helps keep the eye moist.) Though the third eyelid is useless for us modern humans, it once served a purpose for our prehistoric ancestors.
Many animals, including dogs, cats, and some birds, reptiles, and fish, still have a functioning nictitating membrane. This translucent membrane protects the eye while still allowing the animal to see, and also essentially acts like windshield wipers by removing debris and maintaining moisture. Birds rely on their nictitating membrane while in flight and fish while swimming. Its purpose in prehistoric humans remains unclear due to the lack of definitive fossil records.
Oops, incorrect!
It's a factDuring development, human embryos briefly grow tails. This posterior appendage forms around the fifth gestational week and is usually absorbed into the body eight weeks after conception. In rare cases — fewer than 40 recorded instances to date — humans have been born with these tails.
In fact, the third eyelid is believed to have lost its usefulness in humans long before the first appearance of Homo sapiens roughly 300,000 years ago. This is due to relaxed selection — an evolutionary phenomenon wherein formerly advantageous traits become less prominent with infrequent use. However, these traits can continue to physically stick around if they don’t actively hinder survival. The plica semilunaris is an example of one of these vestigial structures — a useless, harmless feature that shrunk but never fully disappeared.
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The largest eyes of any living creature belong to the colossal squid.
Charles Darwin waited more than two decades to publish his theory of evolution.
From 1831 to 1836, naturalist Charles Darwin traveled the world researching evolution — but even after his return to England, he didn’t reveal his findings to the public for another two decades. Some claim Darwin feared a negative reaction from scientific and religious communities, while others suggest he used the gap to ensure his theory was irrefutable, hoping to compose an extensive, unassailable treatise before informing the world.
In 1858, Darwin received an essay from naturalist Alfred Russel Wallace that proposed similar evolutionary theories to his own. This unexpected development prompted Darwin to divulge his findings to the scientific community alongside Wallace. In 1859, he introduced his theory of natural selection in his work On the Origin of Species. Later, in 1871, Darwin published The Descent of Man, and Selection in Relation to Sex, in which he first publicly posited that humans descended from apes.
Bennett Kleinman is a New York City-based staff writer for Optimism Media, and previously contributed to television programs such as "Late Show With David Letterman" and "Impractical Jokers." Bennett is also a devoted New York Yankees and New Jersey Devils fan, and thinks plain seltzer is the best drink ever invented.
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