Many animals have highly specialized tails. Horses use them to swat flies, while birds them to steer during flight. But what about us? Why don't humans have tails, too?
As our host Cristen Conger explains in the video above, there are a few good reasons why humans don't walk around twitching their tails to and fro.
Unlike quadrupedal four-legged animals — like cats — that use their tails for balance, people walk on two legs and employ a different system to avoid falling over. Our center of gravity passes vertically down our spines, and we don't need another appendage to counterbalance the weight of our rather large heads.
We also don't require a tail to anchor ourselves onto branches or to swing from tree to tree across jungle canopies like other primates (although how cool would that be?).
Plus, tails take up valuable energy, and they don't make sense for humans who no longer need tails to survive. Wait, does that mean we once had tails? It sure does! And, in a way, we still do. We each have a series of fused vertebrae, called a coccyx, at the end of our spines. This vestigial remnant is leftover from the days when our ancestors sported tails.
In other primates, the coccyx still leads to a fully developed tail. And, humans continue to sprout an embryonic tail around day 30 of development in the womb, but this appendage is reabsorbed before birth — in most cases.
In extremely rare instances, babies are born with actual tails. It's a type of atavism, a trait of distant ancestors that randomly reappears. Usually, these tails are just a few centimeters long and are removed shortly after birth.
Still curious about human tails? See what else Cristen has to say about them in this BrainStuff episode — it's quite a tale about a tail.