Whether they’re swatting away horseflies or helping a monkey swing through the forest, tails are a pretty amazing adaptation. So why don’t humans have them?
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Heads… or tails? As far as appendages go, tails are pretty much amazing. Over time, different animals have evolved various, highly-specialized tails. A horse uses its tail to swat flies, for instance, while a bird uses its tail to steer during flight. Which leads us to today’s question:
If these specialized limbs are so useful… why don’t humans have them?
There are two answers. First: we don’t really need them. In many quadrupedal, or 4-legged, creatures – like a cat, for example – a tail helps with balance. Fish and marine mammals, on the other hand, use their tails for steering or locomotion.
Some lizards and primates use their prehensile tails to grip things, while crocodile store fat in their tails, kind of similar to the way camel store fat reserves in their humps.
But let’s look at humans: we’re bipedal, meaning we walk on two legs. Our center of gravity passes vertically down our spine, so we don’t need a tail to counterbalance the weight of our heads.
And, unlike some other primates, we don’t need a tail to help us hold onto stuff while we swing through trees, because as a species we don’t regularly Tarzan our way around the forest anymore. And why have a tail if you don’t use it? It’s just another thing that takes energy from the rest of the body.
But here’s the second answer. Our ancestors did have tails, and, at some point, you had a tail, too. You can find evidence of our 5-limbed past in the skeleton of every human being. Each of us has a coccyx, or tailbone, made of fused vertebrate. In other primates, this coccyx leads to the tail, but, again, we don’t really need it. It’s a vestigial organ.
Now, I know what you’re saying: “C’mon, Cristen! I may not be a doctor, but I’m pretty sure I don’t have a tail.” Well, maybe not now – but you did while you were in the womb.
All mammals have a tail at some point in development. When you were about 30 days old in the womb, you had a tail-like structure sprouting out of your body. If you’re like most people, you reabsorbed this structure as you developed.
It’s extremely rare, but a few modern people have been born with actual tails. This is what’s called an ‘atavism’, a trait of distant ancestors that reappears in the modern day. Usually these tails are a just a few centimeters long, and often removed shortly after birth.
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