• Most birds, mammals, reptiles, and even fish have tails.
  • But humans and other apes don't, even though our close primate relatives do.
  • That's because while most mammals use their tails for balance, we don't walk on four legs. So we don't need them.
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Since tails first evolved at least 500 million years ago, they've taken on every role imaginable. Geckos use them to store fat. Birds use them to steer through the air. And rattlesnakes use them to scare off predators. But for most mammals, they serve one major purpose: Balance.

Yet, as you get closer to humans on the evolutionary tree, tails disappear. Gorillas don't have them. And neither do chimps or any other ape.

To understand why, you need to look at how we walk. Some primates crouch with our chests held diagonally to the ground. Others, like gibbons and humans, can walk completely upright.

Now, walking like this gives us a huge advantage Because unlike four-legged animals, which must pour energy into every step they take, two legs take advantage of gravity, which does some of the work for us. Each time we take a step, gravity pulls us forward. The result is that when we walk, we use around 25% less energy than walking on all fours.

In the wild, every ounce of energy you save can mean the difference between survival and starvation. But this way of getting around also eliminates the need for a tail. Because even though a human head weighs a hefty 5 kilograms, it sits on top of the body when you walk - not in front.

So, you don't need a tail as a counterbalance.

You can still see a reminder of a time when our ancient primate ancestors had one. Just look at a human spine. You can see how the last few bones are partially fused together. That's your tailbone - it's all that's left of our tail. And yes, it's sad and pathetic, and you can't wag it.

Now, in rare cases, babies are born with what looks like a tail. But that's not what's really going on. Most often, these "tails" are tumours, cysts, or even a parasitic twin. Even more occasionally, they're a true outgrowth of the spine, but are completely boneless, a soft tube made entirely of fat and tissue. These types of tails usually form as a birth defect, a deformity of the spine called spina bifida. And in these situations, doctors will surgically remove the tail with no harm to the baby.

But as cool as it might sound to have an extra limb to swing through the trees or keep mosquitos away, we are who we are today because, well, we don't.

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