Subtitles section Play video Print subtitles Narrator: Tails are like opinions. Basically, everyone has them: fish, birds, most reptiles, and even some of our closest relatives. So why are we missing out? 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 apes, including us, of course. To understand why, take a look at how we walk. Some of us primates crouch with our chest 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 have to pour energy into every step they take, two legs take advantage of gravity, which does some of the work for us. You see, each time we take a step, gravity pulls us forward. The end result is that when we walk, we use around 25% less energy than walking on all fours. And in the wild, every ounce of energy you save can mean the difference between survival and starvation. But this way of getting around also totally 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. Pretty disappointing, huh? That being said, 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 actually tumors, 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 tails 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 mosquitoes away, we are who we are today because, well, we don't.