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  • 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.

Narrator: Tails are like opinions.

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B1 US tail spine walk energy gravity fat

Why Humans Don’t Have Tails

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    Liang Chen posted on 2019/05/08
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