WEBVTT - Why Don't Humans Have Tails?

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<v Speaker 1>Hey, everybody. As you might be able to tell from

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<v Speaker 1>That's one more time the Great Courses plus dot Com

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<v Speaker 1>slash brain Stuff. Welcome to brain Stuff from How Stuff Works. Hey,

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<v Speaker 1>brain Stuff, it's Christian Seger. As far as appendages go,

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<v Speaker 1>tails are pretty much amazing. Over time, different animals have

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<v Speaker 1>evolved various highly specialized tails. A horse uses its tail

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<v Speaker 1>to swap flies, for instance, while a bird uses its

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<v Speaker 1>tail to steer during flight, which leads us to today's question.

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<v Speaker 1>If these specialized limbs are so useful, why don't humans

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<v Speaker 1>have them? Why don't people you know me, you, Benedict

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<v Speaker 1>cumber Patch, Why don't we have tails? Well there's two answers. First,

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<v Speaker 1>we don't really need them. In many quadrupedal or four

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<v Speaker 1>legged creatures like a cat, for example, a tail helps

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<v Speaker 1>with balance. Fish and marine mammals, on the other hand,

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<v Speaker 1>use their tails for steering or look emotion. Some lizards

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<v Speaker 1>and primates use their prehensile tails to grip things, while

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<v Speaker 1>crocodiles store fatten their tails, kind of similar to the

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<v Speaker 1>way camels store fat reserves in their humps. But let's

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<v Speaker 1>look at humans. We're bipedal, meaning we walk on two legs.

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<v Speaker 1>Our center of gravity passes vertically down our spines, so

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<v Speaker 1>we don't need a tail to counterbalance the weight of

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<v Speaker 1>our heads. And unlike some other primates, we don't need

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<v Speaker 1>a tail to help us hold onto stuff while we

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<v Speaker 1>swing through trees, because as a species, we don't regularly

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<v Speaker 1>tarzan our way around the forest anymore. And why have

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<v Speaker 1>a tail if you don't use it. It's just another

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<v Speaker 1>thing that takes energy away from the rest of your body.

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<v Speaker 1>And as our ancestors evolved away from an arboreal lifestyle,

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<v Speaker 1>a tail just became less and less useful. But here's

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<v Speaker 1>the second answer. Our ancestors did have tails, and at

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<v Speaker 1>some point you had a tale too. You can find

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<v Speaker 1>evidence of our five limbed past in the skeleton of

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<v Speaker 1>every human being. Each of us has a co six

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<v Speaker 1>or tailbone, made of fused vertebrate and other primates. This

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<v Speaker 1>co six leads to the tail, but again, we don't

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<v Speaker 1>really need it. It's a vestigial organ. Now I know

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<v Speaker 1>what you're saying, Come on, Christian, I may not be

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<v Speaker 1>a doctor, but I'm pretty sure I don't have a tail. Well,

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<v Speaker 1>maybe not now, but you did while you were in

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<v Speaker 1>the womb. All mammals have a tail at some point

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<v Speaker 1>in development. When you were about thirty days old in

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<v Speaker 1>the womb, you had a tail like structure sprouting out

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<v Speaker 1>of your body, and if you're like most people, you

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<v Speaker 1>reabsorbed the structure as you developed. It's extremely rare, but

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<v Speaker 1>a few modern people have been born with actual tales.

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<v Speaker 1>This is what's called an atavism, a trait of distant

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<v Speaker 1>ancestors that reappears in the modern day. Usually, these tails

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<v Speaker 1>are just a few centimeters long and often removed shortly

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<v Speaker 1>after Burton. Check out the brainstuff channel on YouTube, and

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<v Speaker 1>for more on this and thousands of other topics, visit

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<v Speaker 1>how stuff works dot com.