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As far as appendages go, 27 00:01:36,120 --> 00:01:39,800 Speaker 1: tails are pretty much amazing. Over time, different animals have 28 00:01:39,840 --> 00:01:44,520 Speaker 1: evolved various highly specialized tails. A horse uses its tail 29 00:01:44,560 --> 00:01:47,080 Speaker 1: to swap flies, for instance, while a bird uses its 30 00:01:47,080 --> 00:01:50,880 Speaker 1: tail to steer during flight, which leads us to today's question. 31 00:01:51,360 --> 00:01:55,480 Speaker 1: If these specialized limbs are so useful, why don't humans 32 00:01:55,480 --> 00:01:59,320 Speaker 1: have them? Why don't people you know me, you, Benedict 33 00:01:59,320 --> 00:02:04,559 Speaker 1: cumber Patch, Why don't we have tails? Well there's two answers. First, 34 00:02:04,880 --> 00:02:08,160 Speaker 1: we don't really need them. In many quadrupedal or four 35 00:02:08,240 --> 00:02:11,239 Speaker 1: legged creatures like a cat, for example, a tail helps 36 00:02:11,280 --> 00:02:14,720 Speaker 1: with balance. Fish and marine mammals, on the other hand, 37 00:02:15,040 --> 00:02:19,079 Speaker 1: use their tails for steering or look emotion. Some lizards 38 00:02:19,080 --> 00:02:22,679 Speaker 1: and primates use their prehensile tails to grip things, while 39 00:02:22,720 --> 00:02:26,240 Speaker 1: crocodiles store fatten their tails, kind of similar to the 40 00:02:26,280 --> 00:02:30,080 Speaker 1: way camels store fat reserves in their humps. But let's 41 00:02:30,080 --> 00:02:33,400 Speaker 1: look at humans. We're bipedal, meaning we walk on two legs. 42 00:02:33,680 --> 00:02:36,920 Speaker 1: Our center of gravity passes vertically down our spines, so 43 00:02:36,960 --> 00:02:39,359 Speaker 1: we don't need a tail to counterbalance the weight of 44 00:02:39,400 --> 00:02:43,200 Speaker 1: our heads. And unlike some other primates, we don't need 45 00:02:43,240 --> 00:02:45,800 Speaker 1: a tail to help us hold onto stuff while we 46 00:02:45,880 --> 00:02:49,440 Speaker 1: swing through trees, because as a species, we don't regularly 47 00:02:49,520 --> 00:02:52,680 Speaker 1: tarzan our way around the forest anymore. And why have 48 00:02:52,760 --> 00:02:55,280 Speaker 1: a tail if you don't use it. It's just another 49 00:02:55,320 --> 00:02:57,840 Speaker 1: thing that takes energy away from the rest of your body. 50 00:02:58,400 --> 00:03:01,799 Speaker 1: And as our ancestors evolved away from an arboreal lifestyle, 51 00:03:01,919 --> 00:03:05,519 Speaker 1: a tail just became less and less useful. But here's 52 00:03:05,560 --> 00:03:10,280 Speaker 1: the second answer. Our ancestors did have tails, and at 53 00:03:10,320 --> 00:03:13,520 Speaker 1: some point you had a tale too. You can find 54 00:03:13,600 --> 00:03:17,480 Speaker 1: evidence of our five limbed past in the skeleton of 55 00:03:17,639 --> 00:03:21,160 Speaker 1: every human being. Each of us has a co six 56 00:03:21,360 --> 00:03:25,880 Speaker 1: or tailbone, made of fused vertebrate and other primates. This 57 00:03:26,040 --> 00:03:29,079 Speaker 1: co six leads to the tail, but again, we don't 58 00:03:29,080 --> 00:03:32,080 Speaker 1: really need it. It's a vestigial organ. Now I know 59 00:03:32,120 --> 00:03:34,200 Speaker 1: what you're saying, Come on, Christian, I may not be 60 00:03:34,280 --> 00:03:38,160 Speaker 1: a doctor, but I'm pretty sure I don't have a tail. Well, 61 00:03:38,200 --> 00:03:40,440 Speaker 1: maybe not now, but you did while you were in 62 00:03:40,440 --> 00:03:43,400 Speaker 1: the womb. All mammals have a tail at some point 63 00:03:43,440 --> 00:03:46,480 Speaker 1: in development. When you were about thirty days old in 64 00:03:46,520 --> 00:03:49,360 Speaker 1: the womb, you had a tail like structure sprouting out 65 00:03:49,360 --> 00:03:52,040 Speaker 1: of your body, and if you're like most people, you 66 00:03:52,160 --> 00:03:56,760 Speaker 1: reabsorbed the structure as you developed. It's extremely rare, but 67 00:03:56,840 --> 00:04:00,480 Speaker 1: a few modern people have been born with actual tales. 68 00:04:01,000 --> 00:04:04,360 Speaker 1: This is what's called an atavism, a trait of distant 69 00:04:04,400 --> 00:04:09,120 Speaker 1: ancestors that reappears in the modern day. Usually, these tails 70 00:04:09,160 --> 00:04:13,119 Speaker 1: are just a few centimeters long and often removed shortly 71 00:04:13,240 --> 00:04:20,640 Speaker 1: after Burton. Check out the brainstuff channel on YouTube, and 72 00:04:20,680 --> 00:04:23,120 Speaker 1: for more on this and thousands of other topics, visit 73 00:04:23,120 --> 00:04:38,320 Speaker 1: how stuff works dot com.