Do you have a tail？ Of course not， we know． But tails are like opinions． actually， everyone has them： fishes， birds， most reptiles， and even some of our closest relatives．

Since tails first evolved（进化） at least 500 million years ago． Geckos （壁虎）use them to store fat， birds use them to steer through the air． But for most mammals （哺乳动物），（1） 　　Yet， as you get closer to humans on the evolutionary tree， tails disappear， Gorillas（大猩猩）don' t have them， and neither do chimps（黑猩猩） or any other apes （类人猿）， including us， of course． To understand why， take a look at how we walk．

（2） 　　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（重力），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 this way of getting around also totally eliminates（淘汰） the need for a tail．

It's said that you can still see a reminder of a time when our ancient ancestors had one． Just look at a human spine（脊椎）．（3） 　　That's your tailbone． It's all that's the rest of our tail．

Now， babies are born with what looks like a tail， but that's not what's really going on． Most often， these tails are actually a parasitic twin（寄生物）． These types of tails usually form as a birth defect（瑕疵）．（4） 　　

So the tail of humans has gone， it's the result of evolution millions of years Will humans be much taller， stronger or smarter in the future？

A． Nowadays， humans can walk completely upright． B． And in these situations， doctors will advise to remove the tails with no harm to the babies． C． they serve one major purpose﹣﹣balance． D． You can see how the last few bones are partly fused（融合） together．