我们都听说过一些“返祖现象”，比如某个婴儿身上的毛发特别多，或是长了尾巴；还有本来没有脚的动物比如鲸鱼，却多出了两只像脚一样的东西。那么，”返祖“究竟是怎么回事呢？有人认为这是基因的问题。

Four-year-old Jiaxue, of China, was born with hairy black moles¹ covering parts of her back, chest, neck and face. The media reported her condition as an atavism.
 
Perhaps you've heard of the world's hairiest man. The media loves to show his picture. Newspapers call conditions like this and others atavisms（返祖现象）. Is the world's hairiest man really a throwback to our simian²（猿） ancestors? Not quite. Ape faces are clear of hair, as Michael Le Page notes in his New Scientist article on atavisms. So hypertrichosis（多毛症）, a condition in which excessive amounts of hair cover the body, isn't an atavism -- unless our ancestors were werewolves（狼人）. If it is an avatism, it's not from the animals we point to when we talk about evolution, like chimps³.

What's the difference between atavisms and vestigial（发育不全的，退化器官的） structures? They're actually pretty close. Vestigial structures are body parts that survive as degenerate⁴, imperfect versions of what they should be. Think of the ostrich⁶: It has wings, but it can't fly. Ostriches⁷ use their wings for other purposes, like balance, but their wings can't function as wings. Their wings are vestigial -- they're used for a purpose, but not the purpose for which they were designed.

Atavisms are traits of distant ancestors that reappear in the modern day. In order for the trait to be an atavism, an organism's parents can't have the trait, and neither can recent ancestors. The atavism you've probably heard of most often? The human tail. It's not just a gag to pull in a Farrelly brothers movie -- it really happens. There are two kinds of human tails, however: the pseudo-tail and the much rarer, true human tail. The pseudo-tail doesn't have any bones or cartilage（软骨） -- it's skin and fat. But the true human tail has nerves and muscles and sometimes even cartilage or vertebrae（椎骨）, although there seems to be some scientific debate about this last point. All vertebrates have the ability to make a tail, and guess what? Humans are vertebrates. Way back when, maybe we all had tails, and they provided some important function for us, perhaps to help us balance.

So how do these traits crop up? One idea is that instead of genes⁸ being lost during evolution, they're merely silent - they're still there, they're just not doing anything. But maybe these genes can be turned back on. Studies have shown species that lose and regain⁹ wings millions of years afterward¹⁰. Are these species just evolving once again? Or did a silent gene⁵ labeled "grow wings" suddenly switch back on?

Scientists aren't sure how atavisms work -- not yet. They point to embryos¹¹ to help decipher the answer. Embryos develop characteristics in the womb that later disappear. Whale embryos do grow buds where their legs would be, but they lose them as they develop. When you were a fetus¹², you had a little bud where a tail would be, but it disappeared before you were born. But on rare occasions, they don't disappear, and a baby whale is born with leg-like appendages¹³ or a human baby is born with a tail, which is usually amputated.

So why do atavisms happen? What place do they have in evolution? We don't know the answer. Often when people think of genetic¹⁴ information, they think about cloning, designer children and other controversial topics. But as scientists delve¹⁵ more deeply into our genetic information, this could be one of the most exciting areas of research -- finally figuring out where we came from and how we got to where we are.