The description of any animal as an ' evolutionary throwback’ is controversial. 把任何一种动物描述成“进化返祖”是有争议的。 For the better part of a century, most biologists have been reluctant to use those words, mindful of a principle of evolution that says ‘ evolution cannot run backwards’. 近一个世纪以来，大部分生物学家不情愿去使用这个词，因为他们信守着“进化是不可逆转的”这一进化原理。 But as more and more examples come to light and modern genetics enters the scene,that principle is having to be rewritten. 但是随着越来越多的返祖例子为人所知以及现代基因学的出现，这个原理就必须被重写。 Not only are evolutionary throwbacks possible, they sometimes play an important role in the forward march of evolution. 进化逆转现象不仅是可能的，而且在进化的前进过程中有时还会起到重要的作用。

The technical term for an evolutionary throwback is an ‘atavism’, from the Latin atavus, meaning forefather. 进化逆转的专业术语是“返祖”，该词来自于拉丁语的“返祖”，意指“祖先”。 The word has ugly connotations thanks largely to Cesare Lombroso, a 19th-century Italian medic who argued that criminals were born not made and could be identified by certain physical features that were throwbacks to a primitive, sub-human state. 这个词的负面含义在很大程度上是源自 Cesare Lombroso，一位 19 世纪的意大利医师，他认为罪犯是天生的而不是后天的，而且可以通过某些返祖到原始的类人状态的体貌特征来识别。

While Lombroso was measuring criminals, a Belgian palaeontologist called Louis Dollo was studying fossil records and coming to the opposite conclusion. 当 Lombroso 在测定罪犯的时候，一位叫做 Louis Dollo 的比利时古生物学家在研究化石记录，并且得出了相反的结论。 In 1890 he proposed that evolution was irreversible: that ‘an organism is unable to return, even partially, to a previous stage already realised in the ranks of its ancestors’. 他在 1890 年提出了进化是不可逆转的：“一个生物不可能倒退（即使是局部地）到以前已经在祖先中出现过的阶段。 Early 20th-century biologists came to a similar conclusion, though they qualified it in terms of probability, stating that there is no reason why evolution cannot run backwards—it is just very unlikely. ”20 世纪早期的生物学家得出了一个相似的结论，虽然他们用可能性来限制这个结论，这个结论认为没有进化不能逆转的理由，只是这种情况极不可能。 And so the idea of irreversibility in evolution stuck and came to be known as ‘Dollo’s law’. 所以进化的不可逆性的想法搁浅了，并且被称为“多洛定律”。

If Dollo’s law is right, atavisms should occur only very rarely, if at all. 如果多洛定律是正确的，那么即便有返祖现象的出现，概率也应该极低。 Yet almost since the idea took root, exceptions have been cropping up. 但是几乎从这个理念建立开始，例外就层出不穷。 In 1919, for example, a humpback whale with a pair of leg-like appendages over a metre long, complete with a full set of limb bones, was caught off Vancouver Island in Canada. 比如说，在 1919 年，一头有着一对一米多长的腿状物而且有一副完整四肢骨架的座头鲸在加拿大温哥华岛被抓到了。 Explorer Roy Chapman Andrews argued at the time that the whale must be a throwback to a land-living ancestor. 探险家 Roy Chapman 那时认为这头鲸鱼肯定是对一个陆生祖先的返祖。 ‘I can see no other explanation,’ he wrote in 1921. “我找不到其他解释”，他在1921年写道。

Since then, so many other examples have been discovered that it no longer makes sense to say that evolution is as good as irreversible. 从那以后，很多其他例子被发现了，以至于再说进化不可逆就没有意义了。 And this poses a puzzle: how can characteristics that disappeared millions of years ago suddenly reappear? In 1994, Rudolf Raff and colleagues at Indiana University in the USA decided to use genetics to put a number on the probability of evolution going into reverse. 这就提出了一个疑问：那些几百万年前就消失的特征怎么会突然间重新出现呢？在 1994 年，Rudolf Raff 以及美国印第安纳大学的同事决定使用基因学来用数字表明进化逆转的可能。 They reasoned that while some evolutionary changes involve the loss of genes and are therefore irreversible, others may be the result of genes being switched off. 他们推论出虽然一些进化的改变包含基因的丢失并因此不可逆，其他改变可能是基因不活动的结果。 If these silent genes are somehow switched back on, they argued, long-lost traits could reappear. 他们认为如果这些不活动基因重新活跃起来，丢失已久的特征就会重新出现。

Raff's team went on to calculate the likelihood of it happening. Raff 的团队继续计算这种情况出现的可能性。 Silent genes accumulate random mutations, they reasoned, eventually rendering them useless. 他们推论不活动基因不断随机突变，最终致使它们变得没有用。 So how long can a gene survive in a species if it is no longer used? The team calculated that there is a good chance of silent genes surviving for up to 6 million years in e a few individuals in a population, and that some might survive as long as 10 million years. 所以如果一个基因不再被使用，它能在一个物种中存活多久呢？这个团队计算出不活动基因极有可能在一个种群的部分个体中存活长达 600 万年之久，并且有些不活动基因甚至能存活长达千万年之久。 In other words, throwbacks are possible, but only to the relatively recent evolutionary past. 换句话说，逆转是有可能的，但仅限于相对较近的进化过程。

As a possible example, the team pointed to the mole salamanders of Mexico and California. 这个团队提出了拿墨西哥和加利福尼亚的鼹鼠蝾螈作为一个可能的例子。 Like most amphibians these begin life in a juvenile ‘tadpole’ state, then metamorphose into the adult form—except for one species, the axolotl, which famously lives its entire life as a juvenile. 像很多两栖动物一样，他们以一种幼体“蝌蚪”的形式开始生命，然后变形成成年形式——除了一个物种，美西螈，它们知名于以一种幼体的形式持续一生。 The simplest explanation for this is that the axolotl lineage alone lost the ability to metamorphose, while others retained it. 最简单的解释就是只有美西螈家系失去了变形能力，而其他家系都保持着这种能力。 From a detailed analysis of the salamanders' family tree, however, it is clear that the other lineages evolved from an ancestor that itself had lost the ability to metamorphose. 然而，从一个详细的蝾螈的家谱分析可以看出，其他家系是从一个本身就失去变形能力的祖先进化而来的。 In other words, metamorphosis in mole salamanders is an atavism. 换句话说，鼹鼠蝾螈的变形就是返祖。 The salamander example fits with Raff’s 10-million-year time frame. 这个蝾螈的例子符合 Raff 的千万年时间框架的理论。

More recently, however, examples have been reported that break the time limit, suggesting that silent genes may not be the whole story. 然而，最近有一些打破了这个时间限制的例子被报道，表明不活动基因可能不是唯一的因素。 In a paper published last year, biologist Gunter Wagner of Yale University reported some work on the evolutionary history of a group of South American lizards called Bachia. 在去年发表的一篇论文中，耶鲁大学的生物学家 Gunter Wagner 报道了一些关于一群被称为 Bachia 的南美蜥蜴的进化史的工作。 Many of these have minuscule limbs; some look more like snakes than lizards and a few have completely lost the toes on their hind limbs. 这些蜥蜴大部分有极小的四肢；一些看上去更像蛇而不是蜥蜴，还有一些完全失去了后腿的脚趾。 Other species, however, sport up to four toes on their hind legs. 然而，另一些物种突变到了后腿有四个脚趾。 The simplest explanation is that the toed lineages never lost their toes, but Wagner begs to differ. 最简单的解释是有脚趾的家系从未失去脚趾，但 Wagner 不同意这个解释。 According to his analysis of the Bachia family tree, the toed species re-evolved toes from toeless ancestors and, what is more, digit loss and gain has occurred on more than one occasion over tens of millions of years. 根据他对 Bachia 家谱的分析，有脚趾的物种从没有脚趾的祖先重新进化出了脚趾，另外，趾头的得到和失去在几千万年间不止出现了一次。

So what's going on? 所以发生了什么呢？ One possibility is that these traits are lost and then simply reappear, in much the same way that similar structures can independently arise in unrelated species, such as the dorsal fins of sharks and killer whales. 一个可能性就是这些特征失去了，然后又重现了，和相似结构可以独立出现于不相关物种的方式几乎相同，比如鲨鱼和虎鲸的背鳍。 Another more intriguing possibility is that the genetic information needed to make toes somehow survived for tens or perhaps hundreds of millions of years in the lizards and was reactivated. 另外一个更加有趣的可能性是产生脚趾所需的基因信息在蜥蜴中存活数千万甚至上亿年，然后被重新激活了。 These atavistic traits provided an advantage and spread through the population, effectively reversing evolution. 这些返祖特征提供了一个优势并且传播到整个物种，有效地逆转了进化。

But if silent genes degrade within 6 to 10 million years, how can long-lost traits be reactivated over longer timescales? 但是如果不活动基因在 600~1000 年间退化，这些久失的特征又如何在更长的时间尺度内被重新激活呢？ The answer may lie in the womb. 答案可能存在于子宫中。 Early embryos of many species develop ancestral features. 很多物种的早期胚胎会发展祖先的特征。 Snake embryos, for example, sprout hind limb buds. 比如说，蛇的胚胎萌发了后肢芽。 Later in development these features disappear thanks to developmental programs that say ‘lose the leg’. 后来，这些特征因为“失去腿”的发展程序而消失了。 If for any reason this does not happen, the ancestral feature may not disappear, leading to an atavism. 如果因为某个原因这种现象没有发生，这种祖先的特征就可能不会消失，产生返祖。