第1段

The description of any animal as an ‘evolutionary throwback’ is controversial. 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’. But as more and more examples come to light and modern genetics enters the scene, that principle is having to be rewritten. Not only are evolutionary throwbacks possible, they sometimes play an important role in the forward march of evolution.

    第1段：

将任何动物描述为"进化倒退"都是有争议的。在过去的一个世纪中，大多数生物学家一直不愿使用这些词，意识到进化原理中存在一条"进化不能后退"的规律。但随着越来越多的例子浮出水面，现代遗传学的出现，这一原理不得不被重新书写。进化倒退不仅可能发生，而且有时在进化的前进中起着重要作用。

第2段

The technical term for an evolutionary throwback is an ‘atavism’, from the Latin atavus, meaning forefather. 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.

第2段：

"进化倒退"的技术术语是"复古现象"，源自拉丁语"atavus"，意为祖先。这个词有丑陋的含义，主要要归功于19世纪的意大利医生切萨雷·隆布罗索，他认为罪犯是天生的，而不是后天形成的，并可以通过某些身体特征来辨认出一种倒退到原始、亚人状态的人。

第3段

While Lombroso was measuring criminals, a Belgian palaeontologist called Louis Dollo was studying fossil records and coming to the opposite conclusion. 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’. 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. And so the idea of irreversibility in evolution stuck and came to be known as ‘Dollo’s law’.

第3段：

当隆布罗索在测量罪犯时，比利时的古生物学家路易斯·多洛正在研究化石记录，并得出相反的结论。1890年，他提出进化是不可逆转的观点："一种生物无法返回甚至部分地返回到已经在其祖先中出现的先前阶段"。20世纪初的生物学家得出类似的结论，尽管他们用概率问题来修饰，认为进化不能后退，只是非常不可能。因此，进化不可逆的观念定型，并被称为"多洛定律"。

第4段

If Dollo’s law is right, atavisms should occur only very rarely, if at all. Yet almost since the idea took root, exceptions have been cropping up. 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. Explorer Roy Chapman Andrews argued at the time that the whale must be a throwback to a land-living ancestor. ‘I can see no other explanation,’ he wrote in 1921.

第4段：

如果多洛定律是正确的，复古现象应该非常罕见，如果存在的话。然而，自从这个想法落地以来，就不断出现例外。例如，1919年，在加拿大温哥华岛附近捕获了一只庞大的座头鲸，它有一对长约一米的类似腿的附肢，完整地具有一套肢体骨骼。探险家罗伊·查普曼·安德鲁斯当时认为，这只座头鲸必定是一种退化到陆地生活祖先的复古现象。他在1921年写道："我找不到其他解释"。

第5段

Since then, so many other examples have been discovered that it no longer makes sense to say that evolution is as good as irreversible. 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. 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. If these silent genes are somehow switched back on, they argued, long-lost traits could reappear.

第5段：

自那以后，发现了如此多的其他例子，以至于说进化几乎是不可逆转的已经不再合理。这引发了一个谜题：消失了数百万年的特征如何突然重新出现？1994年，美国印第安纳大学的鲁道夫·拉夫和他的同事决定使用遗传学来计算进化逆转的可能性。他们推断，虽然一些进化变化涉及基因的丧失，因此是不可逆转的，但其他变化可能是由于基因被关闭。他们认为，如果这些沉默基因某种方式重新被激活，久违的特征可能会重新出现。

第6段

Raff’s team went on to calculate the likelihood of it happening. Silent genes accumulate random mutations, they reasoned, eventually rendering them useless. 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 at least a few individuals in a population, and that some might survive as long as 10 million years. In other words, throwbacks are possible, but only to the relatively recent evolutionary past.

第6段：

拉夫的团队接着计算了这种情况发生的可能性。他们推理，沉默基因会积累随机突变，最终使它们失去功能。所以，如果一个基因不再被使用，它在物种中能存活多久呢？该团队计算出，在一个种群中，沉默基因能有很大几率存活长达600万年，有些甚至可能存活1000万年。换句话说，复古现象是可能的，但只限于相对较近的进化历史。

第7段

As a possible example, the team pointed to the mole salamanders of Mexico and California. 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. The simplest explanation for this is that the axolotl lineage alone lost the ability to metamorphose, while others retained it. 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. In other words, metamorphosis in mole salamanders is an atavism. The salamander example fits with Raff’s 10-million-year time frame.

第7段：

作为一个可能的例子，该团队指出了墨西哥和加利福尼亚的鼹蝾螈。像大多数两栖动物一样，它们在幼年时期处于"蝌蚪"阶段，然后发生变态进化为成体形态，但有一种叫做"水獭蝾螈"的物种却以幼年状态一直生活。最简单的解释是，水獭蝾螈的祖先是唯一失去了变态能力的，而其他物种保留了这种能力。然而，通过对蝾螈家族树的详细分析，可以明确的是，其他物种是从已经失去变态能力的祖先进化而来的。换句话说，在鼹蝾螈中，变态是一种复古现象。这个鼹蝾螈的例子符合拉夫提出的1000万年的时间范围。

第8段

More recently, however, examples have been reported that break the time limit, suggesting that silent genes may not be the whole story. 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. 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. Other species, however, sport up to four toes on their hind legs. The simplest explanation is that the toed lineages never lost their toes, but Wagner begs to differ. 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.

第8段：

然而，最近报道的例子打破了这个时间限制，表明沉默基因可能并不是全部的答案。去年，耶鲁大学的生物学家冈特·瓦格纳报告了其对一群南美蜥蜴巴奇亚的进化历史的研究工作。其中许多蜥蜴具有微小的四肢；有些看起来更像蛇而不是蜥蜴，还有几个完全失去了后肢上的趾。然而，其他物种的后肢上有最多四个趾。最简单的解释是，有趾的物种从未失去它们的趾，但瓦格纳对此持不同看法。根据他对巴奇亚家族树的分析，有趾的物种重新从无趾的祖先进化出了趾，而且在数千万年间，趾的损失和增长发生了不止一次。

第9段

So what’s going on? 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. 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. These atavistic traits provided an advantage and spread through the population, effectively reversing evolution.

第9段：

那么到底发生了什么呢？一个可能性是这些特征被失去然后再次出现，就像无关物种中可能独立出现类似结构一样，例如鲨鱼和虎鲸的背鳍。另一个更有趣的可能性是，构成这些物种趾的遗传信息在蜥蜴中以某种方式存活了数千万甚至数亿年，并被重新激活。这些复古现象提供了优势，并通过物种中传播，有效地逆转了进化。

第10段

But if silent genes degrade within 6 to 10 million years, how can long-lost traits be reactivated over longer timescales? The answer may lie in the womb. Early embryos of many species develop ancestral features. Snake embryos, for example, sprout hind limb buds. Later in development, these features disappear thanks to developmental programs that say ‘lose the leg’. If for any reason this does not happen, the ancestral feature may not disappear, leading to an atavism.

第10段：

但是，如果沉默基因在6到10百万年内会退化，那么如何在更长时间尺度上重新激活久违的特征呢？答案可能在胎儿中。许多物种的早期胚胎会发展出祖先特征。例如，蛇的胚胎会长出后肢芽。然而，在发育的后期，这些特征会消失，这要归功于发育程序的作用，即"丢失腿部"。如果由于任何原因，这一过程没有发生，祖先特征就不会消失，导致了复古现象的发生。