A   The passenger pigeon was a legendary species. Flying in vast numbers across North America, with potentially many millions within a single flock, their migration was once one of nature's great spectacles. Sadly, the passenger pigeon's existence came to an end on 1 September 1914, when the last living specimen died at Cincinnati Zoo. Geneticist Ben Novak is lead researcher on an ambitious project which now aims to bring the bird back to life through a process known as 'de-extinction'. The basic premise involves using cloning technology to turn the DNA of extinct animals into a fertilised embryo, which is carried by the nearest relative still in existence -- in this case, the abundant band-tailed pigeon -- before being born as a living, breathing animal. Passenger pigeons are one of the pioneering species in this field, but they are far from the only ones on which this cutting-edge technology is being trialled.

A   候鸽是一个传奇的物种。他们的迁徙曾经是大自然的奇观之一，他们大规模地飞越北美洲，一次迁徙数量可达到数百万只。可悲的是，候鸽于1914年9月1日灭绝了，当时最后一个活着的标本在辛辛那提动物园死亡。遗传学家本·诺瓦克是项目首席研究员，该项目极具野心，目前旨在通过名为“灭绝物种复活“的方式使这种鸟类复活。基本前提是使用克隆技术将灭绝动物的DNA转换为受精胚胎，在出生之前，由存活的近亲动物携带胚胎一在这个案例中是用数量充足的斑尾鸽。候鸽是该领域的先驱物种之一，但它们并非是被用来试验这种尖端技术的唯一物种。

B   In Australia, the thylacine, more commonly known as the Tasmanian tiger, is another extinct creature which genetic scientists are striving to bring back to life. 'There is no carnivore now in Tasmania that fills the niche which thylacines once occupied,' explains Michael Archer of the University of New South Wales. He points out that in the decades since the thylacine went extinct, there has been a spread in a 'dangerously debilitating' facial tumour syndrome which threatens the existence of the Tasmanian devils, the island's other notorious resident. Thylacines would have prevented this spread because they would have killed significant numbers of Tasmanian devils. 'If that contagious cancer had popped up previously, it would have burned out in whatever region it started. The return of thylacines to Tasmania could help to ensure that devils are never again subjected to risks of this kind.'

B   在澳大利亚，袋狼一俗称塔斯马尼亚虎，是另一种已经灭绝的生物，遗传学家正在努力让它复活。新南威尔士大学的迈克尔·阿彻解释说:“塔斯马尼亚州现在没有食肉动物可以填补曾经被袋狼占据的生态龛。“他指出，自袋狼灭绝以来的几十年中，一种让动物衰弱的面部肿瘤综合症正在蔓延，威胁着袋獾的存在--袋獾是岛上另一位臭名昭著的居民。袋狼能够阻止这种疾病传播，因为它们可以杀死大量的袋獾。“如果这种传染性癌症以前曾出现过，那么它将会在其开始蔓延的地方就消失殆尽。袋狼的回归可有助于确保塔斯马尼亚州的袋獾不再遭受这种风险。

C   If extinct species can be brought back to life, can humanity begin to correct the damage it has caused to the natural world over the past few millennia? 'The idea of de-extinction is that we can reverse this process, bringing species that no longer exist back to life,' says Beth Shapiro of University of California Santa Cruz's Genomics Institute. 'I don't think that we can do this. There is no way to bring back something that is 100 per cent identical to a species that went extinct a long time ago.' A more practical approach for long-extinct species is to take the DNA of existing species as a template, ready for the insertion of strands of extinct animal DNA to create something new; a hybrid, based on the living species, but which looks and/or acts like the animal which died out.

C   如果灭绝的物种能够重获新生，人类能否开始纠正过去几千年来对自然界造成的破坏?加州大学圣克鲁斯分校的基因组学研究所的贝丝·夏皮罗说:“灭绝物种复活是指我们可以逆转这一过程，让不复存在的物种复活。但我认为我们无法做到这一点。我们没有办法完全复制一些很久以前就灭绝的物种。“更现实的方法是，以现有物种的DNA作为模板，结合灭绝物种的基因链，来创造新的物种;一种基于现有生物物种，但其外观和/或行为类似于灭绝的动物的杂交物种。

D   This complicated process and questionable outcome begs the question: what is the actual point of this technology? 'For us, the goal has always been replacing the extinct species with a suitable replacement,' explains Novak. 'When it comes to breeding, band-tailed pigeons scatter and make maybe one or two nests per hectare, whereas passenger pigeons were very social and would make 10,000 or more nests in one hectare.' Since the disappearance of this key species, ecosystems in the eastern US have suffered, as the lack of disturbance caused by thousands of passenger pigeons wrecking trees and branches means there has been minimal need for regrowth. This has left forests stagnant and therefore unwelcoming to the plants and animals which evolved to help regenerate the forest after a disturbance. According to Novak, a hybridised band-tailed pigeon, with the added nesting habits of a passenger pigeon, could, in theory, re-establish that forest disturbance, thereby creating a habitat necessary for a great many other native species to thrive.

D   这一复杂的过程以及令人争议的结果抛出了一个问题:这项技术的真正意义是什么?诺瓦克解释说:“对我们而言，目标是用合适的替代品来替代灭绝的物种。从繁殖方面来讲，斑尾鸽是散居动物，每公顷可筑巢一到两个，而候鸽非常适合群居，每公顷可筑巢一万个或更多。“自从这种关键物种消失之后，美国东部的生态系统就遭受了重创，没了成千上万的旅鸽对树木和枝条进行破坏，这意味着树木再生需求很少。这使得森林的发展停滞不前，也少了很多在破坏后帮助森林进化再生的动植物。根据诺瓦克的说法，从理论上讲，杂交斑尾鸽具有候鸽的筑巢习性，可以重新建立起对森林的干扰，从而创造出许多其他本土物种繁衍生息的栖息地。

E   Another popular candidate for this technology is the woolly mammoth. George Church, professor at Harvard Medical School and leader of the Woolly Mammoth Revival Project, has been focusing on cold resistance, the main way in which the extinct woolly mammoth and its nearest living relative, the Asian elephant, differ. By pinpointing which genetic traits made it possible for mammoths to survive the icy climate of the tundra, the project's goal is to return mammoths, or a mammoth- like species, to the area. 'My highest priority would be preserving the endangered Asian elephant,' says Church, 'expanding their range to the huge ecosystem of the tundra. Necessary adaptations would include smaller ears, thicker hair, and extra insulating fat, all for the purpose of reducing heat loss in the tundra, and all traits found in the now extinct woolly mammoth.' This repopulation of the tundra and boreal forests of Eurasia and North America with large mammals could also be a useful factor in reducing carbon emissions -- elephants punch holes through snow and knock down trees, which encourages grass growth. This grass growth would reduce temperatures, and mitigate emissions from melting permafrost.

E   这项技术中另一个受欢迎的候选物种是猛犸象，哈佛医学院教授，猛犸象复活计划的负责人乔治·丘奇一直致力于研究抗寒性，这是区分绝种的猛犸象及其最近的近亲亚洲象的主要方式。通过精确定位哪些基因特征使得猛犸象能够在冻土带的冰冷气候中生存下来，该项目的目标是将猛犸象或类猛犸象物种送回冰冷气候地区。丘奇说:“我的首要任务是保护濒临灭绝的亚洲象，将其活动范围扩大到苔原的巨大生态系统。必要的调整包括较小的耳朵，浓密的毛发和额外的绝缘脂肪，所有这些目的都是为了减少象在苔原的热量散失，而这些所有特征发现都源于现已灭绝的猛犸象。“还原在欧亚大陆和北美地区的苔原及北方森林的哺乳动物数量也可能有利于减少碳排放一大象在雪地上打洞并击倒树木，从而促进了草的生长。这种草的生长可以降低温度并减少永久冻土融化所产生的排放。

F   While the prospect of bringing extinct animals back to life might capture imaginations, it is, of course, far easier to try to save an existing species which is merely threatened with extinction. 'Many of the technologies that people have in mind when they think about de-extinction can be used as a form of "genetic rescue",' explains Shapiro. She prefers to focus the debate on how this emerging technology could be used to fully understand why various species went extinct in the first place, and therefore how we could use it to make genetic modifications which could prevent mass extinctions in the future. 'I would also say there's an incredible moral hazard to not do anything at all,' she continues. 'We know that what we are doing today is not enough, and we have to be willing to take some calculated and measured risks.'

F   虽然复活已灭绝的动物的前景会激发人们的想象，但当然，试图拯救一个仅面临灭绝威胁的现有物种要容易得多。夏皮罗解释说:“人们在考虑反灭绝时会想到的许多技术都可以用作“基因拯救”。她倾向于通过探讨如何利用这种新兴技术，以便充分理解各种物种灭绝的原因，从而得知我们要如何利用它进行遗传修饰，以防止未来的大规模灭绝。她又说:“我也要补充，什么都不做存在着很大的道德风险。我们知道，我们如今做的还不够，我们必须承担一些经过计算和衡量的风险。