段落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.

段落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.’

段落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.

段落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.

段落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.

段落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.’

候鸽是一种传奇的物种。它们曾经在北美大陆上以极其庞大的数量飞翔，一个群体中可能有数百万只鸟。它们的迁徙曾经是自然界的壮观景象之一。然而，候鸽的存在在1914年9月1日结束，当时最后一只活体样本在辛辛那提动物园死亡。遗传学家本·诺瓦克（Ben Novak）是一个雄心勃勃的项目的首席研究员，该项目旨在通过一种称为“复活”的过程将这种鸟类再次带回生活。基本的原理是利用克隆技术将灭绝动物的DNA转化为受精卵，然后由最接近的现存亲属（在这种情况下是丰富的带尾鸽）携带，最终以一个有生命、有呼吸的动物诞生。候鸽是这个领域的先驱物种之一，但它们远远不是唯一一个进行试验的尖端技术。

在澳大利亚，袋狼，更常被称为塔斯曼尼亚虎，是另一种灭绝动物，遗传科学家正努力将其复活。新南威尔士大学的迈克尔·阿彻（Michael Archer）解释说：“塔斯曼尼亚没有肉食动物填补袋狼曾经占据的生态位。”他指出，自袋狼灭绝以来的几十年里，塔斯曼尼亚的恶性面部肿瘤综合征蔓延开来，威胁到该岛的另一种臭名昭著的居民——袋熊的生存。袋狼本来会阻止这种蔓延，因为它们会杀死大量的袋熊。阿彻说：“如果这种传染性癌症此前出现过，它将在它开始的任何地区消失。袋狼回到塔斯曼尼亚可能有助于确保袋熊再也不会遭受这种风险。”

如果已经灭绝的物种可以复活，人类是否可以开始纠正过去几千年对自然界造成的破坏呢？加州大学圣塔克鲁兹分校的贝丝·夏皮罗（Beth Shapiro）说：“复活的想法是我们可以扭转这个过程，将不再存在的物种复活。”她接着说：“我不认为我们可以做到这一点。没有办法让某物回到过去灭绝的物种的100%相同。”对于已经灭绝的物种，一个更实际的方法是以现存物种的DNA为模板，准备好插入灭绝动物DNA链条，创造出一种看起来和/或表现出已经灭绝动物的特征的新物种；这种新物种是基于现存物种的，但却看起来像是灭绝了的动物。

这个复杂的过程和可疑的结果引发了一个问题：这项技术的实际义是什么？诺瓦克解释说：“对我们来说，目标一直是用合适的替代物种取代灭绝的物种。”他继续说：“在繁殖方面，带尾鸽会分散并在每公顷土地上建造一到两个巢穴，而候鸽非常社交，会在一个公顷土地上建造1万个或更多个巢穴。”自从这个关键物种消失以来，美国东部的生态系统遭受了破坏，因为成千上万只候鸽破坏树木和树枝所带来的扰动减少了，导致树林生长减缓。这使得森林停滞不前，对于进化来帮助森林在扰动之后再生的植物和动物来说，森林变得不适宜。诺瓦克表示，一个混合的带尾鸽，具有候鸽的筑巢习性，理论上可以重新建立森林的扰动，从而为许多其他本土物种提供必要的栖息地。

猛犸象是另一个受欢迎的候选物种。哈佛医学院的教授乔治·丘奇（George Church）是猛犸象复兴项目的领导者，他一直致力于研究寒冷适应性，这是灭绝的猛犸象和其最近的现存亲属——亚洲象的主要差异。通过确定哪些遗传特征使猛犸象能够在冰冻的苔原气候中生存，该项目的目标是将猛犸象或类似猛犸象的物种重新引入该地区。丘奇说：“我最优先考虑的是保护濒危的亚洲象，将它们的范围扩大到巨大的苔原生态系统。必要的适应性包括较小的耳朵、更厚的毛发和额外的绝缘脂肪，这些特征都存在于现已灭绝的猛犸象中，目的是减少苔原地区的热量损失。”在欧亚大陆和北美的苔原和针叶林地区重新引入大型哺乳动物也可能有助于减少碳排放，因为大象可以在雪地上开辟道路并击倒树木，这有助于草地生长。草地的生长将降低温度，并减轻融化的永久冻土带来的排放。

虽然复活灭绝动物的前景可能引起人们的想象力，但当然，试图拯救那些仅面临灭绝威胁的现存物种要容易得多。夏皮罗解释说：“当人们谈到复活时，他们心目中所想的许多技术可以用作‘基因拯救’的形式。”她更愿意将辩论的重点放在这项新兴技术如何用于全面理解各种物种最初灭绝的原因，从而如何利用它来进行基因修改以预防未来的大规模灭绝。“我还想说，完全不采取任何行动是一种令人难以置信的道德风险。”她继续说道，“我们知道今天我们所做的还不够，我们必须愿意冒一些经过计算和权衡的风险。”