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全人类只剩一男一女如何复育人口?

  The alien predators arrived by boat.Within two years,everyone was dead.Almost.

  外来捕食者乘船驾到。短短两年内,几乎所有人都命丧黄泉。

  Could just two people repopulate Earth?

  全人类只剩一男一女如何复育人口?

  The tiny islet of Ball’s Pyramid lies 600km east of Australia in the South Pacific,rising out of the sea like a shard of glass.And there they were–halfway up its sheer cliff edge,sheltering under a spindly bush–the last of the species.Two escaped and just nine years later there were 9,000,the children and grandchildren and great-grandchildren of Adam and Eve.

  波尔斯金字塔(Ball’s Pyramid)是南太平洋上的一座小岛,坐落于澳大利亚东部600公里处,它就像一块玻璃尖塔一样耸立于大洋之中。在悬崖峭壁的半山腰上,一丛细长的灌木之下,隐藏着这种生物最后的幸存者。这两个侥幸逃脱的“亚当”和“夏娃”只用了短短9年时间就繁殖了9,000个后代,有孩子,有孙子,还有重孙子。

  No,this isn’t a bizarre take on the story of creation.The lucky couple were tree lobsters Dryococelus australis,stick insects the size of a human hand.They were thought to be extinct soon after black rats invaded their native Lord Howe Island in 1918,but were found clinging on in Ball’s Pyramid 83 years later.The species owes its miraculous recovery to a team of scientists who scaled 500ft of vertical rock to reach their hiding place in 2003.The lobsters were named“Adam”and“Eve”and sent to start a breeding programme at Melbourne Zoo.

  这并不是创世纪里的故事,这对幸运的夫妻也并非人类,而是一种又被称为“树龙虾”(tree lobsters)的豪勋爵岛竹节虫(Lord Howe Island stick insect),体型跟人类的手掌大小相仿。它们原本居住在豪勋爵岛,但那里却在1918年遭到黑鼠的入侵。很多人认为这种生物已经灭绝,但83年后,人们却在波尔斯金字塔上找到了它们。这项奇迹般的发现归功于一组科学家,他们2003年爬上了500英尺的悬崖,到达了这些竹节虫的藏身之地。这两只豪勋爵岛竹节虫被命名为“亚当”和“夏娃”,还被送到墨尔本动物园进行人工复育。

  Bouncing back after insect Armageddon is one thing.Female tree lobsters lay 10 eggs every 10 days and are capable of parthenogenesis;they don’t need a man to reproduce.Repopulating the earth with humans is quite another matter.Could we do it?And how long would it take?

  经历了“世界末日”之后还能重新恢复种群数量,可能只是昆虫世界的个案:雌性豪勋爵岛竹节虫每10天就能产10枚卵,而且能够单性生殖。也就是说,这种竹节虫不需要与雄性交配便可繁衍后代。但如果换成人类,要恢复人口数量却并非如此简单。我们能否做到这一点呢?需要花费多长时间才能做到?

  The answer is more than a whimsical discussion for the pub.From Nasa’s research on the magic number of pioneers needed for our move to another planet,to decisions about the conservation of endangered species,it’s a matter of increasing international importance and urgency.

  这个问题可不是在酒吧里闲聊几句就能回答的。美国航空航天局(NASA)曾经专门研究过移民外星球所需派遣的志愿者人数,还有一些机构就濒危物种的保护计划制定了许多决策,由此看来,这个国际问题的重要性和紧迫性都在与日俱增。

  So let’s fast-forward 100 years.Humanity’s endeavours have gone horribly wrong and a robot uprising has wiped us off the face of the Earth–a fate predicted by Stephen Hawking in 2014.Just two people made it.There’s no way around it:the first generation would all be brothers and sisters.

  让我们抛开现实,设想一下100年后的情形。在这期间,人类的发展方向发生了严重错误,机器人的崛起将我们在地球上赶尽杀绝——史蒂芬·霍金(Stephen Hawking)早在2014年就预言过这番景象。只有两个人侥幸存活。因此,人类别无选择:他们生下的第一代人都是兄弟姐妹关系。

  Sigmund Freud believed incest was the only universal human taboo alongside murdering your parents.It’s not just gross,it’s downright dangerous.A study of children born in Czechoslovakia between 1933 and 1970 found that nearly 40%of those whose parents were first-degree relatives were severely handicapped,of which 14%eventually died.

  西格蒙德·弗洛伊德(Sigmund Freud)认为,除了杀父弑母之外,乱轮是唯一一项人类社会通行的禁忌。乱伦不仅令人心生厌恶,而且十分危险。一项针对1933至1970年间出生在捷克斯洛伐克的儿童进行的研究显示,在父母双方为一级亲属的儿童中,有40%都患有严重残疾,14%最终死亡。

  Recessive risks

  隐性风险

  To understand why inbreeding can be so deadly,we need to get to grips with some genetics.We all have two copies of every gene,one from each parent.But some gene variants don’t show up unless you have two exactly the same.Most inherited diseases are caused by these“recessive”variants,which sneak through the evolutionary radar because they are harmless on their own.In fact,the average person has between one and two lethal recessive mutations in their genome.

  要理解近亲繁殖为何如此致命,就必须了解一些基因知识。我们的每一个基因都分成两套,分别来自父母双方。但有些基因只有在两套完全相同的情况下才会显现变异性状。多数遗传疾病都是由这些“隐性”基因决定的。隐性基因之所以能够躲开进化雷达的监视,是因为它们本身是无害的。事实上,普通人的基因组上都包含一两个致命的隐性基因。

  When a couple are related,it doesn’t take long for the mask to slip.Take achromatopsia,a rare recessive disorder which causes total colour blindness.It affects 1 in 33,000 Americans and is carried by one in 100.If one of our post-apocalyptic survivors had the variant,there’s a one in four chance of their child having a copy.So far,so good.After just one generation of incest,the risk skyrockets–with a one in four chance of their child having two copies.That’s a 1 in 16 chance that the original couple’s first grandchild would have the disease.<纽约时报中英文网 http://www.qqenglish.com/>

  当一对夫妻有亲属关系时,隐性基因就很难继续隐藏。以色盲为例,这种罕见的隐性疾病会导致患者完全无法分辨色彩。每3.3万美国人中约有1人是色盲,但每100人就有1人携带色盲基因。如果灾难发生之后的两名人类幸存者中有1人携带色盲基因,他们的孩子便有四分之一的可能继承这种基因。如果只是到这一步,情况还算不错。但只要有一代人近亲繁殖,风险就会急剧上升——后代有四分之一的可能携带两套色盲基因。照此计算,人类的两位幸存者的第一代孙就有十六分之一的概率患上色盲症。

  This was the fate of the inhabitants of Pingelap,an isolated atoll in the western Pacific.The entire population is descended from just 20 survivors of a typhoon which swept the island in the 18th Century,including a carrier of achromatopsia.With such a small gene pool,today a 10th of the island’s population is totally colour blind.

  这正是平吉拉普(Pingelap)居民的命运,那是一座位于西太平洋上的环礁。18世纪时,一场突如其来的台风席卷了这座小岛,岛上只有20人幸免于难,其中包括1名色盲基因携带者。由于基因库太小,所以岛上现有的居民中有十分之一都是色盲。

  Even with these hideous risks in mind,if the survivors had enough children the chances are at least some of them would be healthy.But what happens when inbreeding continues for hundreds of years?It turns out you don’t have to be stuck on an island to find out,because there’s one community that just can’t get enough of their close relatives:European royalty.And with nine generations of strategic marriages between cousins,uncles,and nieces in 200 years,the Spanish Habsburgs are a natural experiment in how it all adds up.

  即使蕴含着巨大的风险,但如果幸存者有足够多的后代,那至少还有一些人有机会获得健康的身体。但如果近亲繁殖持续数百年的时间,又会发生什么事情呢?不必盯着这座小岛,因为有一群人始终对近亲结婚乐此不疲,那就是欧洲王室。在长达200年的时间内,有9代人都出于战略目的而与自己的表亲堂亲、叔父伯父、侄子侄女联姻。西班牙哈布斯堡家族便是一个活生生的例子。

  Charles II was the family’s most famous victim.Born with a litany of physical and mental disabilities,the king didn’t learn to walk until he was eight years old.As an adult his infertility spelled the extinction of an entire dynasty.

  查尔斯二世是这个家族中最著名受害者,他生来便患有一系列生理和心理残疾,这位国王直到8岁才学会走路。成年之后,他又因为患有不育症而导致了整个王朝的覆灭。

  In 2009 a team of Spanish scientists revealed why.Charles’ancestry was so entangled,his“inbreeding coefficient”–a figure reflecting the proportion of inherited genes that would be identical from both parents–was higher than if he had been born to siblings.

  2009年,一组西班牙科学家揭示了背后的原因。查尔斯的血统十分混乱,以至于他的“近亲系数”(inbreeding coefficient,指的是从父母双方继承相同基因的概率)比亲兄弟姐妹生下的孩子还高。

  It’s the same measure used by ecologists to assess the genetic risks faced by endangered species.“With a small population size everyone is going to be related sooner or later,and as relatedness increases inbreeding effects become more important,”explains Dr Bruce Robertson from Otago University.He studies New Zealand’s giant,flightless parrots,called the kakapo,of which there are only 125 left on the planet.

  生态学家还使用这一指标来评估濒危物种面临的基因遗传风险。“如果种群数量太小,每个个体迟早都会与其他个体产生血缘关系,而随着血缘关系越来越近,近亲繁殖的影响也会越发明显。”新西兰奥塔哥大学(Otago University)的布鲁斯·罗伯森(Bruce Robertson)博士说。他研究了全世界仅存的125只鸮鹦鹉,这是一种生活在新西兰的不会飞的巨型鹦鹉。

  Of particular concern are the effects of inbreeding on sperm quality,which has increased the proportion of eggs that will never hatch from 10%to around 40%.It’s an example of inbreeding depression,Robertson says,caused by the exposure of recessive genetic defects in a population.Despite plenty of food and protection from predators,the kakapo might not make it.

  近亲繁殖对精子质量的影响尤其引人担忧,这已经导致无法孵化的鸮鹦鹉蛋比例从10%上升到40%左右。罗伯森表示,这是近交衰退的典型例子,而近交衰退则是由种群中的隐性基因缺陷造成的一种现象。尽管拥有大量的食物,而且可以免受天敌的威胁,但鸮鹦鹉或许仍然难逃灭绝的命运。

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  Immune mix

  免疫混合

  Endangered species also run the gauntlet of longer-term risks.Although they may already be well adapted to their environment,genetic diversity allows species to evolve their way around future challenges.Nowhere is this more important than immunity.“It’s something that most species seem keen to promote diversity in,even humans.We pick mates with a very different immune composition so our offspring have a diverse array of immune locks,”says Dr Philip Stephens from Durham University.Back in our evolutionary past,it’s thought that pairing with Neanderthals may have given our immune systems a genetic boost.

  濒危物种还要面临更加长期的风险。尽管它们可能已经适应了自己的生存环境,但基因多样性却可以帮助物种在进化的过程中应对未来的挑战。现在看来,没有什么比免疫力更加重要的了。“多数物种似乎都在努力提升多样性,人类也不例外。我们会选择与自己的免疫构成差异极大的人做配偶,这样便可获得丰富的免疫力。”英国杜伦大学的菲利普·史蒂芬斯(Philip Stephens)博士说。回到我们的进化史上,有人认为,如果能与尼安德特人交配,或许能够极大地改善我们的免疫系统基因。

  Even if our species makes it,it could be unrecognisable.When small pockets of individuals remain isolated for too long they become susceptible to the founder effect,in which the loss of genetic diversity amplifies the population’s genetic quirks.Not only would the new humans look and sound different–they could be an entirely different species.

  即使人类做到了这一点,实际的影响可能也微乎其微。如果一小群个体与世隔绝的时间太长,就很容易受到“奠基者效应”的影响——基因多样性的丧失会扩大该种群的基因缺陷。这些新人类不仅外形和声音与我们不同,甚至有可能是跟我们截然不同的另一个物种。

  So how much variety do you need?It’s a debate that goes right back to the 80s,says Stephens,when an Australian scientist proposed a universal rule of thumb.“Basically you need 50 breeding individuals to avoid inbreeding depression and 500 in order to adapt,”he says.It’s a rule still used today–though it’s been upped to 500-5,000 to account for random losses when genes are passed from one generation to the next–to inform the IUCN Red List,which catalogues the world’s most threatened species.

  那么我们究竟需要多大程度的多样性呢?史蒂芬斯表示,这一话题早在20世纪80年代就已经引发了热议。当时,澳大利亚科学家提出了一条经验法则。“大致来说,需要50个能够生育的个体才能避免近交衰退,需要500个才能充分适应环境。”他说。国际自然保护联盟(IUCN)濒危物种红色名录至今仍然沿用这一法则——但由于基因从一代遗传给下一代时可能发生随机损失,所以这一数字已经上调到500至5,000。

  Increasingly,the concept is leading those in the field to question the policies of large conservation charities,which prioritise the most endangered species.“It’s conservation framed in the context of triage–you sift casualties and ask is there a chance of saving them.It can be used to say well,can we forget about species?”

  逐渐地,这个概念也令人们开始质疑大型动物保护组织的政策,因为这些组织往往优先救助最濒危的物种。“这种保护政策还是沿用了‘伤员分检’的理念——首先了解伤亡情况,然后判断是否有救活的希望。这听起来似乎不错,但如果用来保护物种的话,效果究竟怎样呢?”

  But before you write off our couple,as one scientist pointed out,we’re living proof of the concept’s inherent flaws.According to anatomical and archaeological evidence,our ancestors wouldn’t have made our own population targets,with 1,000 individuals in existence for nearly a million years.Then between 50,000 and 100,000 years ago,we hit another rough patch as our ancestors migrated out of Africa.As you would expect,we’ve been left with astonishingly low genetic diversity.A 2012 study of the genetic differences between neighbouring groups of chimpanzees found more diversity in a single group than among all seven billion humans alive today.

  不过,正如一位科学家所说,在你否定一男一女能够恢复人口数量之前,我们人类其实正是这个遗传缺陷概念的活生生的例子。解剖学和考古学证据显示,我们的祖先本来就没有达到我们自己的人口目标:在接近100万年的时间里,只有1,000个人类个体。之后,在5万至10万年前,当我们的祖先离开非洲时,我们又陷入另一个难关。如你所料,我们当时的基因多样性极低。科学家在2012年针对相邻的黑猩猩种群之间的基因差异展开了研究,他们发现,仅仅是一个种群的黑猩猩的基因多样性都高于如今地球上的70亿人。

  Looking to our ancestors may be our best bet.Anthropologist John Moore’s estimate,which was published by Nasa in 2002,was modelled on small migrating groups of early humans–around 160 people.He recommends starting with young,childless couples and screening for the presence of potentially dangerous recessive genes.Alas,Moore was contemplating long-term space travel,not repopulating the planet.His number only allows for 200 years of isolation before the pioneers head back to Earth.

  效仿祖先或许是最好的做法。NASA 2002年发布的一项研究显示,人类学家约翰·摩尔(John Moore)的测算数据是以一小群早期人类(160人)为模型计算而来的。他建议从无儿无女的年轻夫妇开始,筛选潜在的危险隐性基因。摩尔计算这项数据的目的是为了规划长期的太空旅行,而不是在地球上恢复人口。根据他的测算,探索太空的先驱回到地球之前,只能与世隔绝200年。

  So what of the last man and woman?It’s impossible to say with any certainty,though Stephens is tentatively optimistic.“The evidence for the short-term effects of low genetic diversity is very strong,but all these things are probabilistic.There are stories of incredible journeys back from the brink–anything is possible.”

  那么,如果地球上只剩下一男一女会怎样?无法给出确切的答案,但史蒂芬斯仍然保持谨慎乐观:“关于基因多样性过低所产生的短期影响有着明确的证据,但这都只是概率问题。我们听过很多濒临绝境却起死回生的故事——一切皆有可能。”

  As long as the apocalypse doesn’t destroy the foundations of modern civilisation,humanity could bounce back surprisingly fast.At the turn of the 20th Century,the Hutterite community of North America–which is,incidentally,highly inbred–achieved the highest levels of population growth ever recorded,doubling every 17 years.It’s a tough ask,but if each woman had eight children,we’d be back to seven billion people and our current population crisis in just 556 years.

  只要末日灾难没有摧毁现代文明的根基,人类就能以不可思议的速度恢复活力。在20世纪之初,近亲繁殖率极高的北美哈特人创造了有史以来最高的人口增速——每17年便会翻一翻。虽然很难实现,但如果每名女性都生8个孩子,全球人口数量只需要短短556年便可恢复到70亿,并再次陷入如今的人口危机。

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