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马尼拉湾的热带港湾，两伙人正在小心翼翼地接洽，他们目光冷峻，手把着武器。这些人都是常年在世界各处游走的商人，他们来自相隔万里的地球两端：西班牙和中国。西班牙商人手中有大量由印第安和非洲裔奴隶在美洲开采的白银；那些中国人则带来了上好的丝绸和瓷器，其精良的制作工艺在欧洲闻所未闻。这笔在马尼拉进行的丝绸换白银的交易发生在1571年的夏天，此后它还将继续下去，并延续近250年之久。这一交易为如今被我们称作"全球化"的进程拉开了序幕。欧亚美三大洲第一次被一根经济纽带系在了一起。
这些丝绸在西班牙引起了轰动，就像白银在中国的礼遇一样。而迎接各自商船返航的人们并不清楚这些商船真正带来的是什么。如今我们在谈论全球化问题时，通常只探讨其经济内涵，但实际上，这也是个生物学问题。越来越多的研究人员认为，这些早期跨洋商船上所承载的最重要的货物不是丝绸或者白银，而是那些不受人类控制的动物植物，其中有不少是些无心的"偷渡客"。纵观历史，全球化过程中的生物变迁对世界人类和国家命运的影响可能是更为深远的。
大约两亿五千万年前，地球只有一块大陆，名曰泛大陆。地质运动使得这块大陆被割裂，欧亚大陆和美洲大陆从此永远分开。随着时间的推移，两块大陆上分别衍生出了全然不同的动植物体系。
在哥伦布(Columbus)横跨大西洋之前，只有极少数具备冒险精神的大陆生物曾飞跃海洋，在地球的另一端构建家园，其中绝大多是昆虫和鸟类。除此之外，地球基本上被割裂为各自独立的生态领域。哥伦布的一个卓著贡献就在于──按照历史学家克洛斯比(Alfred W. Crosby)的说法──将泛大陆的裂缝重新缝合起来了。
1492年之后，随着欧洲商船将成千上万的物种带到大洋彼岸安家落户，全球的生态系统开始交错融合。要感谢这种被克洛斯比称作"哥伦布大交换"(Columbian Exchange)的现象，我们如今才能在意大利看到西红柿、在佛罗里达品到橙子、在瑞士吃到巧克力、在泰国尝到红辣椒。
越来越多的学者相信，哥伦布远航引发的生态变迁是奠定当代世界格局的若干重大因素中的一个。欧洲为何能够崛起？昔日国力最为富足、社会最为文明的中国为何败落？传统奴隶制度为何在美洲大行其道？工业革命为何发源于英国？上述问题的答案都与"哥伦布大交换"有着千丝万缕的联系。
从哪儿说起呢？先说说蠕虫吧。确切地说就是蚯蚓──特别是黑蚯蚓和红蚯蚓──在1492年之前，这种生物在北美大陆根本不存在。
在跨过太平洋开始进行丝绸白银贸易之前很长一段时间里，西班牙和葡萄牙的殖民者们都在穿越大西洋寻找稀有金属。最终他们从玻利维亚、巴西、哥伦比亚和墨西哥运走了大量黄金和白银，大大增加了欧洲的货币供应。在这些返乡船只上还有其他一些同样重要的物品：一种来自亚马逊河流域的植物，也就是我们今天所说的烟草。
令人兴奋、使人上瘾的烟草，引爆了第一次真正意义上的全球性消费时尚。在1607年英国于弗吉尼亚建立其第一块殖民地之前，伦敦已经有了七千多家"烟馆"。这些烟馆类似于咖啡馆，为这座城市中队伍日益壮大的瘾君子们提供购买和吸食烟草的场所。为满足需求，英国的商船停靠在弗吉尼亚码头，装上成桶卷好的烟叶。这些大桶一般有4英尺高、2英尺半粗，每个桶至少有半吨重。为平衡重量，船员们会将砂石泥土等压舱物清出。他们用弗吉尼亚的烟草换掉了英国的泥土。
这些泥土中很可能就藏着黑蚯蚓和红蚯蚓。几乎可以确定的是，那些殖民者们带进来的植物的根系中也有。在欧洲人到来之前，美国的中西部的北部区域、新英格兰和加拿大全境都没有蚯蚓。早在上个冰川期，蚯蚓就在这里绝迹了。
在没有蚯蚓的林地，叶子会堆积在地表，乔木和灌木依靠这些落叶提供营养。蚯蚓出现后，很快就将那些落叶清理干净，并将营养以排泄物的形式深藏到地下。彷佛一夜之间，植物们发现无法自己获取营养了，它们原本相安无事的浅层根系似乎长错了地方。野生撒尔沙、野生燕麦、黄精以及大量林下叶层植物消失了，取而代之的是宾夕法尼亚莎草这样的草类物种。糖枫几乎不再生长，口木的幼苗则开始茁壮成长。
现如今，蚯蚓随着农民、花农和垂钓者们四处旅行安家。这些兢兢业业的地下工程师们正重新耕耘着明尼苏达、艾伯塔和安大略湖区的广袤土地。生态学家眼中这场庞大、毫无计划并将历经多个世纪的实验还在继续着，没有人知道接下来会发生什么。
哥伦布航海之前，引发疟疾的寄生虫在欧亚大陆和非洲相当猖獗，但在美洲却不见踪迹。可能早在哥伦布第二次远航时，疟疾便随着船员的身体来到了大洋的这一端。疟疾常见的并发症──黄热病──很快也跟随而至。
十七世纪前，在从美国华盛顿特区到巴西、厄瓜多尔这一带的沿海地区，疟疾和黄热病肆虐，使得这里成为欧洲移民的危险禁区。许多欧洲移民到达这些地方仅数月就一命呜呼了。而对于这些疾病，大多数西非人体内存在先天或后天获得的免疫力。
起初，美国大农场主们喜欢从欧洲输入劳力，因为欧洲劳工和他们说同样的语言，还了解欧洲的农耕方法。而且，从欧洲雇佣劳工比从非洲买奴隶要便宜。不过欧洲人身体远没有非洲人强健，因此雇欧洲劳工这笔投资的风险要高一些。历史学家柯廷(Philip Curtin)算过一笔账，仅从经济学角度讲，哥伦布大交换带来的疾病使得非洲奴隶的价值高达欧洲劳工的三倍。
哥伦布大交换导致传统奴隶制度在美洲盛行？这样说有些欠妥。人性本善，且人们在做出决定时会权衡很多因素。不过，只要了解市场的运行方式，就不难理解奴隶制度在经济上的优越性是如何在美国农场主们做出选择时发挥作用的。
哥伦布大交换在催生大不列颠联合王国过程中所发挥的作用则更为直接。1698年，野心勃勃的商人佩特森(William Paterson)成功说服富足的苏格兰人拿出半数资产用来在巴拿马建立殖民地，希望借此控制这个太平洋和大西洋之间往来贸易的咽喉要道。按照历史学家麦克尼尔(J.R. McNeill)在其著作《蚊子帝国》(Mosquito Empires)中的描述，疟疾和黄热病很快夺去了2,500名殖民者中近九成人的性命。这场灾难导致苏格兰的财务状况江河日下。
在那时，英格兰和苏格兰虽然共戴一君，但各自仍是独立的王国。此前数十年间，较为强大的英格兰一直在竭力促成两国的合并。出于对经济主导权旁落英格兰的担心，苏格兰一直在顽强抵抗。但如今英格兰承诺一旦两国合并，他们可以对苏格兰投资者们在巴拿马投资中的损失做出补偿。正如麦克尼尔所述，"由此，在巴拿马热病的协助下，大不列颠联合王国诞生了。"
即便如此，苏格兰人恐怕也不会对哥伦布大交换带来的所有后果充满怨言。苏格兰并入英国时，当地居民的日常口粮是一种来自南美的植物块茎，也就是我们今天所熟知的马铃薯。
同谷类相比，块茎产量会更高。如果小麦或者稻子的穗子长得很大，植株就会倒伏死掉。而块茎生长在地下，不存在这类问题。与种小麦或大麦所收获的粮食相比，十八世纪农民种植马铃薯的收成能多出三倍左右。
那时，饥饿对欧洲人而言并不陌生。据法国历史学家布罗代尔(Fernand Braudel)称，法国在十六世纪到十九世纪间曾发生过40次全国性的大饥荒，相当于每十年就发生不止一次。英格兰则更为频繁。在欧洲这片土地上根本无法实现自给自足。
马铃薯的存在让欧洲的大部分地区──从爱尔兰到乌克兰之间横跨2000英里的区域──的人解决了吃饱肚子的问题。（另一种来自美洲的作物玉米则在意大利和罗马尼亚扮演了类似的角色。）其结果便是政治稳定、收入增加、人口繁盛。来自秘鲁的马铃薯成了欧洲崛起的动力。
红薯在中国也担任了一个类似的角色。在十六世纪九十年代，借助跨越太平洋的白银贸易，红薯连同玉米一道从南美进入中国。这之后，中国的农民们便能够对山上不生稻米的荒地进行耕作了。在新作物的滋养下，大清王朝日益繁盛起来。但这场盛宴很快就变了味道。
由于中国农民之前从未耕耘过山上干旱的土地，他们犯下了初学者的错误。耕地侵蚀了土壤，导致洪水泛滥，民情涌动，政权动荡。那些为欧洲强盛立下汗马功劳的新作物成了撼动中国统治者江山的重要因素。
哥伦布大交换还让另外一些人付出了代价。哈佛大学昆虫学者威尔逊(Edward O. Wilson)指出，1516年当西班牙殖民者将非洲的大蕉引入伊斯帕尼奥拉岛时，他们还带去一些大蕉的寄生虫：介壳虫。这类寄生虫从大蕉根部吸食汁液。
威尔逊称，介壳虫在伊斯帕尼奥拉岛没有天敌，于是其数量一定会激增，这种现象被称作"生态释放"。介壳虫的出现应该会让当地一种名为热带火蚁的虫子颇为高兴。热带火蚁喜欢吃介壳虫甜滋滋的排泄物。因此介壳虫的泛滥很可能会导致火蚁数量的激增。
以上仅是根据事实所做的推断。但在1518年和1519年间发生的事情证明这不只是推断。根据一位见证了当年情形的牧师描述，在伊斯帕尼奥拉岛，西班牙人的家里和种植园内都被"无数只蚂蚁"侵入，这些蚂蚁的叮咬"比那些能蜇伤人的黄蜂还要疼"。不堪其扰的西班牙人不得不离开圣多明各，将他们的家拱手让给这些小虫子。这是当代史上的第一次生态灾难。
两个世纪后，一次更为严重的生态灾难卷土而来。当时欧洲的船只不慎将一种源自秘鲁、会导致马铃薯晚疫病的细菌带了进来。这种病菌最初于1845年6月在佛兰德斯出现，8月份被风吹到了巴黎周边的马铃薯种植园。几周后，荷兰、德国、丹麦和英格兰的田地均未能幸免。到9月13日，爱尔兰的马铃薯也染上了晚疫病。
爱尔兰比其他任何西方国家都更加依赖马铃薯。不到两年的时间，一百多万人死亡，数百万人逃离家园。爱尔兰从此一蹶不振。直到今天，爱尔兰的不幸仍昭然在目──它是欧洲、乃至全世界唯一一个在同样疆域内的人口数量要少于150年前的国家。
哥伦布大交换现今仍在继续着。原产自巴西的帕拉胶树如今占据着东南亚的大片土地。这种树上产的橡胶作为生产轮胎、皮带、密封圈和垫片的原料，成了维系工业文明的无名英雄。（迄今为止，同等质量的人工合成橡胶仍无法投入批量生产。）
橡胶树能在亚洲安家落户，要归功于一个名叫维克汉姆(Henry Wickham)的英国人。维克汉姆于1876年将七万颗橡胶树种子从巴西偷运到位于伦敦的英国皇家植物园(Kew Gardens)。如今在其亚马逊河流域的家乡，橡胶树的种植几乎是不可能完成的任务，因为当地一种名叫南美叶疫病的强大病菌已将那里的橡胶树消灭殆尽。就像当年马铃薯晚疫病菌度过大西洋那样，南美叶疫病早晚有一天也会穿越太平洋给彼岸的橡胶树带来可以预见的巨大灾难。
生物物种总会借助偶然事件或是有利的环境条件到处游走。而哥伦布大交换就像一张生物互联网，将自然界的每个部分联系起来，并以惊人的速度对其进行着改造。改造的结果可能更好，也可能更糟。
就像全球化进程本身那样，其带来的生物变迁结果也很难预料。就在巴西的橡胶树种植园占据了东南亚热带森林之际，一种原产自中国的豆类植物──大豆──正在亚马逊河南部近八万平方英里的广袤土地上生根发芽，其种植面积几乎相当于整个英国的大小。在巴西干旱的东北地区，种着逾1.5万平方英里的澳大利亚桉树。而另一方面，如今澳大利亚的开拓者们正试图将一种巴西的棕榈树──阿萨伊树──引入当地，这种树的果实一直被社会名流们奉为超级健康食品。
所有这些变化都将产生积极的经济效果──比如大豆出口正帮助巴西成为农业大国，改变着该国边远地区无数贫苦农家的命运。不过如今还在进行中的哥伦布大交换所带来的负面结果也很触目惊心。美国的森林正被大量外来虫害和疫病侵毁：比如橡树突死病，其致病菌很可能来自中国南部，和马铃薯晚疫病病菌同属；花曲柳窄吉丁虫害，这种虫子可能是藏在船只集装架里从中国北部来到美国的；还有白松疱锈病，这种病来自西伯利亚，于1920年在美国西北部的太平洋西北地区首次被发现。
枯树遍布的森林很容易招致毁灭性的森林大火，而这往往是巨变的诱因。新的物种会迅速取代那些在大火中消失的旧物种，物种更替则将带来无法预先获知的后果。我们只有耐心等待才能知道,我们的后代未来将会看到什么样的风景。
如今的我们新闻版面被债务危机、最新电脑应用软件以及中东动荡之类的消息占据。但数百年后，历史学家对我们这个时代的看法，可能正如我们开始看待当代西方世界的崛起那样：不过是哥伦布大交换这本热闹大书所展开的又一页新篇章。
（本文作者曼恩是新书《1493年：发现哥伦布新世界》(1493: Uncovering the New World Columbus Created)的作者。）
CHARLES C. MANN
In the great tropical harbor of Manila Bay, two groups of men warily approach each other, their hands poised above their weapons. Cold-eyed, globe-trotting traders, they are from opposite ends of the earth: Spain and China.
The Spaniards have a big cache of silver, mined in the Americas by Indian and African slaves; the Chinese bring a selection of fine silk and porcelain, materials created by advanced processes unknown in Europe. It is the summer of 1571, and this swap of silk for silver─the beginning of an exchange in Manila that would last for almost 250 years─marks the opening salvo in what we now call globalization. It was the first time that Europe, Asia and the Americas were bound together in a single economic network.
The silk would cause a sensation in Spain, as the silver would in China. But the crowds that greeted the returning ships had no idea what they were truly carrying. We usually describe globalization in purely economic terms, but it is also a biological phenomenon. Researchers increasingly think that the most important cargo on these early transoceanic voyages was not silk and silver but an unruly menagerie of plants and animals, many of them accidental stowaways. In the sweep of history, it is this biological side of globalization that may well have the greater impact on the fate of the world's people and nations.
Some 250 million years ago, the Earth contained a single landmass known as Pangaea. Geological forces broke up this vast expanse, forever splitting Eurasia and the Americas. Over time the two halves of Pangaea developed wildly different suites of plants and animals.
Before Columbus sailed the Atlantic, only a few venturesome land creatures, mostly insects and birds, had crossed the oceans and established themselves. Otherwise, the world was sliced into separate ecological domains. Columbus's signal accomplishment was, in the phrase of the historian Alfred W. Crosby, to reknit the seams of Pangaea.
After 1492, the world's ecosystems collided and mixed as European vessels carried thousands of species to new homes across the oceans. The Columbian Exchange, as Mr. Crosby called it, is why we came to have tomatoes in Italy, oranges in Florida, chocolate in Switzerland and chili peppers in Thailand.
A growing number of scholars believe that the ecological transformation set off by Columbus's voyages was one of the establishing events of the modern world. Why did Europe rise to predominance? Why did China, once the richest, most advanced society on earth, fall to its knees? Why did chattel slavery take hold in the Americas? Why was it the United Kingdom that launched the Industrial Revolution? All of these questions are tied in crucial ways to the Columbian Exchange.
Where to start? Perhaps with the worms. Earthworms, to be precise─especially the common nightcrawler and the red marsh worm, creatures that did not exist in North America before 1492.
Well before the start of the silk-and-silver trade across the Pacific, Spanish and Portuguese conquistadors were sailing the Atlantic in search of precious metals. Ultimately, they exported huge supplies of gold and silver from Bolivia, Brazil, Colombia and Mexico, vastly increasing Europe's money supply. But those homebound ships contained something else of equal importance: the Amazonian plant known today as tobacco.
Intoxicating and addictive, tobacco became the subject of the first truly global commodity craze. By 1607, when England founded its first colony in Virginia, London already had more than 7,000 tobacco 'houses'─cafe-like places where the city's growing throng of nicotine junkies could buy and smoke tobacco. To feed the demand, English ships tied up to Virginia docks and took in barrels of rolled-up tobacco leaves. Typically 4 feet tall and 2½ feet across, each barrel weighed half a ton or more. Sailors balanced out the weight by leaving behind their ships' ballast: stones, gravel and soil. They swapped English dirt for Virginia tobacco.
That dirt very likely contained the common nightcrawler and the red marsh worm. So, almost certainly, did the rootballs of plants that the colonists imported. Before Europeans arrived, the upper Midwest, New England and all of Canada had no earthworms─they had been wiped out in the last Ice Age.
In worm-free woodlands, leaves pile up in drifts on the forest floor. Trees and shrubs in wormless places depend on litter for food. When earthworms arrive, they quickly consume the leaf litter, packing the nutrients deep in the soil in the form of castings (worm excrement). Suddenly, the plants can no longer feed themselves; their fine, surface-level root systems are in the wrong place. Wild sarsaparilla, wild oats, Solomon's seal and a host of understory plants die off; grass-like species such as Pennsylvania sedge take over. Sugar maples almost stop growing, and ash seedlings start to thrive.
Spread today by farmers, gardeners and anglers, earthworms are obsessive underground engineers, and they are now remaking swathes of Minnesota, Alberta and Ontario. Nobody knows what will happen next in what ecologists see as a gigantic, unplanned, centuries-long experiment.
Before Columbus, the parasites that cause malaria were rampant in Eurasia and Africa but unknown in the Americas. Transported in the bodies of sailors, malaria may have crossed the ocean as early as Columbus's second voyage. Yellow fever, malaria's frequent companion, soon followed.
By the 17th century, the zone where these diseases held sway─coastal areas roughly from Washington, D.C., to the Brazil-Ecuador border─was dangerous territory for European migrants, many of whom died within months of arrival. By contrast, most West Africans had built-in defenses, acquired or genetic, against the diseases.
Initially, American planters preferred to pay to import European laborers─they spoke the same language and knew European farming methods. They also cost less than slaves bought from Africa, but they were far less hardy and thus a riskier investment. In purely economic terms, the historian Philip Curtin has calculated, the diseases of the Columbian Exchange made the enslaved worker 'preferable at anything up to three times the price of the European.'
Did the Columbian Exchange cause chattel slavery in the Americas? No. People are moral agents who weigh many considerations. But anyone who knows how markets work will understand the pull exerted by slavery's superior profitability.
Much more direct was the role of the Columbian Exchange in the creation of Great Britain. In 1698, a visionary huckster named William Paterson persuaded wealthy Scots to invest as much as half the nation's available capital in a scheme to colonize Panama, hoping to control the chokepoint for trade between the Pacific and the Atlantic. As the historian J.R. McNeill recounted in 'Mosquito Empires,' malaria and yellow fever quickly slew almost 90% of the 2,500 colonists. The debacle caused a financial meltdown.
At the time, England and Scotland shared a monarch but remained separate nations. England, the bigger partner, had been pushing a complete merger for decades. Scots had resisted, fearing a London-dominated economy, but now England promised to reimburse investors in the failed Panama project as part of a union agreement. As Mr. McNeill wrote, 'Thus Great Britain was born, with assistance from the fevers of Panama.'
But Scots could hardly complain about the consequences of the Columbian Exchange. By the time they were absorbed into Britain, their daily bread, so to speak, was a South American tuber now familiar as the domestic potato.
Compared with grains, tubers are inherently more productive. If the head of a wheat or rice plant grows too big, the plant will fall over, killing it. There are no structural worries with tubers, which grow underground. Eighteenth-century farmers who planted potatoes reaped about four times as much dry food matter as they did from wheat or barley.
Hunger was then a familiar presence in Europe. France had 40 nationwide food calamities between 1500 and 1800, more than one every decade, according to the French historian Fernand Braudel. England had still more. The continent simply could not sustain itself.
The potato allowed most of Europe─a 2,000-mile band between Ireland and the Ukraine─to feed itself. (Corn, another American crop, played a similar role in Italy and Romania.) Political stability, higher incomes and a population boom were the result. Imported from Peru, the potato became the fuel for the rise of Europe.
The sweet potato played a similar role in China. Introduced (along with corn) from South America via the Pacific silver trade in the 1590s, it suddenly provided a way for Chinese farmers to cultivate upland areas that had been unusable for rice paddies. The nutritious new crop encouraged the fertility boom of the Qing dynasty, but the experiment soon went badly wrong.
Because Chinese farmers had never cultivated their dry uplands, they made beginners' mistakes. An increase in erosion led to extraordinary levels of flooding, which in turn fed popular unrest and destabilized the government. The new crops that had helped to strengthen Europe were a key factor in weakening China.
The Columbian Exchange carried other costs as well. When Spanish colonists in Hispaniola imported African plantains in 1516, the Harvard entomologist Edward O. Wilson has proposed, they also brought over some of the plant's parasites: scale insects, which suck the juices from banana roots.
In Hispaniola, Mr. Wilson argues, these insects had no natural enemies. Their numbers must have exploded─a phenomenon known as 'ecological release.' The spread of scale insects would have delighted one of the region's native species: the tropical fire ant, which is fond of dining on the sugary excrement of scale insects. A big increase in scale insects would have led to a big increase in fire ants.
This is only informed speculation. What happened in 1518 and 1519 is not. According to an account by a priest who witnessed those years, Spanish homes and plantations in Hispaniola were invaded by 'an infinite number of ants,' their stings causing 'greater pains than wasps that bite and hurt men.' Overwhelmed by the onslaught, Spaniards abandoned their homes to the insects, depopulating Santo Domingo. It was the first modern eco-catastrophe.
A second, much more consequential disaster occurred two centuries later, when European ships accidentally imported the fungus-like organism, native to Peru, that causes the potato disease known as late blight. First appearing in Flanders in June 1845, it was carried by winds to potato farms around Paris in August. Weeks later it wiped out fields in the Netherlands, Germany, Denmark and England. Blight appeared in Ireland on Sept. 13.
The Irish were more dependent on potatoes than any other Western nation. Within two years, more than a million died. Millions more fled. The nation never regained its footing. Today Ireland has the melancholy distinction of being the only nation in Europe, and perhaps the world, to have fewer people within the same boundaries than it did more than 150 years ago.
The Columbian Exchange continues to this day. The Pará rubber tree, originally from Brazil, now occupies huge swathes of southeast Asia, providing the latex necessary to make the tires, belts, O-rings and gaskets that invisibly maintain industrial civilization. (Synthetic rubber of equal quality still cannot be practicably manufactured.)
Asian rubber plantations owe their existence to a British swashbuckler named Henry Wickham, who in 1876 smuggled 70,000 rubber seeds from Brazil to London's Kew Gardens. Rubber-tree plantations are next to impossible in the tree's Amazonian home, because they are wiped out by an aggressive native fungus, Microcyclus ulei. Much as the potato blight crossed the Atlantic, M. ulei will surely make its way across the Pacific one day, with consequences as disastrous as they are predictable.
Species have always moved around, taking advantage of happenstance or favorable circumstances. But the Columbian Exchange, like a biological Internet, has put every part of the natural world in contact with every other, refashioning it, for better or worse, at a staggering rate.
The consequences are as hard to predict as those of globalization itself. Even as plantations of Brazilian rubber take over tropical forests in Southeast Asia, plantations of soybeans, a Chinese legume, are replacing almost 80,000 square miles of the southern Amazon, an area almost the size of Britain. In dry northeastern Brazil, Australian eucalyptus covers more than 15,000 square miles. Returning the favor, entrepreneurs in Australia are now attempting to establish plantations of açaí, a Brazilian palm tree whose fruit has been endorsed by celebrities as being super-healthful.
All of these developments will yield positive economic results─soy exports, for instance, are making Brazil into an agricultural powerhouse, lifting the fortunes of countless poor farmers in remote places. But the downside of the ongoing Columbian Exchange is equally stark. Forests in the U.S. are being devastated by a host of foreign pests, including sudden oak death, a cousin of potato blight that is probably from southern China; the emerald ash borer, an insect from northern China that probably arrived in ship pallets; and white pine blister rust, a native of Siberia first seen in the Pacific Northwest in 1920.
Forests full of dead trees are prone to catastrophic fires, a convulsive agent of change. New species will rush in to replace those that are lost, with effects that cannot be known in advance. We will simply have to wait to see what kind of landscape our children will inherit.
Today our news is dominated by stories of debt deals and novel computer applications and strife in the Middle East. But centuries from now, historians may well see our own era as we have started to see the rise of the modern West: as yet another chapter in the unfolding tumult of the Columbian Exchange.
Mr. Mann is the author of '1493: Uncovering the New World Columbus Created,' which will be published next week.
In the great tropical harbor of Manila Bay, two groups of men warily approach each other, their hands poised above their weapons. Cold-eyed, globe-trotting traders, they are from opposite ends of the earth: Spain and China.
The Spaniards have a big cache of silver, mined in the Americas by Indian and African slaves; the Chinese bring a selection of fine silk and porcelain, materials created by advanced processes unknown in Europe. It is the summer of 1571, and this swap of silk for silver─the beginning of an exchange in Manila that would last for almost 250 years─marks the opening salvo in what we now call globalization. It was the first time that Europe, Asia and the Americas were bound together in a single economic network.
The silk would cause a sensation in Spain, as the silver would in China. But the crowds that greeted the returning ships had no idea what they were truly carrying. We usually describe globalization in purely economic terms, but it is also a biological phenomenon. Researchers increasingly think that the most important cargo on these early transoceanic voyages was not silk and silver but an unruly menagerie of plants and animals, many of them accidental stowaways. In the sweep of history, it is this biological side of globalization that may well have the greater impact on the fate of the world's people and nations.
Some 250 million years ago, the Earth contained a single landmass known as Pangaea. Geological forces broke up this vast expanse, forever splitting Eurasia and the Americas. Over time the two halves of Pangaea developed wildly different suites of plants and animals.
Before Columbus sailed the Atlantic, only a few venturesome land creatures, mostly insects and birds, had crossed the oceans and established themselves. Otherwise, the world was sliced into separate ecological domains. Columbus's signal accomplishment was, in the phrase of the historian Alfred W. Crosby, to reknit the seams of Pangaea.
After 1492, the world's ecosystems collided and mixed as European vessels carried thousands of species to new homes across the oceans. The Columbian Exchange, as Mr. Crosby called it, is why we came to have tomatoes in Italy, oranges in Florida, chocolate in Switzerland and chili peppers in Thailand.
A growing number of scholars believe that the ecological transformation set off by Columbus's voyages was one of the establishing events of the modern world. Why did Europe rise to predominance? Why did China, once the richest, most advanced society on earth, fall to its knees? Why did chattel slavery take hold in the Americas? Why was it the United Kingdom that launched the Industrial Revolution? All of these questions are tied in crucial ways to the Columbian Exchange.
Where to start? Perhaps with the worms. Earthworms, to be precise─especially the common nightcrawler and the red marsh worm, creatures that did not exist in North America before 1492.
Well before the start of the silk-and-silver trade across the Pacific, Spanish and Portuguese conquistadors were sailing the Atlantic in search of precious metals. Ultimately, they exported huge supplies of gold and silver from Bolivia, Brazil, Colombia and Mexico, vastly increasing Europe's money supply. But those homebound ships contained something else of equal importance: the Amazonian plant known today as tobacco.
Intoxicating and addictive, tobacco became the subject of the first truly global commodity craze. By 1607, when England founded its first colony in Virginia, London already had more than 7,000 tobacco 'houses'─cafe-like places where the city's growing throng of nicotine junkies could buy and smoke tobacco. To feed the demand, English ships tied up to Virginia docks and took in barrels of rolled-up tobacco leaves. Typically 4 feet tall and 2½ feet across, each barrel weighed half a ton or more. Sailors balanced out the weight by leaving behind their ships' ballast: stones, gravel and soil. They swapped English dirt for Virginia tobacco.
That dirt very likely contained the common nightcrawler and the red marsh worm. So, almost certainly, did the rootballs of plants that the colonists imported. Before Europeans arrived, the upper Midwest, New England and all of Canada had no earthworms─they had been wiped out in the last Ice Age.
In worm-free woodlands, leaves pile up in drifts on the forest floor. Trees and shrubs in wormless places depend on litter for food. When earthworms arrive, they quickly consume the leaf litter, packing the nutrients deep in the soil in the form of castings (worm excrement). Suddenly, the plants can no longer feed themselves; their fine, surface-level root systems are in the wrong place. Wild sarsaparilla, wild oats, Solomon's seal and a host of understory plants die off; grass-like species such as Pennsylvania sedge take over. Sugar maples almost stop growing, and ash seedlings start to thrive.
Spread today by farmers, gardeners and anglers, earthworms are obsessive underground engineers, and they are now remaking swathes of Minnesota, Alberta and Ontario. Nobody knows what will happen next in what ecologists see as a gigantic, unplanned, centuries-long experiment.
Before Columbus, the parasites that cause malaria were rampant in Eurasia and Africa but unknown in the Americas. Transported in the bodies of sailors, malaria may have crossed the ocean as early as Columbus's second voyage. Yellow fever, malaria's frequent companion, soon followed.
By the 17th century, the zone where these diseases held sway─coastal areas roughly from Washington, D.C., to the Brazil-Ecuador border─was dangerous territory for European migrants, many of whom died within months of arrival. By contrast, most West Africans had built-in defenses, acquired or genetic, against the diseases.
Initially, American planters preferred to pay to import European laborers─they spoke the same language and knew European farming methods. They also cost less than slaves bought from Africa, but they were far less hardy and thus a riskier investment. In purely economic terms, the historian Philip Curtin has calculated, the diseases of the Columbian Exchange made the enslaved worker 'preferable at anything up to three times the price of the European.'
Did the Columbian Exchange cause chattel slavery in the Americas? No. People are moral agents who weigh many considerations. But anyone who knows how markets work will understand the pull exerted by slavery's superior profitability.
Much more direct was the role of the Columbian Exchange in the creation of Great Britain. In 1698, a visionary huckster named William Paterson persuaded wealthy Scots to invest as much as half the nation's available capital in a scheme to colonize Panama, hoping to control the chokepoint for trade between the Pacific and the Atlantic. As the historian J.R. McNeill recounted in 'Mosquito Empires,' malaria and yellow fever quickly slew almost 90% of the 2,500 colonists. The debacle caused a financial meltdown.
At the time, England and Scotland shared a monarch but remained separate nations. England, the bigger partner, had been pushing a complete merger for decades. Scots had resisted, fearing a London-dominated economy, but now England promised to reimburse investors in the failed Panama project as part of a union agreement. As Mr. McNeill wrote, 'Thus Great Britain was born, with assistance from the fevers of Panama.'
But Scots could hardly complain about the consequences of the Columbian Exchange. By the time they were absorbed into Britain, their daily bread, so to speak, was a South American tuber now familiar as the domestic potato.
Compared with grains, tubers are inherently more productive. If the head of a wheat or rice plant grows too big, the plant will fall over, killing it. There are no structural worries with tubers, which grow underground. Eighteenth-century farmers who planted potatoes reaped about four times as much dry food matter as they did from wheat or barley.
Hunger was then a familiar presence in Europe. France had 40 nationwide food calamities between 1500 and 1800, more than one every decade, according to the French historian Fernand Braudel. England had still more. The continent simply could not sustain itself.
The potato allowed most of Europe─a 2,000-mile band between Ireland and the Ukraine─to feed itself. (Corn, another American crop, played a similar role in Italy and Romania.) Political stability, higher incomes and a population boom were the result. Imported from Peru, the potato became the fuel for the rise of Europe.
The sweet potato played a similar role in China. Introduced (along with corn) from South America via the Pacific silver trade in the 1590s, it suddenly provided a way for Chinese farmers to cultivate upland areas that had been unusable for rice paddies. The nutritious new crop encouraged the fertility boom of the Qing dynasty, but the experiment soon went badly wrong.
Because Chinese farmers had never cultivated their dry uplands, they made beginners' mistakes. An increase in erosion led to extraordinary levels of flooding, which in turn fed popular unrest and destabilized the government. The new crops that had helped to strengthen Europe were a key factor in weakening China.
The Columbian Exchange carried other costs as well. When Spanish colonists in Hispaniola imported African plantains in 1516, the Harvard entomologist Edward O. Wilson has proposed, they also brought over some of the plant's parasites: scale insects, which suck the juices from banana roots.
In Hispaniola, Mr. Wilson argues, these insects had no natural enemies. Their numbers must have exploded─a phenomenon known as 'ecological release.' The spread of scale insects would have delighted one of the region's native species: the tropical fire ant, which is fond of dining on the sugary excrement of scale insects. A big increase in scale insects would have led to a big increase in fire ants.
This is only informed speculation. What happened in 1518 and 1519 is not. According to an account by a priest who witnessed those years, Spanish homes and plantations in Hispaniola were invaded by 'an infinite number of ants,' their stings causing 'greater pains than wasps that bite and hurt men.' Overwhelmed by the onslaught, Spaniards abandoned their homes to the insects, depopulating Santo Domingo. It was the first modern eco-catastrophe.
A second, much more consequential disaster occurred two centuries later, when European ships accidentally imported the fungus-like organism, native to Peru, that causes the potato disease known as late blight. First appearing in Flanders in June 1845, it was carried by winds to potato farms around Paris in August. Weeks later it wiped out fields in the Netherlands, Germany, Denmark and England. Blight appeared in Ireland on Sept. 13.
The Irish were more dependent on potatoes than any other Western nation. Within two years, more than a million died. Millions more fled. The nation never regained its footing. Today Ireland has the melancholy distinction of being the only nation in Europe, and perhaps the world, to have fewer people within the same boundaries than it did more than 150 years ago.
The Columbian Exchange continues to this day. The Pará rubber tree, originally from Brazil, now occupies huge swathes of southeast Asia, providing the latex necessary to make the tires, belts, O-rings and gaskets that invisibly maintain industrial civilization. (Synthetic rubber of equal quality still cannot be practicably manufactured.)
Asian rubber plantations owe their existence to a British swashbuckler named Henry Wickham, who in 1876 smuggled 70,000 rubber seeds from Brazil to London's Kew Gardens. Rubber-tree plantations are next to impossible in the tree's Amazonian home, because they are wiped out by an aggressive native fungus, Microcyclus ulei. Much as the potato blight crossed the Atlantic, M. ulei will surely make its way across the Pacific one day, with consequences as disastrous as they are predictable.
Species have always moved around, taking advantage of happenstance or favorable circumstances. But the Columbian Exchange, like a biological Internet, has put every part of the natural world in contact with every other, refashioning it, for better or worse, at a staggering rate.
The consequences are as hard to predict as those of globalization itself. Even as plantations of Brazilian rubber take over tropical forests in Southeast Asia, plantations of soybeans, a Chinese legume, are replacing almost 80,000 square miles of the southern Amazon, an area almost the size of Britain. In dry northeastern Brazil, Australian eucalyptus covers more than 15,000 square miles. Returning the favor, entrepreneurs in Australia are now attempting to establish plantations of açaí, a Brazilian palm tree whose fruit has been endorsed by celebrities as being super-healthful.
All of these developments will yield positive economic results─soy exports, for instance, are making Brazil into an agricultural powerhouse, lifting the fortunes of countless poor farmers in remote places. But the downside of the ongoing Columbian Exchange is equally stark. Forests in the U.S. are being devastated by a host of foreign pests, including sudden oak death, a cousin of potato blight that is probably from southern China; the emerald ash borer, an insect from northern China that probably arrived in ship pallets; and white pine blister rust, a native of Siberia first seen in the Pacific Northwest in 1920.
Forests full of dead trees are prone to catastrophic fires, a convulsive agent of change. New species will rush in to replace those that are lost, with effects that cannot be known in advance. We will simply have to wait to see what kind of landscape our children will inherit.
Today our news is dominated by stories of debt deals and novel computer applications and strife in the Middle East. But centuries from now, historians may well see our own era as we have started to see the rise of the modern West: as yet another chapter in the unfolding tumult of the Columbian Exchange.
Mr. Mann is the author of '1493: Uncovering the New World Columbus Created,' which will be published next week.
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