4 (page 64)p. 64The technological silk road

Koreans are impressive mountain climbers. Even elderly hikers move fast, heedless of steep rocky terrain or enervating humidity, and seldom stop. They are also professionally outfitted, even on a Sunday excursion to Mt. Bukhansan in Seoul: besides ubiquitous sun visors and bandannas imprinted with maps of mountain temple itineraries, many Korean hikers sport Gore-tex® boots, microfiber quick-dry clothing, ultralight backpacks, CamelBak hydration systems, and retractable molybdenum walking sticks. When they do pause, for lunch, they spread out dish after dish of spiced meats, grilled fish, and multiple vegetable kimchis, all washed down with little green bottles of soju—a sweet indigenous vodka. While eating, many sit comfortably on compact folding stools: two rectangles of metal tubing bolted together and pivoting in the middle, a fabric seat stretched across one end.

This kind of camp stool—a classic design, reworked by Koreans in aircraft aluminum—probably originated in North Africa and passed through Central Asia, arriving in China around the second century CE, where it was named, significantly, huchuang: “barbarian chair.” The huchuang was known in Asia from the classical era, but it remained a specialty item, since East Asians did not sit on chairs. The ancient Chinese term we translate as “chairman” when we speak of Chairman Mao—Mao zhuxi—literally means “master of the mat,” since that's what (page 65)p. 65ancient Chinese sat upon in formal settings. All of which shows that even the most quotidian of “technologies” can have a cross-continental history.

Ancient China was of course technologically capable of making chairs, but it was not until the late tenth and early eleventh century that fixed-frame chairs—with hard platform, back, and sides—became common in Chinese interiors. Before that, the earliest references to chairs in China are Buddhist, first in the representation of seated deities (such as at the Longmen, Yungang, and Dunhuang caves), and then in written advice for monks (meditating on chairs kept them safe from snakes and insects). From there, chairs and thrones were taken up in Chinese courts and only later moved down the social ladder to commoner households. It was thus the centuries of communication between India and China, linked to the spread of Buddhism, that brought the chair to China and encouraged its acceptance as a common household item.

There have been several epochs and axes of intensified communication in silk-road history. The connection between India and China, starting in the first century CE and attenuating only from the mid-ninth century, brought not only Buddhism but also many non-religious ideas and items. For example, in addition to the chair, China adopted, from India, by imperial decree, the technique for producing refined sugar from cane. The explosion of Arab armies out of Arabia in the seventh century and creation of an Islamic empire and zone of cultural influence led to similarly intensified contacts and exchange over much of the middle of the continent. Between the eleventh and thirteenth centuries, the Crusades put Western Europe back in touch with the southern Mediterranean and Persia and led to the reintroduction of classical knowledge, enhanced by Islamic learning, to Europe. The Mongol unification of Eurasia from Korea to Hungary spread military technology as well as aspects of mathematics, astronomy, cartography, agronomy, and other arts and technologies in both directions across the continent.

(page 66)p. 66The silk road owes much to such episodes, and despite its prevalent image as a peaceful highway, these intensified communications arose from war and imperial conquest. Even the Sino-Indian connection, though ostensibly in the hands of Buddhist monks, benefited from Chinese military expansion into Central Asia during the Han and Tang dynastic periods as well as imperial sponsorship of envoys and translation projects. This should not surprise us: the most profound technological exchange of modern times, industrialization, was driven not only by individuals or the market but by states with strategic concerns. This remains true for many new technologies today.

Like horses, silkworms have been domesticated for millennia. Like horses, silk is a product closely associated with Eurasian elites. And as a pair, silk and horses made up one of the most common exchanges on the silk road. In early centuries, Chinese silk fabrics were shipped from east to west, but a longer-lived and ultimately more significant exchange was that of silk (and other) textiles south to north between agrarian and pastoral economies. Horses and silk represent the essence of steppe and sown, the former embodying the martial strength of the Central Eurasian nomads, the latter symbolizing the soft, sensual life of the urbanized elites of Eurasian rimlands. The agrarian lands’ need for transport and cavalry mounts was complemented by the nomads’ demand for textiles, especially fine silks, which could confer legitimacy and status, and help nomad elites secure their followers’ support or be resold for a profit. This contrast and complementarity was a key driver of silk-road history.

The human relationship with silk is intimate, domestic, and feminized. Legend attributes the invention of sericulture to Xi Ling, wife of the mythological Chinese progenitor, the Yellow Emperor. (She supposedly lived in the mid-third millennium BCE, though silkworm cocoons have been found dating from 5000 (page 67)p. 67to 4000 BCE). After accidentally dropping a cocoon into a cup of hot tea, Xi Ling realized that she could unwind it into a long continuous strand. In another story, it was a Chinese princess who, in order to assure herself a supply of silk after her marriage to a Central Asian king, smuggled silkworm eggs and mulberry seeds to Khotan in the first century CE by wrapping them in her headdress. A millennium after that, we are told, Italian women were incubating silkworm eggs in bags nestled between their breasts.

Humans have long intervened directly in the reproduction of the silk moth, Bombyx mori, which no longer lives in the wild; nor have the fundmentals of silk production changed much since ancient times. After lovingly guarding and hatching the tiny silkworm eggs (35,000 to the ounce), silk-raising families east and west would light braziers or move out to the barn, yielding their warm houses to larvae in wicker trays in order to protect them from the chill of early spring. Today eggs and larvae are still incubated and fed in trays. Bombyx mori larvae voraciously consume the leaves of just one species of mulberry; it takes some two hundred pounds of leaves to produce a pound of silk. Careful sericulturalists supply the caterpillars with leaves day and night for around five weeks until they are almost 3 inches (7–8 cm) long. The caterpillars then spin silk through glands on their heads, which combine fibroin, a protein, with sericin, a gum. Gyrating their heads in a figure-eight motion, the caterpillars wrap themselves into a cocoon wound from a single, strong thread up to 4,000 feet (1,200 m) in length. Throughout this entire period, traditional silk-raising families tended their tiny charges like new parents, avoiding loud noises, keeping them well fed, and clearing waste from the trays. Superstitions once attended this delicate stage in the process: in China, for example, menstruating women were not to enter rooms where the worms were feeding.

Once the cocoons are spun, however, the nurturing is over: before they can emerge as moths, the caterpillars are swiftly killed in their (page 68)p. 68cocoons by steaming or, traditionally, by immersion in boiling water. As the heat dissolves its gum, the end of a filament can be located and unwound. Filaments of five to seven cocoons are spun together to make a thread. Textiles woven from such thread are soft, strong, and easily dyed with more brilliant colors than woolen or vegetable fibers can hold. Silk can be woven into sheer gauze, as were the silks first favored by the Romans, or into heavy patterned damasks.

Ancient Greeks and Romans understood silk to come from a land and people called Seres—the source of which term, linguists have recently argued, is none other than the Chinese word for silk (today pronounced si) as filtered through Central Eurasian languages. But even though they wore and coveted the fabric, the Romans did not understand the technology behind it. Virgil (page 69)p. 69in his Georgics (first century BCE) reflected the common belief that silk thread was combed from leaves; around the same time, the Greek geographer Strabo said that silk was the dried bark of certain trees found in India. A century later, Pliny the Elder still described the silk of the Seres as “the wool that is found in their forests.” The Seres, or Chinese, soaked it, combed off its down, and “then to the females of our part of the world they give the twofold task of unravelling their textures and of weaving the threads afresh. So manifold is the labour, and so distant are the regions which are thus ransacked to supply a dress through which our ladies may in public display their charms.” It seems that Roman women would modify imported silk fabrics, either by loosening the weave or refashioning it into some kind of lace. Thus, regarding Cleopatra, the Roman poet Lucan wrote that “her white breasts are resplendent through the Sidonian fabric, which, wrought in close texture by the sley of the Seres, the needle of the workman of the Nile has separated, and has loosened the warp by stretching out the web.”

Pliny railed against both the transparency and the expense of such textiles. In this he was joined by the philosopher Seneca, who is said to have lamented:

I see, too, silken clothing—if clothing that can be called, which does not protect, nor even conceal the body—appareled in which, a woman cannot very truly swear that she is not naked. Such tissues are brought to us at enormous cost, from nations so remote that not even their names can reach us; and by the help of this vast expense, our matrons are able to exhibit, to their lovers and in their couches, nothing at which the whole public has not equally gazed.

Such a fuss over caterpillar spit! Yet these eructations of conservative commentators tell us something: silk was new to the Roman empire, its use just then passing from the ruling echelon to a somewhat broader, though still elite, stratum of society. Tiny fragments dating from centuries earlier have been found in (page 70)p. 70northern European sites, but it was not until the last century BCE and first century CE that silk was known and increasingly available in Rome—at this point still traveling all the way from China.

It is wrong to say that Zhang Qian inaugurated the silk road—as we have seen, trans-Eurasian exchanges go back millennia before that. But his trip to Central Asia in search of anti-Xiongnu allies (ca. 129 BCE) did result in an accelerated marketing of Chinese silk in Central Asia, whence it was traded on by way of Bactria (Kushan empire), India, and Parthia. Some, ultimately, ended up in Rome. It was not primarily as a consumer product that Chinese silk moved west, however, but rather as an instrument of imperial policy. The Han empire in these centuries was shipping enormous amounts of silk north to appease the Xiongnu and buy their horses. On a later embassy to a nomadic people known as the Wusun in northern Xinjiang, Zhang Qian brought gold and silk goods worth 100 million cash (a Chinese currency unit). Subsequent Han embassies to other Central Asian lands, dispatched to secure big western horses, involved hundreds of people, many of whom, Sima Qian sniffs in his Shiji, were

sons of poor families who handled the government gifts and goods that were entrusted to them as though they were private property and looked for opportunities to buy goods at a cheap price in the foreign countries and make a profit on their return to China.

This simply means that besides official exchange of diplomatic gifts, there was trading on the side, to the irritation of Chinese chroniclers. The next Chinese state to operate extensively in Central Asia, the Tang, likewise used its comparative advantage in silk production for strategic purposes. Tang supported its administration in “the Western Regions” (modern Xinjiang) largely with shipments of silk piece goods—up to 900,000 bolts a year, which were used as currency. Over the years, it delivered hundreds of thousands of bolts of silk to Turks and Uyghurs in what is now Mongolia.

(page 71)p. 71China did not retain its monopoly on all aspects of silk textile manufacturing. Inhabitants of the Tarim Basin understood basic sericultural techniques by the first century CE. Some scholars believe that India was manufacturing silk by the third century CE. Byzantium had silkworms (allegedly smuggled from China in a bamboo pole by Nestorian monks) by around 550 and launched its own silk textiles industry under state monopoly during the reign of Emperor Justinian I (r. 527–565), even while continuing to purchase silk from Egypt and Sasanid Persia. Silkworm cultivation spread to Sicily in the twelfth century and onto the Italian peninsula by the thirteenth century.

Even before then, though, the character of the transcontinental silk trade had changed. By the sixth century, it was not piece goods but silk floss (thread) that was being sold on the east–west continental and sea routes via India. It was woven in the Mediterranean world (for example, in Damascus, hence the word “damask”) to match local taste; the best pieces were stained purple with a dye produced from the shells of a Mediterranean mollusk in the Phoenician ports of Sidon and Tyre (modern Lebanon).

Although in later centuries the long-distance east–west silk trade involved fewer piece goods, and the technology of sericulture diffused to most southern parts of Eurasia where the climate could support it, the north–south exchange of silk and other textiles for horses long remained a fundamental economic and strategic dynamic between the steppe and settled agrarian lands. In the eighteenth century, for example, the Mughal Empire was importing as many as 50,000 horses annually from the Central Asian steppes and from Iran to support its cavalry. Indian merchants traded dyes and textiles, primarily cottons from the advanced Indian textile industry, for the Central Asian horses. Many of the Indian products were re-traded in Bukhara and other Central Asian cities, winding up in Russian hands as the Russian empire expanded eastward. This Indian–Central Asian–Russian (page 72)p. 72trade of textiles for horses continued until the nineteenth century, when the British undermined India's textile industry.

And in China, too, the textile-for-horse trade continued through Tang, Song, and Ming dynasties, increasingly supplemented by tea sales as this beverage became a necessity for nomad herdsmen and as Russians, too, developed a taste for it. In the late eighteenth and nineteenth centuries, the Qing Empire launched another massive exchange of silks and cottons with the Kazakhs. Qing imperial officials in northern Xinjiang traded silk from southern China for steppe horses and cotton cloth from southern Xinjiang for sheep and cattle. From 1759 to 1853, the Qing shipped nearly 420,000 bolts of imperial silk from south China to Central Asia. The textile-for-horse trade, the warp and weft of the silk road, extended into modern times.

Today, as newspapers become digital, millions read “the paper” on small electronic devices; these devices also help us communicate, navigate by map and compass, record and display music and images, read literature, predict astronomical events, perform mathematical calculations, learn market prices, or translate foreign languages. Anyone alive in the late twentieth century and early twenty-first century understands the transformative quality of such technology. Likewise, while silk is the most glamorous silk-road product, paper has had the most impact, since it first revolutionized most of the activities we now do with smartphones. And you can wrap fish and chips in it too.

Paper consists of vegetable fibers that have been beaten and suspended in water and dried on a screen into a felted sheet. Like many things, it was first invented in China, where credit is traditionally given to Cai Lun in the second century CE. Examples have been found, however, from as early as the first century BCE. It was probably first used to wrap things, and only later for writing (page 73)p. 73on (it replaced bamboo strips and supplemented silk textiles and carved tablets for this purpose). But this flexible and strong material was also put to other uses, including as clothing, hats, shoes, armor, kites, as well as one nonliterary purpose still familiar today: a sixth-century Chinese scholar admonished his family not to use scraps of paper inscribed with the classics in the toilet—which demonstrates how inexpensive paper was by that time. (The same issue arose in China during the Cultural Revolution of the 1960s and 1970s: the ubiquity of the words of “master of the mat” Mao made it hard to put old newspapers to an accustomed purpose.)

Buddhism and the Chinese civil service examination system (which relies on candidates’ knowledge of the Chinese classics) helped spread paper use to neighboring lands. Korea and Vietnam adopted paper in the first centuries CE; a Korean monk is credited with introducing papermaking to Japan in 610. By then, paper was being made in Karakhoja (Gaochang), but archeologists, including Aurel Stein and Sven Hedin, have unearthed paper documents in Xinjiang dating from centuries earlier. Some are official or commercial records. The Soghdian letters were on paper, including the one describing the fourth-century Xiongnu attacks. Many other paper records, including the earliest printed books and other items from the Dunhuang library cave, concern Buddhism. Oddly, though Chinese missionaries brought paper to India, its use did not catch on there until the twelfth century, after Buddhism's decline in India. Perhaps the humid climate and teeming insect life made paper too ephemeral a medium for religious texts, although later Indo-Islamic texts would indeed use paper.

The Battle of Talas, in which Tang met Arab forces in 751, is reported in a later Arabic source to have been the catalyst for the spread of paper to the Islamic world: Tha‘alibi's eleventh-century Book of Curious and Entertaining Information records that craftsmen among the Chinese taken captive in that battle opened the first paper mills in Samarkand. Although paper was probably (page 74)p. 74known and made in Transoxiana decades before the Battle of Talas, Samarkand was the first place in the new Islamic world to develop papermaking technology and became famous for its paper exports. Thereafter, paper spread remarkably quickly throughout the lands under the command of the Abbasid caliphate. It is said that Caliph Harun al-Rashid established a paper mill in the capital, Baghdad, in 794/95, to supply paper for his growing bureaucracy. Over the next two centuries, papermaking spread to Syria, Egypt, North Africa, and Spain. Europe learned of paper at this time and imported it from Arab lands, but the first manufacture of paper in Europe was not until the twelfth (Spain) and thirteenth centuries (Italy).

This paper-gap between medieval Christendom and the Islamicate reflects and contributed to differentials in learning and scholarship. The burgeoning production and circulation of books and maps of all sorts in the Islamic world corresponds to the new prevalence of paper from the ninth century. Paper thus spurred scholarship based on the Greco-Roman tradition and Islamic science, but it also had a popular dimension: the earliest extant text of One Thousand and One Nights was copied on paper in Egypt or Syria in the ninth century—which suggests that this book, in a relatively inexpensive edition, was to some degree available to common folk. By contrast, even centuries later, the greatest libraries of Europe could boast only a few hundred books in their collections, all on very expensive parchment or vellum (both processed animal skin), and many of them, like those in the Sorbonne, chained to desks.

Of course, what truly disseminated the written word—with revolutionary results, especially in the European Renaissance—was a combination of technologies: paper with mechanical printing. Gutenberg's movable type, with which he produced multiple copies of his Vulgate Bible in 1463–1455, is the well-known European milestone. Like paper, printing technology began in East Asia, where both block printing and movable type (page 75)p. 75were known centuries earlier. Developed on many technological precedents (seals, rubbings, printing on textiles), block printing on paper began around 700 CE in China, and both of the earliest extant examples there are Buddhist: a charm scroll from the early eighth century CE, and the earliest book, a block-printed Diamond Sutra scroll from 868 CE, found at Dunhuang and now in the British Library. It may be that the belief that karma could be accrued by repetition of prayers or copying of sutras stimulated the development of mechanical mass-production methods.

Movable type was invented in China in the mid-eleventh century, and the earliest movable metal type developed two centuries later in Korea—still earlier than Gutenberg. The earliest extant book printed with movable type is Korean (the Jikji, a Zen Buddhist primer printed in 1377), but movable type remained of secondary importance in China and East Asia generally. Early movable type could be helpful for printing alphabetic or syllabary scripts, where one needs many of a limited number of different letters. Thus movable type was used for the Uyghur language in an old script derived from Soghdian around 1300. However, the Chinese script, used in China, Japan, Korea, and Vietnam, contains tens of thousands of individual characters, some used frequently, others hardly at all—with the frequencies varying greatly depending on the subject matter of the text. A set of movable type would have to be enormous and still would not fit all needs. It was much more efficient to carve a block in negative—which could print two pages—than to cast, set, and reset pages with thousands of different characters of movable type. Woodblocks also had the advantage of simplifying the integration of text and image on a single page.

Is printing an example of diffusion or convergence? It is not clear whether Gutenberg was inspired by Asian examples when he developed his typographical system. There is a story that his wife, from a Venetian family, had seen Chinese woodblocks at home. On the other hand, Gutenberg combined elements of his (page 76)p. 76technique (ink, ink-pad, type, press) in a unique way. What is without doubt is the travel before Gutenberg's day of printing and printed items over the silk roads from China through the Islamic world and into Europe. The Mongols were instrumental in bringing printed things westward. They adopted a Chinese practice by printing paper money in their domains in Persia in the late thirteenth century. Playing cards are another example. Printed to allow production in quantity, playing cards were first used in China in the ninth century and appeared across the Islamic world over subsequent centuries. By the fourteenth century there are references to them in Europe, where they were possibly brought by the Mongols. Printed religious pictures share a similar timeline and entry point (to Spain and Italy from the Islamic world and to Germany, perhaps due to Mongol activity in Eastern Europe.) In general, the long-distance travel that the Mongol empire facilitated let European missionaries and merchants examine and acquire printed items and write about them in their accounts. Whether the momentous development of European printing arose from a heroic invention, or whether there was a “Chinese background for the European invention of typography” depends largely on one's definition of “invention” and, as for pasta, on the nationality printed on one's passport.

Many have noted that in China, medicine and food are not considered separate categories but rather lie along a continuum of things one ingests that have effects on the body. This is often treated as evidence that the traditional Chinese approach to health is more “holistic” than that of modern medicine. Likewise, the philosophy behind traditional Indian Ayurvedic medicine stresses nutrition and consumption of certain foodstuffs to restore “balance” and health. In both traditions, certain foods, though not always the same ones, are understood to be “heating” or “cooling,” “wet” or “dry,” regardless of physical temperature or juiciness. Flavors and types of food eaten together should be calibrated not (page 77)p. 77just to enhance enjoyment but to ensure the healthiness of the meal. In India, for example, a thin-framed, energetic person whose component parts are dominated by vata, a “humor” or dosha made up of air and ether, might be cautioned that eating dal (beans) will lead to considerable flatulence. In China, if one is feverish, pears (cooling) are advised, while mandarin oranges or ginger (heating) are not; meats and rich stews are good for sickly people or in cold weather, but one should never eat dog meat (highly warming, especially when cooked with garlic and hot chilies) in the summer months.

We often point to such practices, a combination of traditional textual knowledge and evolving folklore, to distinguish “eastern” from “western” medicine. In fact, however, what they really show us are vestiges of an older, pan-Eurasian medical theory current even in the West until the nineteenth century; it was only then that new discoveries and empirical practices really began to create the body of knowledge that we now know and practice globally as modern medicine.

The idea that foods fall into certain categories based on their “humors” and how they affect the body is shared by ancient Greco-Roman, Indian, Chinese, and Islamic medical traditions. Each tradition has unique indigenous features, but they all express one version or another of what is often called the “humoral theory” regarding the composition and proper functioning of the body. The western version of this scheme derives from the work of Hippocrates, later elaborated on by the second-century physician Galen. Here, the four humors are understood to be fluids in the body (black bile, yellow bile, blood, and phlegm), associated with the four “elements” (earth, fire, air, water), seasons (autumn, summer, spring, winter), psychological temperaments (melancholic, choleric, sanguine, phlegmatic), bodily organs (spleen, gall bladder, liver, brain/lungs), and various permutations of the qualities hot, cold, dry, and wet. That there is a relationship between this Galenic and Indian humoral theory seems clear, but (page 78)p. 78the exact mechanism of transmission—and even the direction of transmission—remains controversial. Nevertheless, contacts between the Mediterranean and the Hellenic city-states in Central Asia likely played a role.

As for China, The Yellow Emperor's Classic of Medicine from the Han dynasty (206 BCE–220 CE) contains no humoral theory, but by the fifth century CE Chinese medical texts discuss illness as the result of corporal imbalances of heat, coolness, wetness, and dryness, and point out which foods contain these qualities. Buddhism, as so often, seems to have been the medium bringing these ideas from India to China. Chinese Buddhist writings on natural philosophy describe the body as comprised of four elements: earth, water, fire, and air—imbalances of which were understood to cause illness. Demons, too, made people sick, as could bad behavior in previous lifetimes—ideas also imported from India. These concepts coexisted with China's own theories involving not “four elements” but “five phases” (wood, fire, earth, metal, and water), that correspond in turn to five flavors (sour, bitter, sweet, pungent, salty), the five senses, and two sets of organs. In the great eighteenth-century Chinese novel A Dream of Red Mansions/Story of the Stone, characters fall ill due to demon possession, bad karma, or obsessive thoughts and are treated according to the humoral system. Popular belief in China had thus absorbed Indian medical ideas and reconciled them to ancient indigenous ones, at least as far as the question of balance of humors is concerned. In China today, the notion of “heating” or “cooling” foods remains current. When someone urges you to eat duck broth or lotus seeds and avoid donkey-meat burgers in the summer, the underlying logic of their suggestion is both thoroughly Chinese and a product of silk-road interactions linked ultimately to ancient Greece.

The humoral theory reached northern and western parts of Europe later than it entered China and, as with many other aspects of the classical Mediterranean tradition, did so only after a silk-road (page 79)p. 79sojourn. The Abbasid caliphate, whose capital, Baghdad, was one of the largest and most cultured cities in the world, actively supported preservation and translation of Greek philosophical and scientific texts, including medical works by Galen. Just as the monk Xuanzang learned Sanskrit and traveled to India to gather Buddhist scripture, the ninth-century Arab scholar Hunayn Ibn Ishaq, a Nestorian Christian, learned Greek and went to Byzantium to collect medical texts, including more than a hundred of Galen's treatises. Hunayn was a master translator, rendering these and other works into Syriac and Arabic (some of Galen's writing survived only in Hunayn's translations), thereby laying the foundations for centuries of medical writing in the Islamic world, including that by the Bukharan scholar Ibn Sina, known as Avicenna in Latin. Ibn Sina's Canon on Medicine, a synthesis of Islamic and Greek medical learning, was the backbone of the corpus of translated and new medical works that entered Europe from the mid-eleventh century. They passed first through the Italian city of Salerno; there scholars in touch with Byzantium studied the Galenic tradition along with Aristotle and other classical works via the efforts of another translator, Constantine the African. (How critical to world history has been the work of linguists!) The humoral theory and other medical knowledge thus came from classical Greece and Rome to late medieval Europe via Byzantium, Baghdad, Bukhara, and other silk road information entrepôts.

In a way, humoral theory “works”: it sometimes correctly identifies the nutritional value of certain foods and their appropriateness for certain conditions. But there is another truly effective medical technique in which the silk road played a role in both invention and dissemination. Smallpox (variola) is one of the deadliest diseases to have originated on the Eurasian continent, probably from a rodent virus that jumped species. The disease afflicted people in Egypt and India from the second millennium BCE, whence it spread elsewhere on the continent, scarring and blinding those victims it did not kill. As a “crowd disease,” it was (page 80)p. 80endemic in many densely populated areas, where its mortality rate was close to 30 percent, striking children particularly hard. Those who survived outbreaks, however, were immune for life. Adults in cities and agricultural areas, therefore, were likely to have been exposed to and survived the disease and to have at least some immunity to it. The same was not true, however, for Central Eurasian nomads, whose sparse population and infrequent contact with settled areas meant that most adults had never encountered the disease. Towns and cities, therefore, could be deadly for even the fiercest nomad conquerors.

Possibly for this reason, the first systematic governmental efforts at inoculation to protect against smallpox (by introducing a form of live virus into healthy people) occurred in states with roots on the Eurasian steppes: the Ottoman and Qing empires. The disease was definitively identified as a distinct condition around 500 CE in China, and by Al-Razi in the Islamic world four centuries later (Galen had not noted it). There are some suggestions of inoculation in India in the first or second millennium BCE, but the first clear use is in tenth-century CE China; thereafter it seems that the Chinese practiced various approaches to inoculation (deliberately introducing live virus to stimulate immunity), until by the sixteenth century it was a well-known, if still rather dangerous technique. Versions of the practice were also found in Central Asia and Africa after this period, due either to transmission from China or independent invention. We know that there was a great interchange of medical personnel and information between China and Persia under the Mongol empire.

For the Manchu rulers of China during the Qing dynasty (1636–1911), smallpox posed a political as well as medical challenge. The Manchu military was comprised of tribal Manchus and Mongol allies; the court promoted close diplomatic relations with Mongol princes and the high lamas of Tibet, who were all hesitant to come to the Qing capital of Beijing, lest they contract smallpox. Indeed, the Third Panchen Lama died of the disease (page 81)p. 81following a state visit in 1780. Most important, the Manchu imperial clan itself suffered from the disease. China's longest-lived ruler, the Manchu Kangxi emperor (r. 1661–1722), was chosen as heir and enthroned at a young age in part because he had survived smallpox—which may have been the cause of his father's death. Kangxi developed an inquisitive, eclectic, and empirical mind, and personally experimented in the 1680s with various techniques to safely inoculate his own children as well as elites among the Mongolian units loyal to the Qing. Although the most common Chinese method employed live virus (from powdered pustule scabs), it used scabs only from inoculated people or from those infected by a milder variant of the disease (V. minor) rather than those with full-blown smallpox, and did so only after storing the material in a controlled environment for some time. These tricks attenuated the virus, lowering the risk of infection to acceptable levels.

At the other end of Eurasia, a more direct form of inoculation was commonly practiced in the Ottoman empire. The wife of the British ambassador, Lady Mary Wortley Montagu, learned of the technique in Istanbul in 1716–18, inoculated her own family and vocally advocated use of the method in Britain. In a process reflecting the same empirical habit of mind exhibited by the Kangxi emperor but in a public context characteristic of the European Enlightenment, information about inoculation was then propagated through scientific journals of the Royal Society. Experimentation began in the 1720s on prisoners in Newgate prison and on colonials in Boston, and inoculation eventually gained broad acceptance. In 1796 William Jenner discovered the method of vaccination, using cowpox virus, that afforded humans some immunity with no risk of contracting smallpox.

Following a global campaign to modernize and universalize smallpox immunization, the World Health Organization declared the disease eradicated in 1979. This is one of the foremost success stories of twentieth-century international collaboration in public health. But these global efforts had begun much earlier, albeit in a (page 82)p. 82less coordinated fashion, with the emergence and communication of inoculation methods across Eurasia.

One of the last outbreaks of smallpox occurred in the port town of Aralsk, in Kazakhstan in the heart of Central Eurasia. The Soviet Union maintained a secret biological weapons facility on Vozrozhdeniye Island in the Aral Sea, and in 1971 a test of aerosolized smallpox worked a little too well: it blew shoreward and infected ten people, three of whom died, and prompted a massive, if secret, immunization campaign. The accident, when finally revealed in 2002, heightened fears that terrorists in the post-Soviet era would obtain similar weaponized variola or other diseases.

Among the greatest concerns of our time is the spread of biological, chemical, or nuclear weapons to other entities besides those powers that already possess them. But if history is any guide, this is a losing battle: few things have diffused as readily and pervasively as new military technology. The incentives for any power to adopt—and negative incentives against not adopting—effective weapons are simply too great. The most prominent example from our time has been, of course, atomic and thermonuclear weapons, which have spread through a combination of direct diffusion of technology (through espionage and technology transfer) and by “stimulus diffusion”—the independent invention by one group on the basis of a concept observed among another, rather than direct import of the thing itself.

After the United States developed and first used nuclear devices in 1945, the Soviet Union tested a nuclear weapon in 1949 (in Semipalatinsk, in Central Asia); Britain and France followed in subsequent years. In what may be one of the most significant “silk road” transfers ever, from 1950 to 1960—the decade of warm relations between the USSR and China—Soviet advisors provided (page 83)p. 83Chinese nuclear scientists with atomic research facilities, an experimental reactor, a cyclotron, elements of a gaseous diffusion plant, and other technological and financial assistance. Besides socialist fraternity, one factor motivating this aid was the Soviet shortage of uranium, which China had in abundance in mines in Xinjiang. The USSR had packed up a sample atomic bomb with full documentation in 1957 and was about to ship it to the PRC when relations cooled, and the USSR abrogated the transfer agreement. Nevertheless, Chinese scientists went on to develop and test an atomic bomb by 1964 (again in Central Asia, at the Lop Nor site in Xinjiang: the old silk road has been the preferred testing-ground for horrific modern weapons).

Eurasian transfers of military technology are nothing new. It was the diffusion of the equestrian military complex that shaped the linguistic map of the continent and underpinned the unique, long-running military-political system of the nomad states. This began with the domestication of the horse and its use for riding (4200–4000 BCE), along with wheeled ox- or donkey-carts introduced from Mesopotamia, which made deep-steppe nomadism possible (3300 BCE). The next critical technology was the war chariot, developed not in the Near East as was once thought but in Central Eurasia. The earliest chariot burials date from 2100 BCE in the Sintashta and Petrovka sites on the steppes east of the southern Urals, where they are associated with intensive bronze metallurgy.

The chariot is not a cart, as it has only two light, spoked wheels on a single axle, and a cab designed to carry only one or at most three standing riders. It took great skill and much practice to drive two or four horses at speed in such a light vehicle, while firing arrows or hurling javelins at an opponent at the same time. It required heavy investments in metals (bronze) and other hardware to equip an army with chariots. These inputs of time and wealth, as well as the weapons and fine grave goods accompanying chariot burials, show that the steppe societies producing and deploying chariots were stratified, with power centralized in elite hands.

(page 84)p. 84The chariot was a game-changing technology in other ways as well, arguably as significant as gunpowder would be later. The age of the chariot corresponds to the second millennium BCE waves of migrations and invasions of Indo-European speakers from northern and central Eurasia into the rimlands: the Hittites into Anatolia, Syria, and upper Mesopotamia; the Mycenaeans into the Greek peninsula; the so-called Aryans into Persia and India. The Hyksos, who were not Indo-European speakers, invaded Egypt with chariots around the same time. The chariot shows up suddenly, with no technological predecessors, in tombs from Shang China around 1200 BCE. Two centuries later, the Zhou, a people from the northwest with more access to horses, used massed chariots to overcome the Shang. Ancient elites across Eurasia, then, used chariots in military or symbolic ways: the Norse, Greco-Roman, Persian, and Indian gods all ride chariots.

The technical features of chariots and harness systems identified in archeological sites make it clear that these chariots across the continent share a common source. It is a case of diffusion that contributed to demographic shifts and political upheavals, helping to draw the map of the ancient world and permit the broad spread of Indo-European languages across the continent.

And yet, the chariot's day was relatively short. By the time of the Roman chariot races, or when chariots were buried with the first emperor of Qin or carved in relief in Han tombs, they were mere icons of power. Even when Homer described the chariot warfare on the plains of Troy, he was singing of warfare long obsolete. Chariots could be effectively challenged by infantry in hoplite formation, and proved no match for fast cavalry of mounted archers armed with the short compound bow. And with the invention and spread across Eurasia of the stirrup (from China, in the first centuries ce, with Indian and Kushani precedents), the archer sat all the more firmly in the saddle.

After the dissemination of the stirrup, the next highpoint of military technology transfer occurred, like so many other (page 85)p. 85exchanges, during the Mongol period. To resolve a stalemated siege of southern Chinese cities in 1268, the Mongols brought in Persian siege engineers, who built versions of the European counterweight trebuchet (a kind of catapult) to bombard the cities. The Chinese called these “Muslim bombards,” but to the Persian historian Rashid al-Din they were “Frankish” (that is, European) trebuchets. Such weapons had come to the Levant only decades before and had proved decisive at the Crusaders’ siege of Acre in 1191.

Going the other way, at around the same time, was a momentous military technology: gunpowder and various kinds of guns. Gunpowder was a Chinese invention; an early formulation is described in a Daoist text from 850 CE. The early mixtures of sulfur and saltpeter with carboniferous substances (including honey) were, by the eleventh century, refined into powder that would explode, not just burn. Fireworks inspired various kinds of incendiary weapons (bombs, fire-arrows, rockets, and lances that spewed fire and potsherds), and ultimately by the thirteenth century exploding bombs in bamboo, then iron, casings. Song dynasty forces used these devices to defend their cities, for a time, against the Mongols. Smooth-bore cannons that could hurl projectiles were known from the late thirteenth and early fourteenth centuries in China.

The Mongols themselves did not make particular use of gunpowder weapons. Nevertheless, it is striking that knowledge about gunpowder and firearms took root in India, the Middle East, and Europe precisely during the Mongol era. It is likely that the information channels opened by the Mongol Empire helped spread this technology. The first field guns in Europe, from the early fifteenth century, closely resemble Chinese cannon of a century or so earlier. Again, the transfer of military technology is rapid.

Gunpowder and gunpowder weapons ultimately changed the way wars were fought; they also contributed to a common pattern of (page 86)p. 86political consolidation across Eurasia, with centralization of power in monarchies including the Muscovites in Russia and Siberia, the Ottoman Empire, Safavids in Iran, the Mughals in India—states often dubbed “gunpowder empires”—as well as Tokugawa Japan and Ming China. In Europe, the arms race to create more powerful cannon, stronger fortifications, and more efficient firearm-wielding infantry put an end to feudalism and led to intense interstate competition and technological improvement in the manufacture and deployment of firearms. This in turn spurred the creation of overseas empires by the Spanish, Portuguese, Dutch, and ultimately Britain, France, and other European powers. It is not unreasonable to say that silk-road technology transfer in the Mongol era kick-started many military and political developments we associate with the early modern world.