Deserts are wedged between, and thus hinder exchanges among, civilizations. In response, desert people developed a livelihood that capitalized on this need for commercial exchanges, the livelihood of transportation - guiding and servicing the cross-desert caravans. This expertise in safely and efficiently moving goods through deserts channelled a flow of income from non-desert to desert people, and at the same time economically and culturally benefited the non-desert areas at both ends of the cross-desert transportation routes. This trade often made desert people knowledgeable of the politics of Europe and Asia, more than the other way around. Though the great trade empires founded on cross-desert transport are long gone and desert routes are now far less significant, transport and trade still support desert livelihoods.

Deserts have been crossed by trade routes through millennia

Most deserts have been crossed by trading roads through millennia (Figure 3.4 ). The Silk Roads were already active in the late Bronze Age, though intensive use of cross-desert roads was triggered by the domestication of the camel. The trade through the trans-Saharan roads took off only with the Islamic conversion of West Africa. The two main roads, made of a network of shorter segments between oases, led from Morocco to the Niger Bend and from Tunisia to Lake Chad. Guided by Berber guides to ensure safe passage, caravans included on the average a thousand camels, sometimes reaching 12 000 animals, and runners were sent ahead to oases to ship out water when the caravan was still days away. West African gold and slave servants were exchanged for North African salt and slave soldiers, thus enriching kingdoms and empires of Ghana and Mali south of the Sahara, and Tuareg cities north of it. Similarly, through the Silk Road network goods to and from Xinjiang province of China travelled through Central Asian deserts either to West Asia or to Russia.

Trade through these roads declined (as of the 16th and the 12th centuries in Africa and Asia, respectively) due to political unrest, incursions, and wars, on the one hand, and the development of maritime routes on the other. The independence of African nations in the 1960s and rebellions and civil wars of the 1990s halted the cross- Saharan roads at the national boundaries, and trade through the Silk Road was disrupted by the wars of Genghis Khan. Today most cross-desert transport is through an extensive tarmac road network in addition to transport by air and sea; yet, Tuareg camel caravans still travel on the traditional Saharan routes, carrying salt from the desert interior to communities on the desert edges.

Cross-desert trade routes encouraged significant cultural exchange

The transfer of goods between non-desert lands through deserts enriched desert people, both economically and culturally. The Nabatean Kingdom was moulded and subsisted on controlling roads crossing West Asian deserts, moving spices from southern Arabia and goods from India to their capital Petra, and then to the Mediterranean port of Gaza, to be shipped to Greece and Rome. Other desert trading cultures include those of the Saharan Tuaregs, Fulani and Songhai and the Central Asian Uyghurs and Kazaks. The cross-desert trading routes functioned as communication and information channels between non-desert regions, and between these and the desert people. Through the Sahara rumours of West African treasures prompted the Portuguese to reach Guinea, and West Africans became acquainted with the Arab and Mediterranean world long before the adoption of Islam. Books from Europe travelled to Africa through the trans- Saharan roads, the only means for their transport until the 15th Century, exposing sub-Saharan countries to knowledge generated in Europe (Masonen 1997). European traders met trading partners in Africa and Asia with information far surpassing their own. Along with luxury goods and weapons, religions and knowledge moved through and out of the Asian deserts, such that the cultural scene of Central Asia was moulded by Indian, Greek, Chinese, Tibetan and Arabian cultures and the Buddhist, Christian and Muslim religions. Cross-desert trade routes also promoted gene flow among populations isolated from each other by the desert's reproductive barrier. Shared gene pools among camels, horses and goats on both sides of deserts (Jianlin and others 2004) are attributed to Pleistocene migrations along corridors later to become the Silk Road. human populations in Europe, Northern Africa, the Arabian Peninsula, East Asia and North America share a common genetic history, attributed to cross-desert travel and trading (Yao and others 2004).

Cross-desert transport continues to affect desert and non-desert people

While the trade across the desert today is small, cross-desert pipelines, trains and trucks move minerals and oil from the desert to non-desert destinations. Firewood and charcoal as fuel for desert inhabitants is transported from nondesert areas through desert roads. Altogether, ground transport now provides for a significant transportation sector in deserts. Better road systems have also opened up for tourism, one of the fastest growing economic sectors. This modern transport goes through oases and sky-islands and facilitates the urbanization of major oases, thus contributing to the economy of desert regions but also presenting risks to traditional lifestyles. Desert roads, however, are vulnerable to flash-floods and drifting sand, and programmes directed toward the construction of physical wind barriers or establishing vegetation to reduce drifting sand exist (Figure 3.5). The use of modern cross-desert roads is also often constrained by armed bandits and guerrilla warfare that may include road mining, making travel and trade dangerous and uncertain. Although no statistics are available, much of the cross-desert trade and road use is currently of an illegal nature - drugs, arms and slavery (mostly for prostitution); 80-90 per cent of the heroin consumed in Europe comes from the deserts of Afghanistan, and 60 percent of Afghan opium and heroin travels through Central Asian or West Asian deserts. Products from poaching of endangered species also travel through deserts (Nellemann 2005).