英语听力：自然百科 探索世界奥秘之Wild Africa - Deserts(浩瀚沙漠) Unit14（在线收听）

The Arabian camel--not even a native , was domesticated in the deserts of Asia and brought to Northern Africa as a means of transport. In the past , Tuareg nomads used camels to move slaves, gold and ivory between West Africa and the Mediterranean. Today their caravans still trade in salts, dates and millet.

The Sahara has long been a frontier between Asia and Africa. But there are European pioneers here too--red foxes, which scrape a living 

on the Northern fringes of this vast desert.

Though much of the Sahara is uninhabitable, there are a few special places where life can thrive. The Ennedi Mountains in Chad harness a little more moisture from clouds than lower-lying areas enough to support tiny pockets of Savanna. Here rock hyraxes survive on the leaves of acacia trees, a scene more typical of the African plains. These green islands are relics of an older world, one which is slowly vanishing with the march of the desert sands.

The Sahara , as we know it today, is only 2, 000 years old, the product of a rapidly drying climate in North Africa. But our ancestors also had a hand in this change. By over-cultivating marginal lands, and intensively grazing livestock. Before, things were very different. Try to imagine this land flooded by the ocean, swathed in forest and savannah, swept by mighty rivers.

Ennedi Gorge once fed a vast tributary of the Nile. Today all that remains are a few precious pools, crucial watering holes for passing caravans of camels. In just 10 minutes of drinking , these remarkable animals can put on a third of their body weight in water. It's enough to carry them for well over 100 kilometers to the next oasis.

Tuareg:西撒哈拉和中撒哈拉的柏柏尔人

trade in :a method of buying something new by giving an item you own as part of the payment for it 抵价购物

scrape a living :to only just earn enough money to provide yourself with food, clothing and a place to live 勉强够维持生活

hyraxes: a kind of animal 蹄兔

have a hand in: 干予, 参与, 插手