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Rock art in the Ahaggar Mountains dated to about 3500 to 2500 BCE shows a green Sahara with hippos and giraffes, but a drying climate turned it into a sand barrier between the Mediterranean and the Niger basin. The domesticated camel, appearing around the third century, made crossings possible, and caravans of about 1,000 camels sometimes swelled to 12,000, led by Berber guides. The 14th century traveler Ibn Battuta described the route from Sijilmasa toward Walata, where runners went ahead to an oasis and carried water four days back to the caravan. Northern economies needed gold and West African states such as the Mali Empire needed salt, traded at Timbuktu almost weight for weight.

The routes also carried people: historian John Wright estimates about 5,000 enslaved people a year over 1,250 years, roughly 6 to 7 million in all. Islam gave distant merchants shared commercial rules, Arabic and literacy, and by 1200 many West African ruling elites had converted. In the 16th century Portuguese caravels with lateen sails bypassed the desert, and after the Battle of Tondibi in 1591 the army of Ahmad al-Mansur devastated Timbuktu and Gao. French railways and fixed national borders later cut the north to south routes, though Tuareg azalai salt caravans still cover 1,500 miles.

• At Taghaza enslaved miners cut salt slabs, and houses were built from blocks of rock salt.
• Transport costs consumed almost 80 percent of the salt’s value before it reached the south.
• West African gold reaching Sijilmasa was minted into Almoravid dinars for Mediterranean markets.
• The Garamantes of the Fezzan used enslaved labor to dig foggara irrigation tunnels.
• Sultan Moulay Ismail of Morocco built his power on the Black Guard, an army of enslaved soldiers.