<P> Trans - Saharan trade requires travel across the Sahara (north and south) to reach sub-Saharan Africa from the North African coast, Europe, to the Levant . While existing from prehistoric times, the peak of trade extended from the 8th century until the early 17th century . </P> <P> The Sahara once had a very different environment . In Libya and Algeria, from at least 7000 BC, there was pastoralism, herding of sheep and goats, large settlements and pottery . Cattle were introduced to the Central Sahara (Ahaggar) from 4000 to 3500 BC . Remarkable rock paintings (dated 3500 to 2500 BC), in places which are currently very dry, portray vegetation and animal presence rather different from modern expectations . </P>

Which of the following was used in the elaborate trade system that developed for salt and gold