Trans-Saharan trade requires travel across the Sahara between sub-Saharan Africa and North Africa. While existing from prehistoric times, the peak of trade extended from the 8th century until the early 17th century. The Sahara once had a very different environment. In Libya and Algeria, from at least 7000 BC, there was pastoralism, the herding of sheep, 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 that are currently very dry, portray flora and fauna that are not present in the modern desert environment. As a desert, Sahara is now a hostile expanse that separates the Mediterranean economy from the economy of the Niger basin. As Fernand Braudel points out, crossing such a zone, especially without mechanized transport, is worthwhile only when exceptional circumstances cause the expected gain to outweigh the cost and the danger. Trade was conducted by caravans of camels. According to Ibn Battuta, the explorer who accompanied one of the caravans, the average size per caravan was 1,000 camels, but some caravans were as large as 12,000. The caravans would be guided by highly-paid Berbers, who knew the desert and could ensure safe passage from their fellow desert nomads. The survival of a caravan was precarious and would rely on careful co-ordination. Runners would be sent ahead to oases so that water could be shipped out to the caravan when it was still several days away, as the caravans could not easily carry enough with them to make the full journey. In the mid-14th century, Ibn Battuta crossed the desert from Sijilmasa via the salt mines at Taghaza to the oasis of Oualata. A guide was sent ahead and water was brought on a journey of four days from Oualata to meet the caravan. Culture and religion were also exchanged on the Trans-Saharan Trade Route. Many West African states eventually adopted Arabic writing and the religion of North Africa, resulting in them being absorbed into the Muslim world. Early trans-Saharan trade Ancient trade spanned the northeastern corner of the Sahara in the Naqadan era. Predynastic Egyptians in the Naqada I period traded with Nubia to the south, the oases of the Western Desert to the west, and the cultures of the eastern Mediterranean to the east. Many trading routes went from oasis to oasis to resupply on both food