The horse turned West African distance from a barrier into a road

From the first millennium BCE horses spread almost everywhere the climate would carry them, reaching as far south in Africa as the highlands of Ethiopia and the open grassland of the Sudan belt. A horse moves people, equipment and messages at a speed that changes what a government can do. Campaigns reach further, and orders arrive while they still matter. Horses came into West Africa along trade routes that already crossed the Sahara, carried from Carthage into the western Sudan belt by the middle of the first millennium BCE. At first the wealthy kept them for prestige. During the later first millennium CE they became the basis of cavalry in the emerging Sudanic states, Wagadu above all, the power later known as Ghana. Hundreds of kilometres of open grassland between the Niger bend and the Atlantic had been an obstacle to holding territory together. Mounted men made the same ground a corridor, and Wagadu, Mali, Songhay and Kanem-Bornu were built across it.

At Cerrillo, in the west of Puerto Rico, there is a place where people went to work flint. It reads as a workshop rather than a village, since the ground holds the waste of tool manufacture, the flakes and rejected pieces that come off shaping a nodule into something usable. Cerrillo was busiest during the changeover into the Ceramic age, when pottery was being adopted and settlement was reorganising, and the flint had been quarried there before that. The evidence indicates people were taking nodules out of the same deposit back in the Archaic age. That continuity is the interesting part. A source of good stone gets found once and then used for as long as anyone needs stone, across changes in pottery, diet and social arrangement that archaeologists treat as boundaries between cultures. The people at the flint outcrop in the seventh century were going to a place their predecessors had gone to for centuries, for the same reason.

Christianity had been seeping into Nubia on a small scale for some time, but the formal conversion of the rulers and, soon after, of most of the population came in the middle of the sixth century with missionaries from Byzantium. Julian, a Monophysite encouraged by the empress Theodora, went south in 543. The rival Melkite mission never got there in time. Theodora persuaded or ordered the governor of the Thebaid to hold it up in Upper Egypt while Julian travelled on. The doctrinal quarrel of a Byzantine court decided which form of Christianity took root on the middle Nile. The kingdom that grew from it kept a literate culture with Byzantine and Coptic colouring, its own line of bishops at Faras, and a cathedral there that the fifth bishop, Paulos, rebuilt in 707, recorded on two foundation stones, one in Greek and one in Coptic.

The first Muslims in Africa landed on the Eritrean coast, and the first mosque on the continent stood at Massawa, possibly built in the 620s or 630s by members of the Prophet's own family. By the tenth century Islam was spreading inland from the Somali coast through the country that is now Somalia, Djibouti, Ethiopia and Eritrea. The usual telling makes the following twelve centuries a straight fight between cross and crescent. Trade reports something quieter. Muslim merchants worked the Christian highlands in numbers and formed a recognised class there known as jiberti, growing wealthy as Red Sea traffic expanded. The Ethiopian state meanwhile cultivated the image of a Christian kingdom besieged in its mountains, a second Israel with the Bible at the centre of its self understanding. The Oromo made Harar a Muslim city of consequence, and its sultanate rose on links to the Funj in Sudan. Sufi orders did the spreading, the Qadiriyya out of Harar into Somalia and Eritrea, the Salihiyya and Ahmadiyya among the Somali.