The wind over this ocean blows from the southwest for half the year and the northeast for the other half, which means a ship can sail out and be certain of a way home. Michael Pearson's history argues that this single physical fact organised everything else: a merchant sailing from Gujarat to the Swahili coast had to wait months for the wind to turn, and so lived, married, prayed and built in the port he had reached. Religion travelled as a consequence of waiting, not of conquest.
The first great transfer went east. Indian traders, and the brahmins and monks who accompanied or followed them, carried Hindu and Buddhist ideas of kingship, cosmology and temple form to the coasts and river mouths of Southeast Asia in the first millennium, and the results stand at Borobudur and in the Khmer temple mountains. The transmission was not an empire; it was a long conversation carried on the same winds as the pepper.
Islam followed the same routes and used them more thoroughly. By the thirteenth century Muslim merchant communities were established from Mogadishu to Malacca, and conversion in coastal Southeast Asia proceeded largely through trade and marriage rather than through armies. Engseng Ho's study of the Hadrami sayyids, families claiming descent from the Prophet who spread from the Hadramawt valley in Yemen across the ocean, follows a diaspora held together by genealogy, marriage and the tombs of ancestors, and shows how a religious lineage could function as an ocean-wide institution.