Trade between Arabia and the Indian west coast is older than any of the religions now practised on either side of it, and when Islam appeared it moved along a route already running. Arab merchants settled at the ports of Malabar and the Konkan, married locally, and produced the Mappila communities of Kerala and the Konkani Muslims of the coast north of them, communities that trace their Islam to commerce rather than conquest and that were established centuries before any Muslim army reached the south of India. The mosque at Kodungallur in Kerala carries a tradition of foundation within the Prophet's own century, which is not independently documented but which the community has held for a very long time.
The same water carried the hajj. Pilgrims from Gujarat, Malabar and the Deccan sailed to Jeddah on the monsoon, a journey of months each way that put Indian scholars in Mecca and Medina for extended periods and brought them home as teachers, a circulation Michael Pearson traces as one of the ocean's constant institutions. The pilgrim traffic was large enough that Mughal and later British authorities regulated it.
Zoroastrians came the other way. Tradition places the arrival of a group fleeing Iran at Sanjan on the Gujarat coast in the eighth or tenth century, negotiating settlement with a local ruler, and Mary Boyce's history follows the community from that landing to the Parsi presence in Bombay.