Although Muslim traders first travelled through South East Asia early in the Islamic era, the spread of Islam among the inhabitants of the Indonesian archipelago dates to the 13th century in northern Sumatra. Although it is known that the spread of Islam began in the west of the archipelago, the fragmentary evidence does not suggest a rolling wave of conversion through adjacent areas; rather, it suggests the process was complicated and slow. The spread of Islam was driven by increasing trade links outside of the archipelago; in general, traders and the royalty of major kingdoms were the first to adopt the new religion.