Qatar has rock carvings, inscriptions and fragments of pottery dating from 4000 BC. The early population was swelled by seasonal migration of Arab tribes and the peninsula became a centre for fishing and pearls. Commercial activity declined in the Roman era, when trade was concentrated in the Red Sea, but recovered in the 3rd century AD. Islam was established in Qatar in the mid-7th century AD. During the Abbasid period (750–1258) Qatar enjoyed strong relations with the Caliphs in Baghdad. After briefly coming under Portuguese influence in the early 16th century, Qatar fell under Ottoman sovereignty. For the next four centuries it was nominally part of the Ottoman Empire, though considerable power remained with local tribal sheikhs.