how did Broach Sopara and tamralipti emerge as urban centres after 600 BC

The old ports of lothal,sopar , baroach and muziris , that line the west as far back as 4500yearsago .while these are well know we know little of the ports and port towns in the east of the many ,perhaps the most important was tamralipti ,a great center of trade and commerce.also know as tamralipti, this port was located in the present day town of tamluk,in midnapur, west bengal.the name tamralipta come from tamra or copper , which was mined nearby at ghatsila in singbum distric of bihar and exported thourgh this port.....