According to literary sources, by the post-Vedic period, 800 - 600 BCE, the centre of the Aryan homeland had shifted to the upper Yamuna and Ganga basins. Over a period of time, through conquest and territorial expansion, the eight janapadas increased to sixteen mahajanapadas. This was the iron - age period and scholars have pointed out the importance of iron for urbanization. Iron tools were used for clearing the dense forest and as ploughs which, along with the use of draughts animal generated surplus food. In South India there has been evidence of city life from the third century BCE. The territorial organization known as 'mandalams' were equivalent to the mahajanapadas of the North.