<P> Lukacs and Hemphill suggest an initial local development of Mehrgarh, with a continuity in cultural development but a change in population . According to Lukacs and Hemphill, while there is a strong continuity between the neolithic and chalcolithic (Copper Age) cultures of Mehrgarh, dental evidence shows that the chalcolithic population did not descend from the neolithic population of Mehrgarh, which "suggests moderate levels of gene flow ." Mascarenhas et al. (2015) note that "new, possibly West Asian, body types are reported from the graves of Mehrgarh beginning in the Togau phase (3800 BCE)." According to Narasimhan et al. (2018), the IVC - population likely resulted from a mixture of Iranian agriculturalists and South Asian hunter - gatherers, and came into being between ca . 4700--3000 BCE . </P> <P> The Early Harappan Ravi Phase, named after the nearby Ravi River, lasted from c. 3300 BCE until 2800 BCE . It is related to the Hakra Phase, identified in the Ghaggar - Hakra River Valley to the west, and predates the Kot Diji Phase (2800--2600 BCE, Harappan 2), named after a site in northern Sindh, Pakistan, near Mohenjo - daro . The earliest examples of the Indus script date to the 3rd millennium BCE . </P> <P> The mature phase of earlier village cultures is represented by Rehman Dheri and Amri in Pakistan . Kot Diji represents the phase leading up to Mature Harappan, with the citadel representing centralised authority and an increasingly urban quality of life . Another town of this stage was found at Kalibangan in India on the Hakra River . </P> <P> Trade networks linked this culture with related regional cultures and distant sources of raw materials, including lapis lazuli and other materials for bead - making . By this time, villagers had domesticated numerous crops, including peas, sesame seeds, dates, and cotton, as well as animals, including the water buffalo . Early Harappan communities turned to large urban centres by 2600 BCE, from where the mature Harappan phase started . The latest research shows that Indus Valley people migrated from villages to cities . </P>

Mention briefly the suggested causes behind the decline of the indus valley civilization