Whatever the specific ramifications we will probably never understand, there probably was, for some time, a situation of equilibrium. Symbiosis implies exchange between two parties. When at some stage one of the parties becomes too strong or too weak, the symbiosis is disrupted. In Margiana and Bactria this happened in the late first half of the second millennium BC when pressure from the north upon the oasis settlements of the BMAC simply became too strong for the sedentary population to resist. There was no longer any exchange. The Indo-Iranians who had been in constant contact with BMAC were themselves swept aside by their kinsmen who pushed from behind. Masses of Indo-Iranians settled and passed through the lands that formerly had been inhabited by the BMAC people, without adopting any of their characteristics. It was a matter of numbers. When the Indo-Aryans eventually arrived in the northwest of the Indian subcontinent, their knowledge of the BMAC culture was almost nil. Only recent archaeological research has brought this great civilization from northern Afghanistan back to life.
