The idea of a mass migration from the north has fallen out of favor among scholars not only because it has become so politicized, but also because archaeologists have realized that major cultural shifts in the archaeological record do not always imply major migrations. And, in fact, there is scant archaeological evidence for such a population movement. There are no obvious layers of ash and destruction around thirty-eight hundred years ago suggesting the burning and sacking of the Indus towns. If anything, there is evidence that the Indus Valley Civilization’s decline played out over a long period, with emigration away from the towns and environmental degradation taking place over decades. But the lack of archaeological evidence does not mean that there were no major incursions from the outside. Between sixteen hundred and fifteen hundred years ago, the western Roman Empire collapsed under the pressure of the German expansions, with great political and economic blows dealt to the western Roman Empire when the Visigoths and the Vandals each sacked Rome and took political control of Roman provinces. However, there so far seems to be little archaeological evidence for destruction of Roman cities in this time, and if not for the detailed historical accounts, we might not know these pivotal events occurred. It is possible that in the apparent depopulation of the Indus Valley, too, we might be limited by the difficult archaeologists have in detecting sudden change. The patterns evident from archaeology may be obscuring more sudden triggering events.
What can genetics add? It cannot tell us what happened at the end of the Indus Valley Civilization, but it can tell us if there was a collision of peoples with very different ancestries. Although mixture is not by itself proof of migration, the genetic evidence of mixture proves the dramatic demographic change and thus opportunity for cultural exchange occurred close to the time of the fall of Harappa.