|Before and after|
That’s what new research seems to claim: before c. 1900 BCE there was a stable moonson on the Thar Desert feeding what is now known as Ghaggar-Hakra River (seasonal) but mythologically referred to as Sarasvati River (a major hub of the Indus Valley Civilization or Harappa culture).
This, according to the researchers of Woods Hole Oceanographic Institution (WHOI) caused that people already living in the banks of that river had to emigrate eastwards towards the Ganges, where monsoons were still stable but where the fertility was not good enough to support large cities as in the West.
Source and more details at the press release and at WHOI.
See also: Dispatches from Turtle Island (Andrew has a nice review of some of the wider implications).