Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:There is overwhelming evidence that the Indus, Brahmaputra, and Ganges Rivers, existed as separate river systems soon after the original India-Asia collision (~55–45 Million Years (MY) ago) and even before large-scale elevation of the Himalayan mountains (Clift and Blusztajn 2005).
Category: