Some facts can be derived from religious thoughts:
A simple example is the "interpretation of Seal of Shiva, found in mohanjodaro", when i say " interpretation ", it basically means, it is not certain that this seal is of or depicts "Shiva".
Some people think, it is image of Shiva.
This thought could arise from religious ideologies, who try to give idol form to so called Godly figures.
On the other hand, if we look closely to this seal, it shows a human figure. The facial features of this image do not match with south India people or other tribes of of India rather the facial features are more North Western India and more north beyond that area.
If u look closely to what he wears at the head, it is very tribal in nature, shows two horns on both sides and lot of animals from the nature.
http://en.wikipedia.org/wiki/File:Shiva_Pashupati.jpg
In India religious identities have mixed up facts with mythologies, folklores and other imaginary things to suit to their needs and identities. Therefore antiquity of any thought can only be ascertained by deriving it source. For deriving its source one has to explore other things like religions, history, people, etc.