> I gather the Indus Valley civilisation comprised a couple or more of
>(possibly linked) city states, so they would not be tribal societies as I
>understand it. They probably arose directly from tribal societies though,
>but their customs and social structure would have had to adapt to the
>changed material circumstances.
There wasn't a caste system in the Indus Valley civilisation.
> By the way, I'm curious about these "aryans", were they some kind of
>marauding nomads like the Mongols, or the Vikings, who conquered then
>settled an area? And what was so special about them that they get a
>historical period named after them?
I don't know of any historical period named after them. There is or was no Aryan race. Marauding was certainly an important activity for Aryans. Cattle were the main source of food and measure of wealth. The use of the horse was vital; it provided speed in tactical manoeuvres and superiority in battle. But what justifies use of one name "Aryan" for a large group of people is a common family of languages, viz. Indo-European group of languages. Sanskrit, Latin and Greek were Aryan languages.
Ulhas