Maybe I'm wrong but I have read nothing about Indus Civilization (http://en.wikipedia.org/wiki/Indus_civilization). Just one of the first civilization with Sumer and Egypt....
Even for just a city state (Harappa, Mohenjo Daro).
Strange, don't you think ?
Absolutly a candidate
The issue is we just don't know enough about them for them to be a good candidate for a civilization.