I think that in the evolution of the human species, this must have been a Mutual, two-way stream proces: the more advanced our social intelligence became, the more advanced our society's must have become as well, and the more complex our social environment became, on the other hand, the more important our social intelligence became as well..

An interesting detail here may be, that as far as i know, the only animals that have passed the mirror/self-awarenes test (a test to see whether an animal can recognise itself in the mirror), are social animals.

The proces of evolution is Always based on quality's that are already present, so some of the core-quality's on wich human intelligence is based must be present in solitary animals as well. But i think the social dimension has realy pushed the human evolution into a more intellectual as well as spiritual direction.