What percentage of stars have solar systems that contain or once contained intelligent life?

I believe a complex set of circumstances allowed for intelligent life to develop on our planet.If the dinosaurs had not been wiped out i doubt that man would have had s chance to even evolve let alone exist.Therefore the chances of intelligent life existing elsewhere is slim.Let's face it,we are trying our hardest to find and contact alien life why would'nt they be doing the same?If they are intelligent and advanced it is easy to know that if they had no curiosity they would not have become an advanced civilization.