I highly doubt there was ever life on Mercury or Venus. Mercury has too much solar radiation, and Venus is even hotter than mercury because it's atmosphere is made up of carbon dioxide which creates a greenhouse effect, making it the hottest planet it our solar system, and not to mention the surface is mostly made up of an acid.

I do think though that there is intelligent life out there somewhere. It's inevitable, basically sapce is infinite meaning the stars within it must be infinite as well. These stars have planets orbiting around them just like our star does. So with basically an infinite amount of planets, I think it's impossible that our planet is the only one that has intelligent life.