Why Haven't We Heard From Intelligent Life Yet?

Darwin's warm little pond that spawned life on Earth to his mind could, in fact, have been on Mars and sent life here. We've only been listening for signals from space for the last 85 years so we can only detect new signals coming from a maximum of 85 light years away. If an alien civilization was capable of sending out stuff like radio waves, they travel at the speed of light. It took planet Earth about five billion species, over three and a half billion years to evolve one life form complex enough to send messages out into space. So whilst there could be billions of planets with organisms out there, a lot fewer will be as advanced as us. Alien life might simply be