The Universe is a huge place... expanding constantly, no discernable center or edge, inside or outside, top or bottom... For the planet earth to be the only planet containing intelligent life is an extremely lonely thought. If our planet can support intelligent life, surely there are other planets out there which can sustain intelligent life as well. Maybe life that has different surroundings than ours, perhaps they might breathe nitrogen as opposed to oxygen. But with the vast size of the universe, and with so many star systems, there is almost certainly at least one other planet capable of or already sustaining life with a similar or greater level of intelligence than our own, or that is not far from obtaining this level.