We cannot possibly be alone ... not a chance. But the FORMS of intelligent life that exist elsewhere must be unimagineably different. After all, look at the astonishingly varied life forms on just this planet - and we all evolved in the same gravity, atmospheric pressure, temperature variables, etc.... What kind of intelligent creature would have evolved on a planet with say - a mere 110% our mass (gravity)? Or a mere 90%? And those are just minor variables. What about 140%? 65% And astmospheric pressure would cause unimagineable variants in life forms. Jabba the Hut comes to mind.
And there's another variable we forget: T I M E. If the entire history of intelligent homo-sapiens on Earth was placed on a template of 24 hours, electricity was only harnessed at 11:59 pm. And using that same "50,000 years = 24 hours" template, there were several months' worth of life on Earth before homo sapiens developed intelligence at 12:01 a.m.
If aliens have ever visited Earth, it would have been like winning the "Lottery of every Lottery winner ever" for intelligent life to have been present when they arrived. That's one thing Thor Heyerdahl had wrong, and Erich van Daniken and Graham Hancock had right: visiting astronauts MUST have been ancient. Pre-antiquity by all odds.
If that doesn't boggle the mind, try this:
The Sun is one of 200 Billion stars in our galaxy. There are 180 Billion known galaxies.