By life in general, no we aren't alone, but there's a chance that multicellular creatures are exceptionally rare. On this planet all multicellular life are classified as eukaryotes. Eukaryotic cells boast their own personal "power plants", called mitochondria. These tiny organelles in the cell not only produce chemical energy, but also hold the key to understanding the evolution of the eukaryotic cell. The complex eukaryotic cell ushered in a whole new era for life on Earth, because these cells evolved into multicellular organisms. But how did the eukaryotic cell itself evolve? How did a humble bacterium make this evolutionary leap from a simple prokaryotic cell to a more complex eukaryotic cell? The answer seems to be symbiosis — in other words, teamwork.
Evidence supports the idea that eukaryotic cells are actually the descendents of separate prokaryotic cells that joined together in a symbiotic union. In fact, the mitochondrion itself seems to be the "great-great-great-great-great-great-great-great-great granddaughter" of a free-living bacterium that was engulfed by another cell, perhaps as a meal, and ended up staying as a sort of permanent houseguest. The host cell profited from the chemical energy the mitochondrion produced, and the mitochondrion benefited from the protected, nutrient-rich environment surrounding it. This kind of "internal" symbiosis — one organism taking up permanent residence inside another and eventually evolving into a single lineage — is called endosymbiosis.
The universe could be filled with prokaryotic life, the conditions for the symbiosis to produce eukaryotes might even be unique, or there may be other means for complex life to develop but we have no examples to go by.
I have no problem with the notion that we are probably not alone in this galaxy, and more certainly in the universe. However, it is a little like seeing something odd in the sky that does not lend itself to explanation and coming to the conclusion that it is a vehicle being operated by an extraterrestrial intelligence. That's just the wrong way to do things. It should remain an Unidentified Flying Object until sufficient evidence is available to make a reasonable and supportable guess.
the issue isn't just "where" it's "when".If we think about how thin a sliver of time humanity has/will exist. It's temporal overlap of another intelligent life form's existence seems very unlikely, much less time enough to allow for discovery and travel and stuff.
Unless we can find some seriously radical method of extremely high-speed travel, we may as well be. It is laughable that so many people think we are visited by aliens constantly, when the distances are absolutely staggering. A signal from Voyager 1, traveling at the speed of light, takes 17 hours one way to reach Earth. That's 17 light-hours, not even close to one light year. Even if we are able to contact another civilization, the time it would take messages to get back and forth and the time it would take to make sense of them would be far to long to make much difference. Yet people actually worry about the possibility of alien invasions. Your chances of being hit by lightning are astronomically higher.