Also, consider that all the examples we use of life share similar DNA. A monkey differs very noticably from a human (at least the difference is obvious from a human perspective
) and that's only the difference of a small percent of DNA. Even a tree has DNA that likely shares a lot of commonalities with humans, yet we hardly recognize trees as being alive and these are still terrestrial life forms. Consider that evolution on a distant planet might not even use DNA and then try to imagine the possible differences and that doesn't even include the very likely vast difference in technologies. Humans, on Earth 500 years ago likely wouldn't even recognize a limousine as a human artifact and that's something created by their own offspring a bit over a dozen generations later. I doubt we'd even recognize an alien life form if we saw one much less see an alien that had human traits.
Back to the thread title "Do aliens exist?" - if the universe is eternal and life can come and go, then it's not impossible that life on Earth is a rather uncommon event. It seems we'll never have a good idea whether or not life exists elsewhere until we actually begin exploring more of the universe ... and who knows if we might not walk right past life and not even recognize it.