So by the definition when the first people landed on the moon in 1969, humans became the first aliens factually known to exist. This is interesting because many people argue the existence of "aliens" when a handful of astronauts technically were "aliens" at one point in their lives. As I said, the problem lies with our rigidity of definitions. We have created a mental construct of what something is, and due to stubbornness we refuse to see it any other way; when that construct is not necessarily the truth, but it's just our description and personal understanding.
One may argue that that isn't an alien because its just a human on the moon, but humans did not grow, live, and evolve on the moon. So by definition humans are indeed aliens relative to the moon. There are not any lifeforms living on the moon to observe us and define us as aliens, but based on the common definition an alien is any creature that did not originate from the specific location. It is all relative. That is why some people are so concerned about "illegal aliens" because they feel they don't belong here. The illegal alien was born someplace else so that's where they should stay. So practically everything is relative to the way you perceive it and understand it. Most people wouldn't think of humans as aliens, but if we have to wear a space suit in order to survive, I think its safe to say we don't belong there, therefore are alien.
I hope this article has made you question your definitions of what exactly is alien. Remember, all words are the invention of people. Its just by habit and time that we assume they have always existed.
Check out the latest article about Aliens: The Forbidden Topic and the Mathematical Certainty of their Existence.