That's very difficult to say. If you're referring to the homo sapien variety then of course we are. If it's extraterrestrial then there is that possibility. But unless one of them steps forward and announces their presence, then we'll have no way of knowing.
There is talk, however, of microbial alien life forms that were either brought back via space shuttles, or microbes that were brought in via meteors that were not incinerated in Earth's atmosphere. In fact, if there is life in outer space it's likely in microbial form and not humanoid.
Of course, there is the theory that neither evolution nor Biblical explanations account for human existence. Instead, we are left over from space explorations of eons ago. And there's the theory that Earth is a penal colony used by galactic beings. This would account for the genetic flaw that prevents us from learning from past mistakes. This genetic imbalance disqualified us from living among higher life forms in the solar system.
A few scientists (whose names I forget) contend that they can prove alien life forms exist on earth by using mathematical calculations.
Still another plausible theory is that the Hadron Collider, which has been restarted in Switzerland, will prove beyond a doubt that extraterrestrials exist on Earth. The Collider is also supposed to create a Black Hole that will send us spinning into oblivion. That should happen about the year 2012.