Well, as a matter of strict terminology, we've had "alien life" as part of the game since almost day 1 - Jangala in the Corvus system (the very first, and only for a while, campaign system in the game) has a natural, but aggressive Inimical Biosphere on it that requires burning back every once in a while. Randomly generated planets can also come up with Inimical Biosphere, presumably also natural, alien life.
The icons that scare me (or possibly intrigue me) more are death, and xenoplague.