Yea if you had the game "XCOM Enemy Unknown" in your steam folder you would have gotten Evolve added to your library.
Want to play Evolve's Big Alpha on PC but don't have a code? If you own XCOM: Enemy Unknown you're in luck.
Publisher 2K's community manager Stephen Reid announced via the game's Steam page that if you already own XCOM: Enemy Unknown, you can now download and participate in Evolve's Big Alpha. Just check your Steam library.
To find out what the Big Alpha includes and if your PC can run it, check out developer Turtle Rock Studios' announcement from last week.