Well, the chick grew up into a chicken, but you’re still not sure how the egg got into that cave...

When experiments began in the Aperture canyons, the first subjects were not dragons. Instead, GLaDOS went through nearly a hundred chickens in her first attempts. Subject 42, as her name says, was the forty-second in this line. No one is quite sure what her tests involved, only that she failed them. Like other failures, she was scheduled to become lunch for Emergency Intelligence Incinerator, but instead escaped. She now just tries to get by, hoping to find someone else to help her. (She's considered going to a black mesa, but that's just become a joke now, fat chance she'd ever actually make it there.)