In the offices where you can see through those large windows inside the test chambers there are chairs and computers. Suggesting that there were once workers there. Is every employee that was involved in Aperture Science essentially dead due to the passing of time, leaving only GLaDOS alive because, well, it's self-explanatory? Did they actually watch and monitor the test subjects go through the tests in the beginning, working alongside GLaDOS? Throughout the game you don't stumble upon a single human being either directly or indirectly, only the voice of GLaDOS.

Also, Is Chell a clone? Because throughout the game you can find areas with messages saying "Help", "the cake is a lie", etc. Or are they just different individuals? To be honest to me it makes more sense that she's a clone because the whole story revolves around GLaDOS and its capabilities, but of course this is pure speculation. She could have been abducted. Or she could be one of the employees in Aperture Science who was forced to be a test subject by GLaDOS, whose fate other employees in the facility may also have had.

However, it seems that GLaDOS somewhere in the game it states that it has the main characters' brain backed up in case something happens, so that could suggest she may be a clone. But GLaDOS also has a reputation of being a liar. So maybe it was just another one of its lies.