People get this movie the wrong way round (spoilers)
the belief that the captured Humans have been augmented, they have alien legs, hands, grafted to them , that the individuals are humans with aliens addons.
When the doctor ask's the sample question "are you from earth" and he responds "yes" i think indicates these are actually Alien's with human add-on's.The captives are not from earth they just think they are due to the experiments.
The human has been grafted onto the Alien not the other way round.
The central theme is that these 'people' including the old lady and the trucker they still have human will, i think the captive, experimented aliens have more so adopted human trait's, emotion's, memories as in the case of the doctor who is not human but has a human face, human body.