What makes you human?
SF writers have been exploring this question for a long time. One approach has been to use an android - said machine goes on a lengthy quest to emulate its "superior" human creators. Two famous examples are The Bicentennial Man and Other Stories and Data from Star Trek: The Next Generation. Generally, the android starts out more or less niave and incomprehending of human nature and gradually learns to emulate humans more accurately. Emotion and death seem to be characteristics singled out as definining humanity.
Well, that's been done before, so why not do it with aliens instead? In fact, let's have two
aliens that try to hide amongst the Earthlings and contrast how that affects them. These aliens are not the same species as each other but they both turn out to be physically much more robust than life from Earth in general, so they survive through a looooong time on our planet and see many changes. Both are looking for others who are also not local...
The story is diverting enough and easy to read. I feel that I should have guessed how the ending would play out but I didn't. A competent but not greatly remarkable book.