Let's start again from somewhere else. They appear because they are biologically significant, since indulging pleasant feelings and avoiding unpleasant ones tends to promote the survival of the human species. Such a mechanism is almost certainly present in other higher mammalian species. Of course, this cannot be proved. It is likely that any significant differences in the quality of feelings have intellectual causes - in particular the anticipation of future events from knowledge of the past.
Human beings are the only species to recognise the inevitability, and the causes, of death.
This results in a lot of confusion in the MOOS rights discussion. That is the basis of natural selection. But MOOS don't need anything like intellectual appreciation of death or reproduction for the process to work, and indeed they haven't any. They have behavioural tendencies which do in fact generally favour individual survival and reproduction, but that is all. So killing MOOS for food is not objectionable in principle for any reason comparable with human feelings about human deaths.