Death in dolphins: do they understand they are mortal?
newscientist.com
newscientist.com
For example, when I hold my breath, the urge to breath increases steadily until I am forced to start breathing again. Is this because my body explicitly understands that not breathing eventually results in death, or is it more appropriate to say the response is automatic?
So I disagree that "even the most basic creatures understand this", even for very broad definitions where understand can be interpreted as 'act, instinctively or not, as they would should they have consciousness'.
I suspect many understand pain and have a fear of pain (again, where very loosely pain is 'bad things' they have previously experienced). Death is often a corollary of that pain, so they will act similarly, but I don't believe creatures at a most basic level understand death.
Having said that, this was a fascinating article on a topic that has interested me for years, so I'd love to see more research supporting either your thoughts or mine.