This question already has an answer here:
We know that Thanos
killed half of the universe, but did he only kill advances lifeforms or simpler ones, too?
Is there any evidence or examples where Thanos cut the line between different lifeforms?
I think Thanos is a really consequent character, so it would be reasonable
to disintegrate half of all life.
There was a question (unfortunately I can't find it anymore), if the event affected half of the population of every planet, or the whole universe. So my follow-up question would be:
Did the event affect half of all lifeforms on a planet as a whole or only half of one specific species separately?
Note that on the latter case the chance that all humans die/live can vary much more, because only one litre of water contains millions (or billions) of animals, so the number of humans is insignificant.