It'd be interesting to learn the depth of study related to spectrum of scenarios in which human race will be facing apocalypse. Few things that pop up in my head: - disease - nuclear (or comparable) war - depletion of natural resources - natural disaster - extra terrestrial influence: asteroid or alien invasion - artificial intelligence - anarchy / disruption of social security hmm...anything else?
Doesn't have to robots and test tubes, just separate groups that drift apart far enough that they can't interbreed (definition of species) and one species wipes out the other as often happens, and if the species that got wiped out got to hold the legacy "human race" species, well then...
You have a definition problem where evolution is obviously quite natural, just like comets striking the earth is natural (although very rare), so those are natural disasters, although thats probably not what you meant.
From history, it usually takes a couple things to wipe out "apocalypse" a civilization although it often only takes one thing to get the ball rolling.
My money is on something like "peak phosphorous for fertilizer production for cereal grains causes supply chain shock leading to self destruction"