Alexander of Macedon and his muleteer were leveled in death: either they were taken up into the same generative principles of the universe, or they were equally dispersed into atoms.
Reflect on how many separate events, both bodily and mental, are taking place in each one of us in the same tiny fragment of time: and then you will not be surprised if many more events, indeed all that comes to pass, subsist together in the one and the whole, which we call the Universe.
Marcus outlines our duties to animals and rational people. And he, again, reminds us that we should never be concerned with how long we live, as long as we are performing our duties.
No matter if you are Alexander the Great or his servant, you will return to dust. And it doesn't matter if you believe in a god or gods or a random universe; your end will come and you either return to your god(s) or you return to atoms.
Just as there are dozens or hundreds of processes going on in your body (breathing, blinking, moving, talking, listening, seeing, hearing, touching, digesting, etc), so too there are many cogs working in the universe. And just as all those bodily processes are contained in one body, so too are all the universal events contained in one great whole: the Universe.
(see also Citadel p. 48, 148)