[wcoenen]

This misconception comes from "the great chain of being"[1]. It's an old idea which still influences us to see evolution as a ladder.

Evolution doesn't work like that though. It's a process that finds local optima for fitness by randomly exploring a very high dimensional space. It moves in myriad directions; there is no single ladder to climb "up".

E.g. for humans, our evolutionary path looks like it selected for increased intelligence. For mycoplasma, it selected for the organism to be as simple as possible.

[1] https://en.wikipedia.org/wiki/Great_chain_of_being