To be clear, microbial life in no way entails higher life. It's only by the insanely chance event of the eurkaryotic cell emerging that sophisticated multicellular life became possible on earth. Even if life is common in the universe, sophisticated multicellular life is extremely rare.
The selective pressures on bacterial life favors small size and rapid reproduction, jettisoning any unnecessary genetic material.
Mitochondrial ancestors relieved those pressures in a couple ways. Once the eukaryotic cell developed, multicellularity has evolved several times. All the cool things (multicellularity, eyes, flight) have evolved independently a bunch of times, and the eukaryote has happened only fucking once. Here's a deeper explanation of why it's so rare: http://rstb.royalsocietypublishing.org/content/368/1622/2012...
Even if life is common in the universe, multicellular life is extremely rare because the evolution of eukaryote-esque organisms is extraordinarily rare and chance.
Edit: That is to say, there's a strong case that step 4 is the Great Filter: http://en.wikipedia.org/wiki/Great_Filter#The_Great_Filter
