The emergence of eukaryotes as an evolutionary algorithmic phase transition
pnas.org
pnas.org
I'm willing to speculate that the emergence of intelligence is also not a hard step, although it does take time and maybe the right geoplanetary conditions as [1] argues. So I suspect the fraction of such planets with life that go on to develop intelligent life is close to 1 in the Drake equation.
[1] A reassessment of the "hard-steps" model for the evolution of intelligent life: