  We use the critical step model to study the major transitions in evolution on
Earth. We find that a total of five steps represents the most plausible
estimate, in agreement with previous studies, and use the fossil record to
identify the potential candidates. We apply the model to Earth-analogs around
stars of different masses by incorporating the constraints on habitability set
by stellar physics including the habitable zone lifetime, availability of
ultraviolet radiation for prebiotic chemistry, and atmospheric escape. The
critical step model suggests that the habitability of Earth-analogs around
M-dwarfs is significantly suppressed. The total number of stars with planets
containing detectable biosignatures of microbial life is expected to be highest
for K-dwarfs. In contrast, we find that the corresponding value for intelligent
life (technosignatures) should be highest for solar-mass stars. Thus, our work
may assist in the identification of suitable targets in the search for
biosignatures and technosignatures.
