The hard steps model and whole Earth-system transitions.

Area of Science:

• Astrobiology
• Evolutionary Biology
• Planetary Science

Background:

• The 'hard steps' model simplifies evolution towards complex life as rare, random transitions.
• Previous models overlooked the interplay between life's evolution and planetary environmental changes.
• Earth's history shows critical evolutionary steps are linked to long-term, whole-Earth system transitions.

Purpose of the Study:

• To adapt the hard steps model by incorporating delays for Earth system transitions.
• To re-evaluate the number of critical evolutionary steps required for complex life, specifically humans.
• To predict the potential atmospheric biosignatures observable on exoplanets.

Main Methods:

• Mathematical modeling of evolutionary transitions.
• Incorporation of geological timescales and Earth system dynamics into the model.
• Analysis of evolutionary pathways leading to human-level intelligence.

Main Results:

• The adapted model suggests only two or three critical 'hard steps' are necessary for human evolution.
• These critical steps are associated with prolonged, planet-wide environmental transitions, such as major glaciations.
• Exoplanet atmosphere searches are likely to detect early-Earth-like (Archean) biosignatures, not later (Proterozoic or modern) ones.

Conclusions:

• The evolution of complex life, including intelligence, may depend on a few, long-duration, planet-altering events.
• The model's predictions have implications for the search for extraterrestrial life and biosignatures.
• Future astrobiological searches should consider the likelihood of finding only early-stage complex life signatures.