The hard steps model and whole Earth-system transitions.

The hard steps model is a 'toy' mathematical representation of evolution towards complex life on Earth or Earth-like planets. It assumes that, at the longest time scale, the rate of evolution towards increased complexity is governed by unlikely transitions that happen randomly and rarely. Applied to Earth, the model suggests a small number of such transitions in the pathway to 'intelligent observers'-humans. The transitions are usually envisaged as occurring instantaneously, but this ignores the reality that on Earth, the evolution of life and the planetary environment have been inextricably linked. The critical steps should be seen as initiating whole Earth-system transitions that take hundreds of millions of years to complete, as in the events that caused, and followed after, the Paleoproterozoic and Neoproterozoic glacial episodes. These were both caused by, and drivers of, evolutionary advances that were necessary for complex life to arise. I adapt the model to include such delays and show that it then suggests just two or three hard steps to humans. The model predicts that the search for biosignatures in exoplanet atmospheres may find planets with Archean-like atmospheres, but probably will not find the signature of a planet with a Proterozoic or modern Earth-like atmosphere.This article is part of the discussion meeting issue 'Chance and purpose in the evolution of biospheres'.