Searching for signs of alien life—biosignatures—on distant planets requires general hypotheses on the evolution of biospheres, and how life interacts with and influences its planet. We will not be able to visit a distant planet to directly observe any alien life and thus measuring properties of a biosphere such as organism abundance or reproduction rate is impossible. This paper provides an overview of two life–environment coupled models and analyses how their results might aid in guiding our search for biosignatures. The first model is of a simple methanogen biosphere and demonstrates that the ‘strength’ (i.e. the concentration of a gas in the atmosphere) of the biosignature is only very minimally impacted by the population dynamics of the biosphere. This reduces the number of assumptions needed to predict biosignatures for a given metabolic pathway. The second model is of an evolving life–environment coupled system—the Tangled Nature model—and demonstrates that the most probable trajectory for an exo-biosphere is to diversify over time owing to a mechanism known as sequential selection with memory. These models can inform our search for alien life in the galaxy. This article is part of the discussion meeting issue ‘Chance and purpose in the evolution of biospheres’.
Arwen Nicholson (Thu,) studied this question.