Alternative mechanisms for Gaia.

Highlights • We explore a simple quantitative model of selection by survival - the hypothesis that biospheres can acquire persistence-enhancing adaptations by chance over time. • We compare the survival rates of a set of biospheres undergoing selection by survival to a null hypothesis of survival by pure chance, and to biospheres with added feedback from the environment to the biosphere. • We find that selection by survival alone can increase a biosphere's survival probability. • We find that feedback can either hinder or enhance the persistence of a biosphere depending on the strength of the feedback and how closely initial conditions match those ideal for life. Abstract A long-standing objection to the Gaia hypothesis has been a perceived lack of plausible mechanisms by which life on Earth could come to regulate its abiotic environment. A null hypothesis is survival by pure chance, by which any appearance of regulation on Earth is illusory and the persistence of life simply reflects the weak anthropic principle - it must have occurred for intelligent observers to ask the question. Recent work has proposed that persistence alone increases the chance that a biosphere will acquire further persistence-enhancing properties. Here we use a simple quantitative model to show that such 'selection by survival alone' can indeed increase the probability that a biosphere will persist in the future, relative to a baseline of pure chance. Adding environmental feedback to this model shows either an increased or decreased survival probability depending on the initial conditions. Feedback can hinder early life becoming established if initial conditions are poor, but feedback can also prevent systems from diverging too far from optimum environmental conditions and thus increase survival rates. The outstanding question remains the relative importance of each mechanism for the historical and continued persistence of life on Earth. [ABSTRACT FROM AUTHOR]