Other Earths, Gaian bottlenecks and Darwinized Gaias.

In our galaxy, tens of billions of wet, rocky Earth-like planets orbit in the habitable zones of their host stars. Stellar spectra and models of devolatilization yield ranges of plausible chemistries on these Earths. The early emergence of biology on Earth from similar chemistry has a chance of being universally relevant to the origins of life elsewhere. The Gaian bottleneck suggests that most life does not establish Gaian regulation quickly enough to counter positive feedback of runaway greenhouse or runaway ice albedo. Methane (a strong greenhouse gas) was produced by methanogens and oxidized by anaerobic methanotrophs. Their combined effects could have influenced global temperature. As the branches of the phylogenetic tree of life diverge and get pruned, the last universal common ancestor (LUCA) changes and evolves. This adds a time-dependence to LUCA. We argue that the source of LUCA's adaptations is evolution by natural selection at the sub-global level through the extinction of competitors. By surviving a persistence/non-persistence-based selection on the global level, these adaptations qualify as global-level adaptations or Gaian regulation. Thus, within a Darwinian framework, LUCA's sub-global adaptations can become Gaia's global adaptations: Darwinized Gaias may be rare but they are plausible.This article is part of the theme issue discussion meeting issue 'Chance and purpose in the evolution of biospheres' .