A trampoline effect occurring in the stages of planetary reseeding

Abstract

Impactors have hit the Earth since its formation and have continued to be infrequent guests throughout the Earth's history. Although the early part of the Earth's history was marked by these violent events, life was present early, possibly existing already in the Hadean Eon. It is possible that life has been, and still is, transported between the worlds of the solar system, owing to impacts leading material away from the impact region. Beyond this lithopanspermia hypothesis, in the so-called ‘refugium hypothesis’, ejected material has been suggested to also return to its home planet and 'reseed' life after the world has recovered after a global impactor, thus restarting evolution. In addition to such impactors, more frequent impacts from smaller non-sterilizing impactors existed during the Heavy Bombardment epoch, feeding material potentially harbouring viable organisms into near Earth space. During the three stages of planetary self-reseeding, the encapsulated microbial population experiences abiotic stressors; specifically, they experience pressure and heat shock twice, in stage 1 and after a recovery phase in stage 2, and again in stage 3. Although many circumstances have played a role in the endurance of life in the early history of the Earth, a particular biological effect could potentially be conferred on a microbial population in this scenario. Thus, the surviving population not only would experience an increase in the frequency of robust genotypes but also would be expected to have greater stress tolerance than non-stressed organisms of the same species. Hence, because of the trampoline effect, the mean robustness of the microbial population towards these stressors is higher in stage 3 than stage 1. In principle, the time between the impactor and the reimpactor need not be long before this trampoline effect appears. Experiments simulating stage 1 must take this effect into consideration in estimating the survival probabilities of a population of organisms in worlds such as the past Earth. Thus, the stages of planetary self-reseeding can themselves be considered facilitators of a process that enhances the stress capacity of the collected microbial organisms and thus their survival capacity. This process may have played a role in the survival of life through violent periods of Earth's history and thus may affect inhabited worlds in general.