Aarhus University, Ny Munkegade 116, DK-8000, Denmark.
Biosystems. 2021 Jul;205:104412. doi: 10.1016/j.biosystems.2021.104412. Epub 2021 Mar 27.
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.
自地球形成以来，撞击物便不断地光临地球，而且在地球历史的整个过程中，它们一直是稀客。尽管地球历史的早期充满了这些剧烈的事件，但生命很早就出现了，可能早在冥古宙就已经存在了。由于撞击会将物质从撞击区带走，因此生命有可能已经并仍然在太阳系的各个世界之间传播。除了这种撞击散布假说之外，在所谓的“避难所假说”中，还提出了被抛出的物质也会回到其母行星，并在全球撞击后使生命“重新播种”，从而重启进化。除了这些撞击物之外，在重轰炸期还存在着更频繁的、来自较小非灭菌撞击物的撞击，这些撞击将潜在含有存活生物的物质送入近地空间。在行星自我播种的三个阶段中，被包裹的微生物种群经历着非生物胁迫因素；具体来说，它们在第一阶段经历了两次压力和热冲击，在第二阶段的恢复阶段后再次经历，在第三阶段再次经历。尽管许多情况在地球早期历史中对生命的生存起到了作用，但在这种情况下，一种特殊的生物学效应可能会赋予微生物种群。因此，幸存的种群不仅会经历更频繁的强壮基因型的增加，而且还会比同一物种的非应激生物具有更高的应激耐受性。因此，由于蹦床效应，与第一阶段相比，在第三阶段微生物种群对这些胁迫因素的平均稳健性更高。原则上，在这种蹦床效应出现之前，撞击物和再撞击物之间的时间不必很长。模拟第一阶段的实验必须考虑到这一效应，以便估计过去地球等世界中生物种群的生存概率。因此，行星自我播种的阶段本身可以被视为一个过程的促进者，该过程增强了收集的微生物生物的应激能力，从而提高了它们的生存能力。这个过程可能在地球历史上生命经历暴力时期的生存中发挥了作用，因此可能会影响一般的可居住世界。
