THE ROLE OF METEORITE IMPACTS IN THE ORIGIN AND EVOLUTION OF LIFE

Impact cratering is a geological process that is common to all planetary bodies in the solar system. It is now widely accepted that meteorite impacts can negatively affect life on a planet, as evidenced by the deleterious effects associated with the formation of the Chicxulub impact structure, Mexico, 65 Myr. ago and its link to the Cretaceous–Paleogene mass extinction event. This impact event had a profound affect on the evolution of life on Earth by ending the age of the dinosaurs and paving the way for mammals to ascend to dominance. Further back in geological time, it is likely that impact events also affected the evolution of life at various stages in Earth history.

In terms of the origin of life, it is likely that it did so during one of the harshest, most inhospitable times in Earth history: the Late Heavy Bombardment Period ~4.0–3.8 Ga. During this time, asteroid and comet impacts were ~10–20 times as frequent as they are at the present day. This may seem counterintuitive at first until one considers that perhaps these cataclysmic, initially destructive impact events also had beneficial effects. Indeed, research conducted over the past decade has revealed that impact events produce several beneficial effects with respect to microbial life. First, impact events process planetary surfaces producing suitable substrates, such as clays, that have been proposed as a focus for prebiotic chemistry. Second, impact events are now known to produce several habitats that are highly conducive to life. Major habitats include 1) impact-generated hydrothermal systems, which could provide habitats for thermophilic and hyperthermophilic microorganisms, 2) impact-processed crystalline rocks, which have increased porosity and translucence compared to unshocked materials, improving microbial colonization, 3) impact glasses, which provide an excellent readily available source of bioessential elements, and 4) impact crater lakes, which form protected sedimentary basins with various niches and that increase the preservation potential of fossils and organic material. Thus, impact craters, once formed on Early Earth – and by analogy on Mars and other planets – may have represented prime sites that served as protected niches where life could have survived and evolved and, more speculatively, perhaps originated.