The effects of heavy meteorite bombardment on the early evolution--the emergence of the three domains of life.

A characteristic of many molecular phylogenies is that the three domains of life (Bacteria, Archaea, Eucarya) are clearly separated from each other. The analyses of ancient duplicated genes suggest that the last common ancestor of all presently known life forms already had been a sophisticated cellular prokaryote. These findings are in conflict with theories that have been proposed to explain the absence of deep branching lineages. In this paper we propose an alternative scenario, namely, a large meteorite impact that wiped out almost all life forms present on the early Earth. Following this nearly complete frustation of life on Earth, two surviving extreme thermophilic species gave rise to the now existing major groups of living organisms, the Bacteria and Archaea. [The latter also contributed the major portion to the nucleo-cytoplasmic component of the Eucarya]. An exact calibration of the molecular record with regard to time is not yet possible. The emergence of Eucarya in fossil and molecular records suggests that the proposed late impact should have occurred before 2100 million years before present (BP). If the 3500 million year old microfossils [Schopf, J. W. 1993: Science 260: 640-646] are interpreted as representatives of present day existing groups of bacteria (i.e., as cyanobacteria), then the impact is dated to around 3700 million years BP. The analysis of molecular sequences suggests that the separation between the Eucarya and the two prokaryotic domains is less deep then the separation between Bacteria and Archaea. The fundamental cell biological differences between Archaea and Eucarya were obtained over a comparatively short evolutionary distance (as measured in number of substitution events in biological macromolecules). Our interpretation of the molecular record suggests that life emerged early in Earth's history even before the time of the heavy bombardment was over. Early life forms already had colonized extreme habitats which allowed at least two prokaryotic species to survive a late nearly ocean boiling impact. The distribution of ecotypes on the rooted universal tree of life should not be interpreted as evidence that life originated in extremely hot environments.