What Earth Probably Was, and Most Likely Was Not, in the Earliest Times
Life emerged on Earth by unquantified mechanisms. Several of such mechanisms have been proposed so far, Stephen J. Mojzsis will demonstrate in his talk that four of these can be ruled out: First, a "prebiotic soup" could not have been generated, because Earth’s primordial atmosphere was not fed by a more reduced mantle. Second, Earth was never entirely covered by oceans. Dry land can therefore be considered as a primordial phenomenon. Third, comets such as those of the Late Heavy Bombardment (LHB) or the Late Veneer (LV) could not have been the main source bringing water to Earth. Evidence exists for a hydrosphere well before the LHB. Fourth, it is inconceivable that such a bombardment could have erased microbial life. Evidence shows that both sedimentary rocks and a biosphere existed at LHB time.
Stephen J. Mojzsis is Professor for Geology at the University of Colorado in Boulder, USA.