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The new methods employed here will eventually become applicable to other worlds when samples become available for study early in the 21st century. INTRODUCTION If the presence of a stable liquid water veneer on Earth is necessary for the origin, evolution, and propagation of life (Fig. M., 1999, Geochronological studies of the oldest known marine sediments: 9th Annual V. Goldschmidt Conference, Abstract 7602: Houston, Lunar and Planetary Institute, LPI Contribution 971 (CD-ROM). One possible explanation for this zircon age distribution is that the ~2700 Ma grains are metamorphic in origin, the ~3650 Ma grains are igneous, and the oldest grains are xenocrysts inherited from an older rock. In the latter interpretation, the actual intrusive age of the tonalitic protolith of the gneiss would be only 3650 Ma. G., 1989, The effect on Earth's surface temperature from variations in rotation rate, continent formation, luminosity, and carbon dioxide: Journal of Geophysical Research, v. 1), then the best sources of information to understand this phenomenon are water-laid sediments preserved in the geologic record. Ancient (3800 Ma) water-sculpted terrains have been recognized on Mars, and liquid water appears to exist beneath the icy crust of Jupiter's moon Europa. 3850 Ma, interpreted as the age of crystallization of the magmatic protolith by Nutman et al. If this interpretation is correct, then the BIF is at least as old as 3850 Ma. Carbon isotopic evidence of bio-organic activity during deposition of the BIF sediments (Mojzsis et al., 1996) would then suggest that the emergence of life on this planet occurred much earlier than previously thought (Hayes, 1996; Holland, 1997).
Complicating this opportunity is their protracted metamorphic history; the oldest known sediments have been recrystallized under high-grade metamorphism (Griffin et al., 1980) and do not contain recognizable microfossils (Bridgwater et al., 1981). During this bombardment era, conditions may have favored the survival of certain bacteria that survive (and even thrive) in environmental extremes of temperature, pressure, and p H before diversifying into wider ecological niches throughout the planet. Phylogenetic studies using highly conservative ribosomal RNA sequences reveal that the deepest branches of life derive from "heat-loving," or thermophilic, bacteria (Pace, 1997). Several factors were unique to the early Archean surface, including a higher ultraviolet flux from a Sun 30% less luminous at 3800 Ma than today (Kuhn et al., 1989) and impact rates from asteroids and comets many orders of magnitude greater. Together, these conditions would presumably have restricted the number of suitable environments for life to emerge.
More detailed field studies through the 1980s and 1990s, combined with geochronological work on the intruding gneisses in the coastal regions of southern West Greenland, demonstrated that the rocks around Godthåbsfjord (in the southwestern coastal area), including those of the Akilia association, contain older components (Kinny, 1986) than most rocks in the Isua district, 150 km distant.