View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002Earth, and from a few crystals incorporated intolater-formed rocks. Based on crater counts onnearby bodies in the Solar System, Earth wassubjected to intense bombardment by bolides inits early history. The impacts were sufficient toimpede the accumulation of water or oceans andthe sustained development of life (Nisbet andSleep, 2001). After half a billion years or so, thebombardment slowed and decreased in intensityas the bolides became smaller and fewer. Thesecomets, asteroids, and meteorites likely brought tothe Earth’s surface considerable amounts of organicmaterial—sufficient for life to start (Anders, 1989;Chyba et al., 1990). The organic molecules servedas the foundation of life; and once life started, thosecompounds, and the continued synthesis of carbonmolecules in the waters of Earth, served as nutrientsfor the newly evolving prokaryotes. Crystals inyounger rocks provide evidence for surface waterat least by 4.3 Ga (Mojzsis et al., 2001). Thus, lifehad what was needed to form and survive very earlyin Earth history: the proper chemicals, a source ofenergy, liquid water, and a trophic resource. Alongwith the first life came the first microbial trophicinteractions.ARCHEAN: 4.0 TO 2.5 GAArchean metasedimentary rocks make up asmall percentage of exposures on the continentstoday, and the oceans of most of this eon areessentially unknown (Lowe, 1992). Life, too, ispoorly known. Thick exposures at Isua, Greenland,dated at about 3.8 Ga, contain carbon isotopicsignatures interpreted as having been fractionatedby biologic activity (Rosing, 1999), although atleast some of that signal may be of metasomaticorigin (Fedo and Whitehouse, 2002; see alsoSchidlowski, 2001). No definitive fossils areassociated with these deposits. Prokaryotic fossilswere described in rocks from about 3.5 Ga (Schopfand Packer, 1987; Schopf, 1993), although thesehave been questioned recently (Brasier et al., 2002).In spite of these fossils’ status, life’s presence isindicated by isotopic evidence (Rosing, 1999) andby other fossil occurrences at about this same timeor slightly later (see Schopf, 1992a for summary).Organisms present in the Archean werebacteria, probably Archaea, and possibly earlyeukaryotes (Brock et al., 1999; Summons et al.,1999), likely both benthic and planktic (Table 3,4).No evidence directly links protists to these earlysystems, but they could have been present—wesimply have no way to tell, other than biomarkersof uncertain reliability. Some molecularphylogenies suggest an early divergence ofeukaryotes (Gu, 1997; Hedges et al., 2001), butthis is an indirect conclusion subject to alternativeinterpretations (Katz, 1999).Benthic trophic structures were likely confinedto prokaryote aggregations, films and mats, someof which made stromatolites and laminatedsedimentary rocks (Simonson and Carney, 1999;Nisbet and Sleep, 2001). A prokaryotic trophicstructure also probably operated in open waters.Both benthic and pelagic trophic structures wouldhave been much simpler than later structures (Table3, 4), as they were surely dominated by prokaryoteswithout larger or more complex organisms. Theorigins of these prokaryotic trophic structures mayhave been benthic, associated with hydrothermalvents, because molecular phylogenies indicate thatthermophilic bacteria and archaeans are basal toother extant groups (Reysenbach and Shock, 2002).This scenario has been seriously questioned onseveral grounds (Bada and Lazcano, 2002; Brocierand Philippe, 2002), and origins and evolution incooler waters were also possible. The basal positionof thermophilic bacteria in the current phylogeniesdoes not necessarily indicate origins, but, if correct,indicates only that the last common ancestor ofliving bacterial lineages was thermophilic. Suchan interpretation excludes all fossil and livinglineages yet to be discovered or studied—far morework needs to be done on living and fossilindicators of prokaryotic and protistan presencebefore confident reconstructions can be made. Yetthe earliest ecosystems probably did not haveparticularly unusual kinds of trophic interactions,although they likely were limited and lackedoxygenic photoautotrophs (Nisbet and Sleep,2001). These benthic and pelagic ecosystems were74