- Oceans present 4.4-4.2 Ga
Continental crust 4.5-4.2 Ga :—Age of the Earth 4.53 Ga
FIGURE 2.2 Summary of major events in Precambrian history. (Modified from Schopf et al., 1983; and Taylor and Taylor, 1993.)
proliferation of life is thought to have been hampered by early bombardment of Earth by asteroids, which continued until around 4.1-3.8 Ga, the Late Bombardment phase, although this idea is still controversial (Koeberl, 2006). Most theories on the origin of life on Earth suggest that life began with the synthesis of organic compounds, both in the atmosphere and on the surface of the Earth (Chang et al., 1983). These became increasingly complex, and eventually molecules arose that had the ability to duplicate themselves (self-replicating) and to perform other complex syntheses. The reports of organic matter in carbonaceous chondrites, a type of meteorite, confirm that organic synthesis has occurred in our solar system and beyond (Ehrenfreund et al., 2001), and numerous laboratory experiments have been performed to replicate these early syntheses. Many researchers have suggested that an RNA (ribonucleic acid) world preceded the DNA (deoxy-ribonucleic acid) world that exists today (Gesteland et al., 2006). The RNA world hypothesis is based in part on the fact that RNA is a simpler molecule than DNA. It can also replicate itself, encode and build proteins, and function to catalyze reactions (Joyce, 2002; W. R. Taylor, 2006). Moreover, RNA is the principal component of ribos-omes, the intracellular bodies where proteins are synthesized, thus suggesting that RNA, rather than DNA, was the original information-containing molecule governing protein synthesis. Others suggest that a pre-RNA molecule, perhaps a peptide nucleic acid, may have been important in the early development of life (Nelson et al., 2000). This chemical synthesis of life is termed abiogenesis.
ORIGIN OF LIFE: THEORY AND BIOLOGY
Darwin hypothesized in the late nineteenth century that life on Earth probably arose in a "warm little pond," and since then this idea has been widely believed. There are several barriers to the origin of life in shallow pools, however, including the necessity of shielding early life on the surface from damaging ultraviolet rays. There is also the question of what would have happened to early life on the surface during the Late Bombardment phase, although it has been suggested that the craters formed by the bombardment would have provided an excellent environment for early life (Cockell, 2006). One of the problems with the origin of life in a primeval soup of prebiotic, organic molecules is whether a mix of molecules in liquid would be sufficiently concentrated to ensure the reactions necessary to form more complex macromolecules. It has been suggested that organ-ics may have been attached to layers of clay or pyrite, thus providing the close proximity needed to catalyze reactions (Ferris, 2006). If the early Earth was hot, as some researchers suggest (Lowe and Tice, 2007), these surface organisms would most likely have been anoxygenic photosynthetic hyperthermophiles.
In contrast to the warm little pond hypothesis, the idea that life may have arisen on the ocean floor is more recent. This hypothesis gained support when entire ecosystems were discovered in the early 1980s surrounding deep-sea hydrothermal vents (Waldrop, 1990). Subsequent work has shown that bacterial and archaeal life exists deep in the terrestrial subsurface as well (Amend and Teske, 2005; Chapter 1). In the past 20 years, the diversity of life in extreme environments (extremophiles) has been widely demonstrated, ranging from Antarctic ice to hot springs (thermophiles). Studies of microbial evolution based on rRNA propose that a hyperthermophile represents the ancestral condition in the Archaea and possibly in the Eubacteria as well (Woese, 1987). This deep biosphere is chemosynthetic and thus not dependent upon sunlight for energy. Although some modern hyperthermophiles live in an aerobic environment, because of the low solubility of O2 at such high temperatures and the presence of reducing gases such as H2 S, these organisms are basically anaerobic (Stetter, 2006). These findings suggest that Archean life could have survived periods of asteroid bombardment deep within the ocean or within rocks in earth's crust.
As early as 1988, Gunter Wächterhäuser proposed that life may have arisen around deep-sea vents. The Iron-Sulphur World hypothesis suggests that life began in association with pyrite crystals (FeS22 . This idea holds that carbonate, phosphate, and sulfide ions would be attracted to iron pyrite and would rapidly cover every surface. With prebiotic molecules concentrated in this way, and in the high heat and pressure of hydrothermal vents, which are a ready source of iron and sulfur, reactions could proceed much more rapidly than they could at surface temperatures and pressures (Russell and Hall, 2006). Experimental evidence has shown that pyruvic acid (Cody et al., 2000), acetic acid, and peptides readily form in these conditions (Huber and Wächterhäuser, 1997, 1998). More recently, support for the origin of life near hydrothermal vents has come from an interdisciplinary perspective involving biophysics in combination with molecular biology. In a simulation experiment, Baaske et al. (2007) demonstrated that nucleotides concentrated around hydrothermal pores, such as the pore spaces of rocks found at vents. This concentration depended only on the size of the pores and a hydrothermal gradient. These authors suggest that prebiotic molecules could have been concentrated in this way to form the first protocells. Thus, the Iron-Sulphur World hypothesis appears to be a viable alternative to the warm little pond and provides several advantages over that idea, including survival of life during bombardment and changes in surface conditions on the early Earth (Wächterhäuser, 2000).
Although originally considered only in the realm of science fiction, the possibility that the early Earth was seeded with organic matter from comets or asteroids, termed panspermia, has received serious consideration. For example, Anders (1989) calculated that only dust-sized particles would be slowed by the atmosphere sufficiently to prevent destruction of organics on impact. Chyba et al. (1990), however, assumed that early CO2 atmospheres would be denser and estimated that organics could have accumulated at the rate of 102-107kg/yr from 4.5 to 3.9 Ga during the Late
Bombardment phase. The announcement of evidence of life in a Martian meteorite from the Allan Hills, Antarctica (ALH84001; McKay et al., 1996) seemed to support the idea of panspermia. The evidence has been refuted, however, based on several different types of data (Schopf, 1999; Barber and Scott, 2002).
There are a number of excellent reviews on the prebi-otic (i.e., pre-cellular) origin of life, as well as the atmosphere and environment of early Earth, including books on Precambrian geology and life (Coward and Ries, 1995; Lazcano and Miller, 1996; Orgel, 1994; Schopf, 2002; Knoll, 2003b ; Eriksson et al., 2004; Schoonen et al., 2004; Kesler and Ohmoto, 2006; Reimold and Gibson, 2006; Schopf et al., 2007a).
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