The most striking examples of self-assembling molecules are called amphiphiles, because they have both a hydrophilic ('water loving') group and a hydrophobic ('water hating') group on the same molecule. Amphiphilic molecules are among the simplest of life's molecular components, and are readily synthesized by non-biological processes. Virtually any hydrocarbon having ten or more carbons in its chain takes on amphiphilic properties if one end of the molecule incorporates a polar or ionic group. The simplest common amphiphiles are therefore molecules such as soaps, more technically referred to as monocarboxylic acids. A good example is decanoic acid, which is shown below with its ten carbon chain:
McCollom et al. (1999) and Rushdi and Simoneit (2001) demonstrated that a series of alkanoic acids and alcohols in this size range can be produced from very simple organic compounds by Fischer-Tropsch reactions that simulate geothermal conditions on the early Earth. It has been found that such molecules readily form membranous vesicles, as shown in Figure 5.1 (Apel et al., 2002). This fact will become important later when we discuss the emergence of cellular life.
Two possible sources of organic compounds on a primitive planetary surface are delivery during late accretion, followed by chemical evolution, and synthesis by geochemical processes in the early atmosphere or hydrosphere. Early investigations focused on chemical synthesis of monomers common to the primary macromolecules involved in living systems, with the goal of determining whether it was possible that biologically relevant compounds were available on the primitive Earth. Most of these studies emphasized water-soluble compounds such as amino acids, nucleobases, and simple carbohydrates. The classic experiments of Miller (1953) and Miller and Urey (1959) showed that amino acids such as glycine
and alanine could be obtained when a mixture of reduced gases was exposed to an electrical discharge. The mixture was assumed to be a simulation of the original terrestrial atmosphere which, by analogy with the outer planets, would have contained hydrogen, methane, ammonia, and water vapour. At sufficiently high energy fluxes, such mixtures of reducing gases generate hydrogen cyanide and formaldehyde, which in turn react to produce amino acids, purines, and a variety of simple sugars.
The possibility that organic compounds could be synthesized under prebiotic conditions was given additional weight when it was convincingly shown that carbonaceous meteorites contained amino acids, hydrocarbons, and even traces of purines (Kvenholden et al., 1970; Lawless and Yuen, 1979; Cronin et al., 1988). Such meteorites are produced by collisions in the asteroid belt between Mars and Jupiter. Asteroids range up to hundreds of kilometres in diameter, and are examples of planetesimals that happened to avoid accretion into the terrestrial planets. Asteroids and their meteoritic fragments therefore represent samples of the primitive Solar System in which the products of prebiotic chemical reactions have been preserved for over 4.5 billion years. It was reasonable to assume that similar reactions and products were likely to have occurred on the Earth's surface.
In the late 1970s it became increasingly clear that the archaean atmosphere was largely of volcanic origin and composed of carbon dioxide and nitrogen rather than the mixture of reducing gases assumed by the Miller-Urey model (Holland, 1984; Kasting and Broun, 1998). This is consistent with the fact that approximately 65 atmosphere equivalents of carbon dioxide are present in the Earth's crust as carbonate minerals, all of which must have passed through the atmosphere at some point as a gaseous component. Carbon dioxide does not support synthetic pathways leading to chemical monomers, so interest was drawn to the second potential source of organic material, extraterrestrial infall in the form of micrometeorites and comets. This was first proposed by Oro (1961) and Delsemme (1984) and more recently extended by Anders (1989) and Chyba and Sagan (1992). The total organic carbon added by extraterrestrial infall over ~108 years of late accretion can be estimated to be in the rage of 1016-1018 kg, which is several orders of magnitude greater than the 12.5 x 1014 kg total organic carbon in the biosphere (www.regensw.co.uk/technology/biomass-faq.asp). From such calculations it seems reasonable that extraterrestrial infall was a significant source of organic carbon in the prebiotic environment. Even today meteorites and interplanetary dust particles (IDPs) deliver organic materials to the modern Earth at a rate of -106 kg/y-1 (Love and Brownlee, 1992; Maurette, 1998).
The discovery of biologically relevant compounds in meteorites also indicated that organic synthesis can occur in the interstellar medium, which immediately leads to the question of sources and synthetic pathways. The most important biogenic elements (C, N, O, S, and P) form in the interiors of stars, then are ejected into the surrounding interstellar medium (ISM) at the end of the star's lifetime during red giant, nova, and supernova phases. Following ejection, much of this material becomes concentrated into dense molecular clouds from which new stars and planetary systems are formed (Ehrenfreund and Charnley, 2000; Sandford, 1996). At the low temperatures in these dark molecular clouds, mixtures of molecules condense to form ice mantles on the surfaces of dust grains where they can participate in additional chemical reactions. Comparison of infrared spectra of low-temperature laboratory ices with absorption spectra of molecular clouds indicates that interstellar ices are mainly composed of H2O mixed with CO, CO2, CH3OH, NH3, and other components, the last ingredients generally comprising 5-15% of the total. The ices are exposed to ionizing radiation in the form of cosmic rays (and secondary radiation generated by their interaction with matter), and ultraviolet photons from stars forming within the cloud.
Laboratory experiments have shown that illuminating such ices with ultraviolet light leads to more complex molecular species (Greenberg and Mendoza-Gomez, 1993; Bernstein et al., 1995; Gerakines et al., 2000; Ehrenfreund et al., 1995). Hundreds of new compounds are synthesized, even though the starting ices contain only a few simple common interstellar molecules. Many of the compounds formed in these experiments are also present in meteorites, comets, and 10 Ps, and some are relevant to the origin of life, including amino acids (Munoz-Caro et al., 2002; Bernstein et al., 2002; Bernstein et al., 2001), and amphiphilic material (Dworkin et al.,2001).
Although it is now clear that organic molecules are synthesized in dense molecular clouds, the molecules must be delivered to habitable planetary surfaces if they are to take part in the origin of life. This requires that they survive the transition from the dense cloud into a protostellar nebula and subsequent incorporation into planetesimals, followed by delivery to a planetary surface. During the late bombardment period, which lasted until about 4 billion years ago, the amount of extraterrestrial organic material brought to the prebiotic Earth was likely to have been several orders of magnitude greater than current rates of infall (Chyba and Sagan, 1992). Thus, the early Earth must have been seeded with organic matter created in the interstellar medium, protosolar nebula, and asteroidal/cometary parent bodies.
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