SOBRE EL CONOCER EN ESTA CLASE DE CIENCIAS

It has long been proposed that the origins of life were hot, seeded in hydrothermal vent systems deep in Earth’s early oceans (e.g. Orgel 1998; Copley et al., 2007). Similarly,

molecular analyses of 16S RNA present a last universal common ancestor (LUCA) that is buried within the hyperthermophiles of the Bacteria and Archaea, branching among H2/S°

chemolithotrophs (Brack et al., 2010). There is, of course, much contention with this theory: 1) high temperatures make RNA synthesis difficult as these nucleic acids will tend to denature; 2) tree construction using alternate molecules such as RNA polymerases branch out instead within mesophilic organisms; 3) there is no obvious concentration mechanism in hydrothermal vent systems that might allow for the formation of a pre- biotic cell. So how did life occur? Life as we know it requires an energy gradient, or a –

∆G, and a solvent (water). Added to this is the need for encapsulation via some type of

boundary molecules, informational molecules such as RNA or PNA (the purported precursor to RNA), and catalytic molecules such as enzymes (Monnard and Deamer 2002). The formation of all of these molecules requires endergonic reactions and must somehow be derived from a chemical gradient that would force these reactions to occur (e.g. Schwartz 1995; Orgel 1998). A recent paper by Russell et al. (2013) has returned to these hydrothermal vent systems and proposed that cellular life was preceded by abiotic metabolisms derived from the dissipation of physical energy in sea floor fracturing to chemical energy through serpentinization and the formation of precipitated hydrothermal mounds. The authors argue that the release of H2 and CH4 could have reacted with

available electron acceptors (CO2, NO3, NO2, Fe2+, Mg2+) in the acidic Hadean ocean,

where the gradient in pH between the highly alkaline effluent and the acidic ocean could have driven the first primitive proton pumps across the hydrothermal mound. These initial processes could have allowed for increased complexity and the generation of pyrophosphate in ferrous hydroxide layers, which would have acted as permeable “membranes”. A similar argument for a non-biotic membrane was proposed by Cairns- Smith and Hartman (1986), who argued that a complex molecule such as RNA could not have been created de novo and instead proposed the idea of self-replicating clays, where

negative layers are held together by cations in the intervening spaces, spaces that also contain H2O and could eventually act as a template for more complex molecules.

The reliance on hydrothermal vent systems in the early earth to provide the starting mechanisms for the origins of life is restrictive and poses many problems. An alternative to this vein of thought can be found in the relationship between impact craters and life.

As we have discussed, impact craters have been shown to provide habitats for contemporary (and past) endolithic organisms in polar deserts; having important implications for Mars, which can be considered as an extreme version of Earth’s polar regions. The largely unknown facet of impact craters, however, is their ability to support life during a generated hydrothermal stage and the potential for providing a viable chemical system for the origins of life. Impact cratering does, however, provide us with many of the physical and chemical parameters that make hydrothermal vent systems such attractive prospects for the origins of life. Impact into a terrestrial target causes massive fracturing and brecciation of the substrate as well as disruption of the deep subsurface (Cockell et al., 2012), dramatically increasing the surface area of the region. In the event that a hydrothermal system is generated, the dissolution of the fractured, shocked target will occur, resulting in the generation of primary minerals such as quartz, amphibole- group minerals, phyllosilicates and zeolites, carbonates, sulfides, sulfates, oxides and halides (Osinski et al., 2013). Secondary, or weathered, mineral assemblages can also occur in the form of Fe-sulfates, and oxyhydroxides such as goethite (Izawa et al., 2011). In the event that an impact occurs into a mafic target such as basalt, serpentinization will occur, resulting in the release of methane and hydrogen gas (Marzo et al., 2010). The crater itself, especially in an impact into a continental plate, serves as the concentrating mechanism, providing a bowl shaped depression that will tend to collect fluids and potentially serve as an evaporative pond within which life could have arisen (Cockell 2006).

During the early Hadean, the impact flux into Earth, and indeed every other terrestrial body in the solar system was much greater than today (Sleep et al., 1989), indicating that many of these “warm ponds” would have been present, thus increasing the chances for many potential origins. The spike in the cratering record between 4.0 Ga and 3.8 Ga, known as the late heavy bombardment (LHB) would have provided even more available habitat for the above scenario to occur. Some believe that the period of cratering during the LHB was so intense that it would have either precluded the existence of life prior to 3.8 Ga, or acted as a thermal bottlenecking event causing the selection of hyperthermophiles over other present life resulting in the skewed evolutionary tree that we see today (e.g. Nisbet and Sleep 2001; Kring and Cohen 2002; Brack et al., 2010). In a

study by Abramov and Mojzsis (2009), which explores the habitability of the Earth during the LHB, their modeling of the extent of hydrothermal systems revealed that hyperthermophilic regimes would only have consisted of a relatively small portion of the Earth’s crust and that temperatures would have dissipated quickly enough, that life (if it did exist) should have survived the bombardment.

In the aftermath of such extensive cratering as well as the cooling and dissipation of hydrothermal systems, we would be left with significant disruption of the deep subsurface, providing refugia for microbial organisms as they do on Earth (Wanger et al., 2006), extending the depth of the biosphere (Cockell et al., 2012). On Mars, where liquid water is not stable on the surface due to low atmospheric pressure, these deep-seated niches could have provided sanctuary for any putative life long after the surface became inhospitable (Boston et al., 1992; Michalski et al., 2013). Further available habitats remain in the impact breccias which, as we saw in Chapter 5 could have been colonized during a hydrothermal period, and would have had the potential to provide refuge for any endolithic organism after the cessation of the hydrothermal stage.

Though gneisses are by no means a primary rock type on other planets, such as Mars, the understanding of the response of this target to shock and the corresponding colonization by microorganisms can be used as a template for understanding how other typically dense, non-porous crystalline targets, such as basalt, may respond and help to inform further biological investigations of impact shocked lithologies. This work has clearly shown that these impact generated habitats are long-lasting and are capable of providing refuge for microorganisms on a geologic timescale, from the post-impact environment through to the present day.