Abstract

A central interest of chemical evolution is how it led to life. The current belief is that certain chemicals existed on early Earth, and that their reactions led to complex molecules that eventually led to life itself. These compounds were thus biologically important. What is missing, however, is a consideration of chemical evolution that did not necessarily lead to life. Instead, it may have resulted in formation of complex chemicals which, however, were not suitable as informational molecules under certain environmental conditions. We are interested in investigating the preservation of organic molecules from within the transition zone that bridges the primordial organic world and the biotic world and applying this concept to mineral systems. Organic molecules interact with minerals, clays, and oxides that might lead to their sequestration in the deposited mineral matrix. The search for chemicals on Mars should include investigating these systems without the usual dissolution of minerals prior to organic analysis. What we learn about chemicals from a transition zone on Mars might also shed light on the process of chemical evolution on early Earth.

About the Transition Zone

In this zone, novel structures were formed that may or may not have lead to life (Perry and Kolb, 2003c, 2003b). Let us first address the transition from non-living to living systems.

One scenario is a gradual transition. It implies a transition that occurs as a result of a small accumulation of changes over a period of time. Another scenario is a quantum transition. This concept is based on ideas of G. G. Simpson and J. McFadden, and it implies a sudden change (Simpson, 1949; McFadden, 2000). Yet another scenario is punctuated equilibrium, based on work by S. J. Gould and N. Eldredge (Eldredge, 1985; Gould, 1996). It implies long periods of stasis, followed by bursts of changes. The philosophical concept that is close to the sudden change or leap is that of Hegel, whose first law of dialectics addresses quantity to quality transition. We may envisage that a quantity of chemicals may lead to a new quality, that of life.

Gradual transition in a chemical system such as a prebiotic pond, may produce complex and diverse chemical compounds. These, in conjunction with inorganic partners, would lead us to a transition zone, which eventually led to life. The transition zone (Perry and Kolb, 2003c) contains both gradual events, which are long term, and sudden events, which are punctuated or quantum.

About Chemical Properties of the Transition Zone

The transition zone bridged the primordial organic world and the biotic world. The primordial organic world contained many reactive chemicals that were brought to Earth by comets, meteorites, or formed by Miller-Urey type reactions. As time went by, the impacts on Earth were fewer, the supply of new chemicals dwindled, and the reactive compounds that were originally present in the primordial organic world were transformed into more stable, and thus less reactive compounds. These compounds could have reacted further with the help of catalysts. The original catalysts may have been based on clays, various metals, inorganic and organic species that happened to be present and possess some catalytic activity. In the transition zone, as compared to the primordial organic world, evolution of the catalysts would have gone further, the catalyst supply was more diverse, and would probably involve various peptides.

Another feature of the transition zone was that the energy supply for its reactions was steadier, more reliable and somewhat secured, as compared to the primordial organic world. At least some of the chemical systems present in the transition zone had the capability of storing energy and transferring it as chemical energy.

A central problem of any chemical process involves the supply of chemicals, as the supply becomes depleted with time, and may not be replenished in the same amounts with identical chemicals. This would be most likely if the major suppliers of chemicals were comets and meteorites. A chemical system that survives better would be able to make its own starting materials. This would be possible in a chemical cycle. A chemical cycle would have a better chance of reproducing itself if it were coupled with an energy cycle, which would ensure a reliable source of energy. Various chemical cycles were present in the transition zone, involving also molecules that had the capacity to self-replicate. Initially they were primitive and had poor fidelity of replication. An abundance of errors in replication caused the “informational content” to dissipate.

This was not life yet. Replicating systems were prone to errors and were not coupled efficiently to other cycles, those of enzyme production and energy storage/transfer.

An Example of Chemistry in the Transition Zone: Alternative Genetic Systems

Orgel (1987) summarized the difficulties in prebiotic syntheses of the nucleoside components of RNA (nucleo-base + sugar). The condensation of nucleo-bases adenine or guanine with sugar ribose gives only minute amounts of nucleosides with correct stereochemistry, and synthesis of pyrimidine nucleosides is even more difficult. Shapiro addressed the difficulties in prebiotic syntheses of nucleo- bases adenine and cytosine (Shapiro, 1995, 1999). Orgel also suggested that some of the original nucleo-bases may not have been purines and pyrimidines: “RNA is an evolutionary advanced molecule that was preceded by one or more simpler genetic polymers” (Orgel, 1987). Shapiro (1995) holds the same view: “An alternative and attractive possibility is that some other replicator preceded RNA (or RNA-like substances) in the origins of life”. If we are to believe that chemicals evolved to life, we must have a preceding system.

Some examples of alternative nucleo-bases, nucleosides, and nucleic acid with alternative backbones or sugars, prepared in the laboratory are: urazole as uracil mimic and urazole nucleosides (Kolb et al., 1994; Kolb and Colloton, 2003), nucleo-bases that extend the genetic alphabet (Piccirili et al., 1990), nucleic acids with sulfone and peptide backbones, rather than phosphate (Benner and Hutter, 2002), and nucleic acids with sugars with different structures and configurations (Beier et al., 1999). The heterocyclic component of these systems readily complexes with metals that are normally present in rocks. The sugar component of these molecules interacts with Si and Al from the rocks, and especially from the rock coatings (Perry and Kolb, 2003a). These chemical interactions preserve these and many other organic molecules. Striking examples include isolation of amino acids and DNA from rock coatings (Perry et al., 2003; Perry and Kolb, 2003a).

Adapting These Ideas to Looking for Life on Mars

The concept of the transition zone may be helpful in the search for life on Mars or elsewhere, since complex chemicals may have existed in the transition zone but have not lead to fully functioning life. We might find polymers, with some regularity in their structures, self-templating fragments, or remnants of the failed chemistry of reproduction. These would be the chemical signatures of the transition zone on Mars.

Martian rocks and rock coatings should be investigated for the presence of organic compounds. Especially important are compounds that may serve as genetic systems, capable of recognition, templating, and self-replication.

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