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The Limits of Organic Life in Planetary Systems
connection of the six components would be required to generate even a short RNA molecule, let alone one with biological function.
5.6 MINERALS INVOLVED IN THE CONSTRUCTION OF BIOMOLECULES
Minerals can participate in many ways in the synthesis and interconversion of organic species. For example, Martin and colleagues at Harvard have noted that sphalerite can convert photochemical energy into chemical energy.36 Minerals can also provide a direct source of redox chemical energy, such as in the conversion of sulfides to disulfides and vice versa. In addition, they may catalyze reactions and provide compartments to house evolving chemical systems.
5.7 SMALL-MOLECULE (“METABOLISM FIRST”) THEORIES OF LIFE’S ORIGIN
5.7.1 Life Without a Replicator
Replicator theories, which state that life began with the spontaneous formation of RNA or another information-rich genetic polymer that could direct its own replication, face difficulties in fundamental chemistry. These obstacles have long been thought to place them in the category of extremely improbable events. The appearance of such a replicator appears to require the combination of many chemicals in a long reaction sequence in a specific order, interspersed with a number of complicated separations, purifications, and changes of location.37 Physical law does not forbid such a process, but if a replicator initiated life and no natural environments can be found that make its generation favored, life may be very rare in the universe, and life that we encounter elsewhere is likely to be a result of panspermia.
It is reasonable to consider the assumption that life began, somehow, among one of the mixtures of small organic molecules that are produced by abiotic processes. The only natural examples in hand today are the components of meteorites that have fallen to Earth (see Section 5.2.1) and particles returned by the Stardust mission. Spectroscopy has also yielded partial lists of the organic molecules in interstellar space and interplanetary dust clouds.
For such a mixture to move in the direction of life, self-organization would be necessary. That process would increase the concentration of some components of the mixture either at the expense of others or by new synthesis from raw materials, such as carbon monoxide or carbon dioxide. An external source of free energy would be needed to drive the changes, which otherwise would involve an overall negative change in entropy.
That view of the origin of life has commonly been called “metabolism first”; the absence of a genetic polymer has been equated with the lack of any mechanism for heredity. As we have seen, replicator theories center on the spontaneous formation of large, information-bearing organic polymers endowed with the ability to copy themselves. The hereditary information carried in the sequence of such a polymer is called a genome.
In the words of Lancet and colleagues, a “fundamentally different approach has envisaged primordial self-replication as the collective property of ensembles of relatively simple molecules, interconnected by networks of mutually catalytic interactions.”38 The hereditary information in this case would be represented by the identity and concentration of its components. The term compositional genome has been used to describe this system, in which genetic information is not stored in a list, as in DNA, but is represented by the presence or absence of organic components.39,40 As an analogy, consider DNA to be the equivalent of a class list that records the full possible enrollment in a course. The information in a compositional genome would be represented by the presence of students who have turned up on a particular day.
A molecular assembly with a compositional genome would adapt to changes in the surrounding environment by altering the composition of the system and in the reactions used to sustain it. Growth of the system would take place through the acquisition or synthesis of additional quantities of the key components, and reproduction would occur when physical forces split the enlarged system into two or more fragments. For successful reproduction, each “daughter” fragment resulting from the division should contain a sufficient quantity of the key molecules to enable the networks of mutually catalytic interactions to continue. Such networks have been demonstrated in computer simulations,41,42 and “an experimental demonstration that amphiphilic assemblies display self-replication behavior” has been carried out.43 According to Lancet and colleagues, when competition for material and