Did life originate in a mineral?
theories about the origin of life are confronted with two questions to which there are no satisfactory answers: How could organic compounds accumulate in an aqueous solution? And: Where did the first catalysts come from that ensured the regulated construction of molecular chains - proteins, DNA or RNA? Perhaps one particular mineral holds the key to answering both questions: zeolite. "The synthesis of biomolecules from carbon compounds dispersed in an aqueous solution requires a mechanism that concentrated the carbon compounds. In addition, the biochemically important molecular chains - such as proteins and ribonucleic acids - had to be protected from photochemical destruction by the sun's rays protected," explains the geophysicist Joseph V. Smith from the University of Chicago. In the March 31, 1998 issue of the Proceedings of the National Academy of Science (Abstract), he argues that zeolites provide the desired environmental conditions for the emergence of life.
Zeolites were discovered two hundred years ago as a mineral that "walls and foams in the fire in front of the soldering tube almost like borax". What came out of the mineral under heat was water. At the molecular level, the silicon-rich mineral contains a rugged structure with interstices, pores, and channels where water tends to collect. Since the mineral has a large molecular surface, it is predestined for catalytic processes. A new technical development are so-called "guest-host systems": For the construction of microsystems, molecules are fixed in the pores of the mineral in order to take on certain tasks there. Smith's thesis boils down to the fact that "guest-host systems" are not an originally technical development, but that natural "guest-host systems" made the origin of life possible.
Most zeolites are hydrophilic and absorb water from their surroundings. However, there are also synthetic zeolites that are lipophilic and absorb and enrich organic molecules from the surrounding water. Recently, a natural zeolite – the so-called mutinaite – was discovered in the Antarctic, which is also lipophilic. Smith therefore considers mutinait to be a favorable environment for chemical evolution, which preceded biological evolution. Mutinait contains aluminum instead of silicon. Smith speculates that on early Earth the surface mineral lost aluminum through weathering and replaced it with silicon. Remaining aluminum formed catalytic nuclei on which organic molecules lined up to form long chains. During chemical evolution, amino acids were concentrated in the pores of the lipophilic zeolite, which were then assembled into peptides by the catalytic process. These could exist in the zeolite over a long period of time, since the mineral protected the peptides from the decomposing rays of the sun.
In 1954, Stanley Miller at the University of Chicago proved the following: Amino acids could form on earth under the conditions of the primeval atmosphere. So far, however, no experiment has shown how the first proteins could form from amino acids. If Smith's theory is correct, then aluminous zeolites should be able to do this. The scientist is therefore now planning corresponding experiments. Smith's theory also suggests an explanation for a peculiarity of life: for each amino acid there are two possible spatial structures that are like mirror images of each other, a D- and an L-form. But in the building blocks of living beings they appear almost exclusively in the L variant. So far there is no interpretation of this whim of nature. Smith surmises, "It is probably coincidence that almost only the L-shapes are used. It may have been due to a suitably shaped channel in a zeolite." One-dimensional channels in zeolite were therefore the templates for the first peptides from which the evolution of life grew.
Smith is also planning a field trip to Australia, home to some of the oldest and most pristine rock on earth. He hopes to discover more zeolites there that may still show traces of biocatalysis.
