Moreover, a dilute solution of organic compounds in the pre-biotic sea would hardly give rise to a rich “primeval soup” fromwhich life could have arisen, as they suggested.
The importance of the Miller-Urey experiment is not that it elucidated the origin of life, but that it made it a subject of reputable scientific inquiry. It has retained this status ever since!Observing the shortcomings of the Miller Urey experiment and other such experiments on same lines, scientists proposed the contamination of earth by organic molecules from outer space or even meteorites. One such meteorite that was in hit list was the Allan Hills meteorite, but though informed opinions vary,most scientists remain unconvinced by the evidence for Martian life in the Allan Hills meteorite, the meteorite from mars. The Allan Hills story became even less credible when scientists began examining other meteorites. Surprisingly, all meteorites reveal signs of life—but it is Earth life.Anyway, meteorites and the outer space scenarios do not explain, but explain away the phenomenon of origin of life. Another mystery yet to be explained about life is the molecular chicken and egg problem. The problem states that the early Earth was well provided with simple organic molecules, but how were these turned into proteins and nucleic acids? How they were polymerized? This remains unanswered.
There is an even more contentious question: which came first, nucleic acids or proteins – or, perhaps, cell! membranes? Each of these possibilities has been championed. Each entails considerable difficulties.This chicken-and-egg debate was transformed during the 1980s by a novel discovery: Cech and Altman found that RNA can function as an enzyme – no proteins were necessary. Some RNAs catalyzetheir own cleavage and their own polymerization. Orgel had demonstrated some years previously that polynucleotide, particularly RNAs, can catalyze the formation of copies of themselves. These strands of evidence suggested to some scientists that the question “proteins first or nucleic acids first?” was answered: the nucleic acid, specifically RNA, came first. Gilbert and others proposed that for a period on the primitive Earth, RNA molecules manipulated themselves and each other, replicating autonomously. They called this period the “RNA world”.
The RNA world hypothesis is now textbook material. There is a consensus that the “RNA world” gave way to “true” life when the RNA started to translate itself into proteins and DNA succeeded it as the repository of genetic information. But the hypothesis is inadequate. First, it is hard to make key RNA reactions go without external catalysts, and bringing the four bases together for initial synthesis would have been problematic. Second, RNA molecules are fragile and tend to break up unless carefully cosseted. The longer the RNA, the more fragile it is; but a short RNA is relatively useless both as a repository of information and as an enzyme. There are many other difficulties in the model. However note the dogmatic appeal in upholding the model and even giving it way in the text books.
Which came first, the living state or the molecules (proteins, nucleic acidsand membrane components) on which it depends? Nearly everyone wouldsay that the molecules came first and the living state somehow arose fromthem. But alternative is logically possible. The first living things mighthave been made of entirely different materials, which were subsequentlyreplaced by nucleic acids and proteins.Cairns-Smith drew attention more than 30 years ago to the “replicating” properties of certain kinds of clay. Kaolinite, for example, forms large flattish crystals that stack like playing cards. Defects formed in a kaolinite crystal when one atom is replaced by another can be replicated: a new crystal formed adjacent to it repeats the defect, maintaining the same overall shape.
Thus, kaolinite behaves superficially like DNA. It replicates itself, so long as it is supplied with the right ingredients; and it passes on its acquired defects to subsequent “generations”. However, although kaolinite’s crystal structure is quite complicated by mineral standards, it isincomparably simpler than DNA and therefore far more likely to form by ordinary physical and chemical processes. It was probably common on the prebiotic Earth. Cairns-Smith suggests that life began as a replicating clay mineralsystem.
Many clay minerals then bind organic molecules such as nucleic acid bas
