The Origin of Life - Criticism

Criticism

A lack of evidence is often mentioned as the barrier to acceptance.

It's true that more studies would be welcomed - and that more research would probably help with this - but I think that there is enough evidence available now for competent individuals to evaluate the theory.

Many seem to object to the idea of a genetic takeover as being unparsimonious - or contrary to Occam's razor.

Others ridicule the idea of unreactive silicon elements being the basis of life - when carbon chemistry offers such rich opportunities.

I've also encountered the criticism that Cairns-Smith is a reductionist thinker - an "Ultra-Darwinian" far more extreme than (say) Richard Dawkins. This may have some truth to it - but reductionism is great. The "charge" of using reductionism should not adversely affect the assessment of scientific theories - rather it is a positive thing.

Paul Davies

The ideas are summarised, with the comment:

It has to be said that there is very little experimental evidence to support Cairns-Smith's clay theory.

Still, whatever the plausibility of clay as the primal life stuff, the basic principle of genetic takeover is sound.

J. Maynard Smith

Heredity has not been convincingly demonstrated.

At least clay enthusiasts can show that information is being copied with enough accuracy to preserve it many thousands of times, in an inorganic process.

Christopher Wills and Jeffrey Bada

Unfortunately for his idea, it quickly became apparent that minerals simply could not carry enough information to be the genetic material for even the most primitive life. This mineral-first hypothesis also faces another severe problem: If clays were the bases of life, why aren't traces of them still inside us? [...]

Cairns-Smith's notions about clay genes have now been largely rejected by the scientific community. [...]

Stephen C. Meyer

No evidence indicates that a "low tech" clay-life ever existed or was transformed into organic-based substances. More importantly, like the primordial soup approach, the theory does not account for the so-called "information problem" of genetics.

This is where Cairns-Smith's theory breaks down. He hopes to someday prove that what he terms his clay "crystal genes" can store information. He offers no clues, however, as to how either "high tech" amino acids or "low tech" clay crystals might have arranged themselves into complex biochemical messages. In the absence of any mechanisms explaining these mysteries. Cairns-Smith invokes the literary device, to bridge the gap, of endowing evolution with attributes of intelligence. The "machinery of life," he says, was built from "well-chosen components" with "an entire design approach" and a "cleverness that could only have been a product of evolution."

Robert Shapiro

John A Hewitt

The Scottish chemist Cairns-Smith (1971) suggested that prebiosis occurred in defect structures on silicate (rocky) surfaces and this idea has been quite widely support since some such rocks, notably the clay montmorillonite, exhibit a wide range of catalytic activities. The feeling is that these surfaces might help to solve the problem of bounding by keeping reagents together, especially if lipids were also involved.

John Horgan

For more than a decade, Cairns-Smith has been pushing his own hypothesis. Like Wachtershauser, he proposes that life arose on a solid substrate that occurs in vents and almost everywhere else, but he prefers crystalline clays to pyrite. All crystals consist of self-replicating units, Cairns-Smith points out, but clay crystals have enough complexity to mutate and evolve in a lifelike way. Some clays might have become still better breeders by developing the ability to attract or synthesize organic compounds-such as nucleic acids or proteins. Eventually, the organic compounds would become sophisticated enough to begin replicating and evolving on their own.

Unlike some origin-of-life theorists, Cairns-Smith cheerfully admits the failings of his pet hypothesis: no one has been able to coax clay into something resembling evolution in a laboratory; nor has anyone found anything resembling a clay-based organism in nature. Yet he argues that no theory requiring organic compounds to organize and replicate without assistance is likely to fare any better. "Organic molecules are too wiggly to work," he says.

Did Cairns-Smith really say 'wiggly'?

The usual turn of phrase in this context is that "organic molecules are usually too sticky" to self-assemble.

Leslie E. Orgel

Some 30 years ago he proposed that the very first replicating system was inorganic. He envisaged irregularities in the structure of a clay - for example, an irregular distribution of cations (positively charged ions) - as the repository of genetic information. Replication would be achieved in this example if any given arrangement of the cations in a preformed layer of clay directed the synthesis of a new layer with an almost identical distribution of cations. Selection could be achieved if the distribution of cations in a layer determined how efficiently that layer would be copied.

So far no one has tested this daring hypothesis in the laboratory. On theoretical grounds, however, it seems implausible. Structural irregularities in clay that were complicated enough to set the stage for the emergence of RNA probably would not be amenable to accurate self- replication.

Wikipedia (Silicon-based_life)

Life as we know it could not have developed based on a silicon biochemistry. The main reason for this fact is that life on Earth depends on the carbon cycle: autotrophic entities use carbon dioxide to synthesize organic compounds with carbon, which is then used as food by heterotrophic entities, which produce energy and carbon dioxide from these compounds. If carbon was to be replaced with silicon, there would be a need for a silicon cycle. However, silicon dioxide precipitates in aqueous systems, and cannot be transported among living beings by common biological means.

As such, another solvent would be necessary to sustain silicon-based life forms; it would be difficult (if not impossible) to find another common compound with the unusual properties of water which make it an ideal solvent for carbon-based life.

This criticism is illogical and incoherent. Silicon-based life can work in water without any particular difficulty. The fact that silicates precipitate in water is not a relevant objection.

Mark A. S. McMenamin

Cairns-Smith felt that there must have been some sort of inorganic scaffolding on which the earliest life would have started. He proposed clay as the living crystal of Gene-1. More recent research has shown that clay does not have the properties needed to act as the scaffolding of Gene-2.

The claim that clay "does not have the properties needed to act as the scaffolding of Gene-2" is unreferenced and inaccurate.

Massimo Pigliucci

A contemporary discussion of the question of the origin of life cannot be complete without the inclusion of A.G. Cairns-Smith's theory of clay crystals. I hope this will not be the case for much longer (except as a footnote of historical value). Don't get me wrong, I am familiar with Cairns-Smith's research and writing, and I find it excellent. But everybody can make a mistake, and I think the clay theory clearly falls within the cracks of Cairns- Smith's career, as ingenious and superficially enticing as it may be.

Briefly, the idea is that life didn't originate with either nucleic acids or proteins. The original replicators and catalyzing agents were actually crystals found everywhere in the clay that lay around the primitive Earth. There are four cardinal points of Cairn-Smiths hypothesis. First, crystals are structurally much simpler than any biologically relevant organic molecule. Second, crystals grow and reproduce (i.e., they can break because of mechanical forces, and each resulting part continues to "grow"). Third, crystals carry information and this information can be modified. A crystal is a highly regular structure, which tends to propagate itself (therefore, it carries information). Furthermore, the crystal can incorporate impurities while it's growing. These impurities alter the crystal's structure and can be "inherited" when the original piece breaks (hence, the information can be modified). Fourth, crystals have some minimum capacity of catalyzing (i.e., accelerating) chemical reactions.

Cairns-Smith then proposed that these very primitive "organisms" started incorporating short polypeptides (protoproteins) found in the environment - presumably in the soup - because they enhanced the crystals' catalyzing abilities. The road was suddenly open for an increase in the importance of proteins first, and then eventually of nucleic acids, until these two late arrivals on the evolutionary scene completely supplanted their "low-tech" progenitor, and gave origin to the living organisms that we know today.

What is wrong with this picture? First of all, Cairns-Smith seems to completely ignore what a living organism is to begin with. For one thing, crystals don't really have a metabolism, not in the sense defined above for living organisms. The reason for this may have something to do with the fact that not only are crystals structurally much less complex than a protein or a nucleic acid, but also with their silicon-based chemistry, recognizably much simpler than the carbon-based chemistry utilized by living organisms on Earth. The lower complexity and simpler chemistry may be insurmountable "hardware" obstacles to the origination of a true metabolism in clay matter. Second, crystals don't really react to their environment either, another hallmark of every known living creature. Notice that this is a property distinct from metabolism, in that metabolism can be entirely internal, with no reference to the outside world (except for some flux of energy that must come into the organism to maintain its metabolism). On the other hand, living organisms universally and actively respond to changes in external conditions, for example by seeking sources of energy or by avoiding dangers. Furthermore, an argument can be made that crystals are not actually capable of incorporating new information in their inherited "code," unlike what happens with mutations in living beings. True, they can assimilate impurities from the environment and "transmit" such "information" to their "descendants" for some time; but these impurities do not get replicated, they need continually to be imported from the outside, and they do not become a permanent and heritable part of the crystal. Moreover, impurities do not create new types of crystals, the way mutations give rise to entirely new kinds of animals and plants.

Another colossal hole in the clay theory is - of course - that we have no clue to how the "mutiny" of nucleic acids and proteins actually occurred, and in fact we are given very faint hints about how a crystal could possibly co-opt a polypeptide to enhance its growth. Therefore, as much as creationists might like the flavor of a theory of the origin of life in which the first living beings came literally from dust (although Cairns-Smith is certainly no creationist), we're still left with ribonucleo-proteins as our best, albeit fuzzy, option.

Massimo Pigliucci

1968 - A.G. Cairns-Smith publishes a paper suggesting that the first life on earth might have been fine-grained clay crystals. He will publish on this topic several more times before his death, but the experimental evidence will remain scant, perhaps in part because sufficient technology doesn't yet exist to test the hypothesis.

In closing