Introduction

Richard Dawkins once presented an interesting definition of the term "gene":

Insulation

Nearby molecules tend to attach themselves to the edges of growing clays anyway, due to electrostatic forces at the edges of the material.

However crystal organisms will attempt to bond with such molecules in such a manner that results in their sides being covered up while their growing ends remain exposed.

How can a growing crystal selectively control on which surfaces nearby molecules attach themselves?

One crude method which could be employed to this end would be to have charged edges, but neutral faces. This would mean that charged or polarized molecules from the environment would attach themselves to the sides of the crystal - but not to its growing ends.

However more effective solutions seem likely to involve the catalytic grooves along the sides of the growing crystals. A crystal organism could use the to grab molecules and hold them there. This would allow more discrimination to be applied in choosing the types of molecules that were involved.

They are (mainly) divided into monosaccharides, disaccharides, oligosaccharides, Polysaccarides (corresponding to "1", "2", "few" and "many" sugar units).

Polysaccharides consist of that subset of carbohydrates with "many" repeating monosaccharide units in a chain - where "many" should be interpreted as meaning "more than ten or so".

They are ubiquitous in the modern world. Cellulose, starch, chitin, xanthan gum, carrageenan, gum arabic and gum tragacanth are among the more well-known polysaccharides.

Both areas of research involve polysaccharides:

• Oil
  The oil industry uses a wide range of organic compounds to control the materials it has to deal with. It uses emulsifiers, surfactants, detergents, lubricants, and so on.

  Polysaccharide functions in the industry include cooling the drill bit, preventing fluid loss through the borehole, and preserving the properties of the materials which are being drilled through. The drilled materials often include clay minerals.

• Earth
  Polysaccharides in soils are of interest mainly due to their roles of cementing and stabilizing aggregates. They typically originate from bacteria in the soil.

  While they are generally incapable of reaching between individual grains they play an important structural role by combining with clay particles - and bridging the gaps in this way.

As a result of these sorts of studies, we now know much about the properties of clays in conjunction with polysaccharides, in the context of adding polysaccharide material to clays in order to modify their properties.

In addition, there has been some recent research relating to interactions between RNA and montmorillonite - with the origin of life in mind. The idea appears to have been for the clay to take on the role of catalyst - freeing RNA from the role of making enzymes that catalyse its own formation.

They did this mainly for protection against physical disturbance - and to avoid the mutational force represented by sideways growth - which otherwise threatens to obliterate them.

Initially coverage would have been thin - and the protection offered slight.

Short polysaccharides synthesized by a crystalline organism

However some protection is generally better than no protection at all - and even irregular coverage by monomer units would act to inhibit sideways growth, and cushion against physical blows.

Over time, better coverage would be selected for.

There is no shortage of other possible applications which such bonded chains might have.

The polysaccharide chains lengthen

As time passes, specific catalytic grooves could come to form - designed to select specific types of monomer units. Linear grooves in clays can form asymetrically - so the solution to many problems of orientation would have been available.

Crystals can be naturally chiral

Not all of the effects of the polysaccharide chains will be positive.

For example, it seems likely that the addition of polysaccharide material will increase the tensile strength of the crystal. This might sound positive - but in an environment where splitting equates to reproduction, it is rather unlikely to be beneficial. However, growing more slowly is certainly preferable to suffering a genetic error catastrophe - for that represents the end of life.

As with all theories of the origin of long polymer chains the question of the origin of the monomer units arises. Although the theory presented here is no more vulnerable to criticisms in this area than any other, we would like to briefly draw attention to a novel solution involving multiple species.

One species might synthesize sugar bases - perhaps in order to make its local environment more sticky. Another species can then utilise this material in constructing polymers for protection against mutations. Complexity need not be concentrated in any single individual. Instead it can come from a combination of several species in the same environment.

The whole rod

There are several main possibilities:

• A clean break
  

  A clean break

  Here the two crystals part company as they would "normally" do, and continue growing separately.

• Attachment
  

  Attachment

  Here the two crystals remain joined at a "hinge" - perhaps eventually coming to join along their entire length. The chances of this sort of thing happening depend on the relative sizes of various forces - and it is probably not a very likely outcome.

• Single unzip
  

  Single unzip

  More likely is an "unzipping" process. Here the bond between the polysaccharide and the clay is weaker than the bonds between individual monosaccharide units - and as a consequence when force is applied at either end the polysaccharide comes "unzipped" from one of the crystals.

• Double unzip
  

  Double unzip

  Another sort of "unzipping" is theoretically possible: the "double unzip".

  This works because the force applied at the crystal-polymer junction depends on the angle between them. The more obtuse the angle, the greater the force. The result is that the two sides of the zip keep approximately in step with one another.

  A true double zip is not very likely to happen in practice. It only arises in a relatively narrow range of conditions.

If the event is more-or-less continuous, the "single unzip" seems the most likely outcome.

On the other hand, if the crystal is jerked around a bit, an approximate symmetry between the ends (while the unzipping has paused) suggests that unzipping is equally likely to start again at either end. Several factors can break the symmetry:

• the end attached to the smaller crystal has a more variable angle between the polymer and the crystal;

• One end is likely to have "given" most recently. The chances are that the angles have been preserved somewhat since then - and it is most likely to give again;

• One end of the crystal is likely to be anchored down.

When the unzipping is complete, two crystals remain:

The survivors

The one on the left has an attached polysaccharide bundle. We will refer to this as the "mother" crystal.

The one on the right has empty grooves where polysaccharides once lay. That one will be referred to as the "daughter" crystal.

The daughter's empty grooves can be expected to slowly fill up again, creating a new polysaccharide adjacent to the existing ones.

The mother's fate is less certain. There are several possibilities. One is that the trailing polysaccharides will attach itself to some other physical object, and eventually get unzipped - resulting in a situation similar to that of the daughter.

Another possibility is that the polysaccharide will wrap over the the growth face of the crystal and get caught on the other side - effectively preventing growth.

Tangle prevents growth

Lastly there's the possibility that crystal growth will continue - and offer the possibility that the polysaccharides will gradually lie down in new grooves that are constructed beneath them.

Resting down again

This model overcomes a number of difficulties in conventional models of the origin of carbon-based genetic systems.

In particular:

• It provides a mechanism whereby the formation of polysaccharide material might be catalyzed - in what would otherwise be thermodynamically unlikely volumes;

• It suggests that these might be systematically laid down next to one another in parallel grooves designed to hold them there;

• It provides a source of mechanical force necessary to tear them apart - in the absence of specialized machinery to perform this task;

• It removes any obligation to act as a high-fidelity transmitter of information. Genetic functions can form slowly - and it does not matter how pathetic and feeble the results are initially, since they do not need to act as the sole support for an evolving system.

The selective force responsible for the use of the polysaccharide material is reducing mutational load - through providing insulation from the environment and inhibiting sideways growth.

It plays a structural role - and the role of a mutation damper - before any genetic functions are involved.

The origin of genetic functions will lie in some sort of self-perpetuating influence between adjacent polysaccharide chains.

If molecules are more likely to take on the forms of their neighbours, then that can act as the origin of an inheritance mechanism transmitting information through the polymers - largely independently of the crystal substrate.

It does not take much imagination to see that genetic functions would be useful. For example, the ability to influence the form of the coiled-up polysaccharide is likely to be an aspect of the organism's phenotype of some interest to selection.

The crystals can co-operate in facilitating relationships between adjacent polymers - should it prove advantageous for them to do so:

Friendly neighbours

Here the catalytic grooves are shaped in such a way that adjacent polymers are placed in close contact with one another.

The clay genes find the organic ones useful, and the organic genes can't reproduce without the clay ones at all - at least not until they can produce a whole family of fancy enzyme-like structures to catalyse their own replication.

Also, if existing "floating" disaccharides, ogliosaccharides and polysaccharides can join to existing crystals, then the "death" of an existing polysaccharide-coated crystal (e.g. through heating) may liberate the attached polysaccharides - who can then go on to attach to other crystals.

Differences in reproductive opportunities between the crystal organisms and the organic ones seem likely to result in conflicts of interest between them.

Symbiotic organisms exist most harmoniously together when their reproductive systems are tied together - and neither can reproduce in the absnce of the other.

For the polysaccharide/crystal alliance, this condition will not ever have held - the reproduction opportunities of the organic genes may be correlated with those of their crystal forbearers - but the correlation will never have been perfect.

Crystal genes were necessary in order to form the first organisms - for reasons A. G. Cairns-Smith explained eloquently. Organic polymers naturally form sticky tars - not self-organising structures. About as near as they come to spontaneous self-organisation is in creating forms - but foams are next to useless from the point of view of reliably transmitting heritable information.

However - once they come into existence - organic genes can perform all the other functions performed by clay genes much more efficiently.

Ultimately, the ease of self-assembly of the crystalline genes was also their undoing:

Death

They arranged for the existence of better catalysts of their precursors than the clays could manage to build - and found a way to copy themselves using them.

In closing

Of course there are many remaining puzzle pieces in the story of life's origin. For example, we have not looked at the origin of cell walls.

The plausibility of the hypothesis discussed here can be evaluated by examining the details of the underlying chemistry involved.

Ultimately, we see synthesis of living organisms from inorganic precursors as the acid test of origin of life theories.