Every thinking man or woman seems to have their own favourite theory of origins of life. Like clothes fashions and hairstyles, in this case one is sometimes reduced to mere irrational favoritism at the end, devoid of real substantial logic. The reason is straightforward; origins might be the biggest unsolved problem in chemistry and biology of all time, but it deals with things billions of years backwards in time which we can hardly mimic, let alone observe directly. Modern life with its machinery of DNA, RNA and proteins provides tantalizing clues and yet no answers. Which came first? Genes, protein, or something else?
For many years now, I have placed my own bets on an origins theory about which I first read in mathematician John Casti's sweeping survey of the big problems facing modern science- Paradigms Lost. The theory which Casti says is his favourite is mine too because of a sane reason that Casti provides- the venerable (perhaps too much so) principle in science called Ockham's Razor, which simply says that "entities should not be unnecessarily multiplied" or, "Simple is best". When one is confronted with several explanations that lead to the same conclusion, the simplest one is likely to be the correct one. Well, not really, but if we are going to proceed on hunches anyway, why not choose the simplest one. The premise is simple here; DNA, RNA and proteins are too complicated for us to think about how they could have arose on our primordial planet. Better to start with simple, possibly inorganic substances that were abundant on early earth.
Enter the British biochemist Alexander Graham Cairns-Smith (CS from now on) who came up with a "life from clay" theory. CS conjectured that it makes much more sense to think of life evolving from simple inorganic materials, especially crystals, rather than from organic molecules. His hypothesis was straightforward and dealt with two familiar properties of crystals that are very similar to what we think are essential properties for anything to be called "living"- reproduction and natural selection. Crystals by their very nature are periodic, extending regularly in infinite planes, "reproducing" if you will in three dimensions. Crystals are also subject to defects and impurities. The beauty of crystallization is that impurities or defects always exist. The key principle that CS recognized was that these impurities or defects, if they confer some benefit like better 'stickiness' or mechanical properties, would be propagated in the way that beneficial mutations are propagated through evolution. Gradually, the old, decrepit crystal will be left behind and this impure crystal with its superior properties would take over. Ergo crystal evolution.
Which crystal would be preponderant on primitive earth to possibly do such a job? Why, ordinary clay of course. Silicon dioxide in its myriad manifestations. For one thing, it's always been extremely abundant for billions of years on earth. Secondly, it comes in an amazing variety of polymorphs, allotropes and geometries. Silicon is a wonderful element. It can mix and match with an untold number of cations and anions and form quixotic 3D structures. It can act as a scaffold for many other substances. One can spend a lifetime studying silicates. Science fiction writers have long since fantasized about a silicon-based biosphere. But here silicon has been theorized to be the seed of life in quite another fashion.
According to CS, crystals of silicon easily harbor organic impurities in them. With time, these impurities will grow along with the crystal. At some point as noted above, the organic molecules will have an evolutionary benefit possibly because of their greater flexibility, branching power, stickiness due to hydrophobic interactions and multiple bonding characteristics. Gradually, like a snake shedding its old skin, the organic molecules will simply grow faster and stronger and leave their old siliconian parentage behind. In time, what you will have would be an organic crystal. Now think about DNA, RNA, proteins and suchlike forming from such organic entities. At least it's a little easier than before, where one had to conjure up these biochemical wonders from inorganic gunk.
CS now has an article (cited below) expounding upon his "life from clay" theory. The most compelling hypothesis I found in this article is that if crystals are to serve as a template for making copies, replication should have to be edgewise and not in other directions. CS cites DNA as an example and considers a simplified version of it with the sugar-phosphate backbone removed and hydrogen bonds removed too. Now it's just a stack of colored plates, with each color corresponding to a base. It's pretty obvious that copying can take place only along the edge, as CS's figure shows:
CS then gives some example of silicate minerals which could have such edges acting as templates. Edges could come together because of electrostatic or non-polar interactions. They could be jagged or smooth. Finally, you need not have only one kind of edge. A whole panoply of silicates with their varied kinds of edges could compete for sheet formation and consequent duplication. After that, the aforementioned impurities and defects could ensure natural selection and organic crystal formation. Life could then piggyback on the surfaces of these crystals.
CS also ponders if one could have inorganic enzymes that could possibly speed up the arrival of life. One does not imagine such enzymes to have the flexibility of organic ones. On the other hand as CS says, inorganic enzymes are already used in industry as superior catalysts. Clay crystals could similarly act as catalysts and speed up all kinds of reactions. By the way, CS is of the "genes first" camp as opposed to the "metabolism first" camp. However, CS's idea is one of metabolism facilitated by "inorganic genes" that replicate and improve their stock. I personally find the idea of crystal surfaces very alluring; after all so many reactions are speeded up on surfaces- Haber's ammonia synthesis which partly led to the latest Nobel Prize for Gerhard Ertl immediately comes to mind. If one is looking for simple chemical entities that could kick-start and speed up reactions, inorganic crystal surfaces sure seem to be good candidates.
CS's most tantalizing thoughts are about such clay-based life-inducing reactions happening even today, quietly in the nooks and crannies of nature. While such reactions would be hard to discover and compared to our life spans would be trivial and temporary, it is fascinating to think that the echoes of the origins of life still resonate all around us. Score one for Si.
Cairns-Smith, A. (2008). Chemistry and the Missing Era of Evolution. Chemistry - A European Journal, 14(13), 3830-3839. DOI: 10.1002/chem.200701215