Sometimes crystals spontaneously start to form in solution. At other times they have to be ‘seeded’, either by particles of dust or by small crystals dropped in from elsewhere. Cairns-Smith invites us to perform the following experiment. Dissolve a large quantity of photographer’s ‘hypo’ fixer in very hot water. Then let the solution cool down, being careful not to let any dust drop in. The solution is now ‘supersaturated’, ready and waiting to make crystals, but with no seed crystals to start the process going. I quote from Cairns-Smith’s Seven Clues to the Origin of Life:
Carefully take the lid off the beaker, drop one tiny piece of ‘hypo’ crystal onto the surface of the solution, and watch amazed at what happens. Your crystal grows visibly: it breaks up from time to time and the pieces also grow … Soon your beaker is crowded with crystals, some several centimetres long. Then after a few minutes it all stops. The magic solution has lost its power — although if you want another performance just re-heat and re-cool the beaker … to be supersaturated means to have more dissolved than there ought to be … the cold supersaturated solution almost literally did not know what to do. It had to be ‘told’ by adding a piece of crystal that already had its units (billions and billions of them) packed together in the way that is characteristic for ‘hypo’ crystals. The solution had to be seeded.
Some chemical substances have the potential to crystallize in two alternative ways. Graphite and diamonds, for instance, are both crystals of pure carbon. Their atoms are identical. The two substances differ from each other only in the geometric pattern with which the carbon atoms are packed. In diamonds, the carbon atoms are packed in a tetrahedral pattern which is extremely stable. This is why diamonds are so hard. In graphite the carbon atoms are arranged in flat hexagons layered on top of each other. The bonding between layers is weak, and they therefore slide over each other, which is why graphite feels slippery and is used as a lubricant. Unfortunately you can’t crystallize diamonds out of a solution by seeding them, as you can with hypo. If you could, you’d be rich; no on second thoughts you wouldn’t, because any fool could do the same.
Now suppose we have a supersaturated solution of some substance, like hypo in that it was eager to crystallize out of solution, and like carbon in that it was capable of crystallizing in either of two ways. One way might be somewhat like graphite, with the atoms arranged in layers, leading to little flat crystals; while the other way gives chunky, diamond-shaped crystals. Now we simultaneously drop into our supersaturated solution a tiny flat crystal and a tiny chunky crystal. We can describe what would happen in an elaboration of Cairns-Smith’s description of his hypo experiment. You watch amazed at what happens. Your two crystals grow visibly: they break up from time to time and the pieces also grow. Flat crystals give rise to a population of flat crystals. Chunky crystals give rise to a population of chunky crystals. If there is any tendency for one type of crystal to grow and split more quickly than the other, we shall have a simple kind of natural selection. But the process still lacks a vital ingredient in order to give rise to evolutionary change. That ingredient is hereditary variation, or something equivalent to it. Instead of just two types of crystal, there must be a whole range of minor variants that form lineages of like shape, and that sometimes ‘mutate’ to produce new shapes. Do real crystals have something corresponding to hereditary mutation?
Clays and muds and rocks are made of tiny crystals. They are abundant on Earth and probably always have been. When you look at the surface of some types of clay and other minerals with a scanning electron microscope you see an amazing and beautiful sight. Crystals grow like rows of flowers or cactuses, gardens of inorganic rose petals, tiny spirals like cross-sections of succulent plants, bristling organ pipes, complicated angular shapes folded as if in miniature crystalline origami, writhing growths like worm casts or squeezed toothpaste. The ordered patterns become even more striking at greater levels of magnification. At levels that betray the actual position of atoms, the surface of a crystal is seen to have all the regularity of a machine-woven piece of herringbone tweed. But — and here is the vital point — there are flaws. Right in the middle of an expanse of orderly herringbone there can be a patch, identical to the rest except that it is twisted round at a different angle so that the ‘weave’ goes off in another direction. Or the weave may lie in the same direction, but each row has ‘slipped’ half a row to one side. Nearly all naturally occurring crystals have flaws. And once a flaw has appeared, it tends to be copied as subsequent layers of crystal encrust themselves on top of it.
Flaws can occur anywhere over the surface of a crystal. If you like thinking about capacity for information storage (I do), you can imagine the enormous number of different patterns of flaws that could be created over the surface of a crystal. All those calculations about packing the New Testament into the DNA of a single bacterium could be done just as impressively for almost any crystal. What DNA has over normal crystals is a means by which its information can be read. Leaving aside the problem of read-out, you could easily devise an arbitrary code whereby flaws in the atomic structure of the crystal denote binary numbers. You could then pack several New Testaments into a mineral crystal the size of a pin’s head. On a larger scale, this is essentially how music information is stored on the surface of a laser (‘compact’) disc. The musical notes are converted, by computer, into binary numbers. A laser is used to etch a pattern of tiny flaws in the otherwise glassy smooth surface of the disc. Each little hole etched corresponds to a binary 1 (or a 0, the labels are arbitrary). When you play the disc, another laser beam ‘reads’ the pattern of flaws, and a special-purpose computer built into the player turns the binary numbers back into sound vibrations, which are amplified so that you can hear them.
Although laser discs are used today mainly for music, you could pack the whole Encyclopaedia Britannica onto one of them, and read it out using the same laser technique. Flaws in crystals at the atomic level are far smaller than the pits etched in a laser disc’s surface, so crystals can potentially pack more information into a given area. Indeed DNA molecules, whose capacity for storing information has already impressed us, are something close to crystals themselves. Although clay crystals theoretically could store the same prodigious quantities of information as DNA or laser discs can, nobody is suggesting that they ever did. The role of clay and other mineral crystals in the theory is to act as the original ‘low-tech’ replicators, the ones that were eventually replaced by high-tech DNA. They form spontaneously in the waters of our planet without the elaborate ‘machinery’ that DNA needs; and they develop flaws spontaneously, some of which can be replicated in subsequent layers of crystal. If fragments of suitably flawed crystal later broke away, we could imagine them acting as ‘seeds’ for new crystals, each one ‘inheriting’ its ‘parent’s’ pattern of flaws.
So, we have a speculative picture of mineral crystals on the primeval Earth showing some of the properties of replication, multiplication, heredity and mutation that would have been necessary in order for a form of cumulative selection to get started. There is still the missing ingredient of ‘power’: the nature of the replicators must somehow have influenced their own likelihood of being replicated. When we were talking about replicators in the abstract, we saw that ‘power’ might simply be direct properties of the replicator itself, intrinsic properties like ‘stickiness’. At this elementary level, the name ‘power’ seems scarcely justified. I use it only because of what it can become in later stages of evolution: the power of a snake’s fang, for instance, to propagate (by its indirect consequences on snake survival) DNA coding for fangs. Whether the original low-tech replicators were mineral crystals or organic direct forerunners of DNA itself, we may guess that the ‘power’ they exercised was direct and elementary, like stickiness. Advanced levers of power, like a snake’s fang or an orchid’s flower, came far later.
What might ‘power’ mean to a clay? What incidental properties of the clay could influence the likelihood that it, the same variety of clay, would
be propagated around the countryside? Clays are made from chemical building blocks such as silicic acid and metal ions, which are in solution in rivers and streams having been dissolved — ‘weathered’ — out of rocks further upstream. If conditions are right they crystallize out of solution again downstream, forming clays. (Actually the ‘stream’, in this case, is more likely to mean the seeping and trickling of the groundwater than a rushing open river. But, for simplicity, I shall continue to use the general word stream.) Whether or not a particular type of clay crystal is allowed to build up depends, among other things, upon the rate and pattern of flow of the stream. But deposits of clay can also influence the flow of the stream. They do this inadvertently by changing the level, shape and texture of the ground through which the water is flowing. Consider a variant of clay that just happens to have the property of reshaping the structure of the soil so that the flow speeds up. The consequence is that the clay concerned gets washed away again. This kind of clay, by definition, is not very ‘successful’. Another unsuccessful clay would be one that changed the flow in such a way that a rival variant of clay was favoured.
We aren’t, of course, suggesting that clays ‘want’ to go on existing. Always we are talking only about incidental consequences, events which follow from properties that the replicator just happens to have. Consider yet another variant of clay. This one happens to slow down the flow in such a way that future deposition of its own kind of clay is enhanced. Obviously this second variant will tend to become common, because it happens to manipulate streams to its own ‘advantage’. This will be a ‘successful’ variant of clay. But so far we are dealing only with single-step selection. Could a form of cumulative selection get going?
To speculate a little further, suppose that a variant of a clay improves its own chances of being deposited, by damming up streams. This is an inadvertent consequence of the peculiar defect structure of the clay. In any stream in which this kind of clay exists, large, stagnant shallow pools form above dams, and the main flow of water is diverted into a new course. In these still pools, more of the same kind of clay is laid down. A succession of such shallow pools proliferates along the length of any stream that happens to be ‘infected’ by seeding crystals of this kind of clay. Now, because the main flow of the stream is diverted, during the dry season the shallow pools tend to dry up. The clay dries and cracks in the sun, and the top layers are blown off as dust. Each dust particle inherits the characteristic defect structure of the parent clay that did the damming, the structure that gave it its damming properties. By analogy with the genetic information raining down on the canal from my willow tree, we could say that the dust carries ‘instructions’ for how to dam streams and eventually make more dust. The dust spreads far and wide in the wind, and there is a good chance that some particles of it will happen to land in another stream, hitherto not ‘infected’ with the seeds of this kind of dam-making clay. Once infected by the right sort of dust, a new stream starts to grow crystals of dam-making clay, and the whole depositing, damming, drying, eroding cycle begins again.
To call this a ‘life’ cycle would be to beg an important question, but it is a cycle of a sort, and it shares with true life cycles the ability to initiate cumulative selection. Because streams are infected by dust ‘seeds’ blown from other streams, we can arrange the streams in an order of ‘ancestry’ and ‘descent’. The clay that is damming up pools in stream B arrived there in the form of dust crystals blown from stream A. Eventually, the pools of stream B will dry up and make dust, which will infect streams F and P. With respect to the source of their dam-making clay, we can arrange streams into ‘family trees’. Every infected stream has a ‘parent’ stream, and it may have more than one ‘daughter’ stream. Each stream is analogous to a body, whose ‘development’ is influenced by dust seed ‘genes’, a body that eventually spawns new dust seeds. Each ‘generation’ in the cycle starts when seed crystals break away from the parent stream in the form of dust. The crystalline structure of each particle of dust is copied from the clay in the parent stream. It passes on that crystalline structure to the daughter stream, where it grows and multiplies and finally sends ‘seeds’ out again.
The ancestral crystal structure is preserved down the generations unless there is an occasional mistake in crystal growth, an occasional alteration in the pattern of laying down of atoms. Subsequent layers of the same crystal will copy the same flaw, and if the crystal breaks in two it will give rise to a sub-population of altered crystals. Now if the alteration makes the crystal either less or more efficient in the damming/drying/erosion cycle, this will affect how many copies it has in subsequent ‘generations’. Altered crystals might, for instance, be more likely to split (‘reproduce’). Clay formed from altered crystals might have greater damming power in any of a variety of detailed ways. It might crack more readily in a given amount of sun. It might crumble into dust more readily. The dust particles might be better at catching the wind, like fluff on a willow seed. Some crystal types might induce a shortening of the ‘life cycle’, consequently a speeding up of their ‘evolution’. There are many opportunities for successive ‘generations’ to become progressively ‘better’ at getting passed to subsequent generations. In other words, there are many opportunities for rudimentary cumulative selection to get going.
These little flights of fancy, embellishments of Cairns-Smith’s own, concern only one of several kinds of mineral ‘life cycle’ that could have started cumulative selection along its momentous road. There are others. Different varieties of crystals might earn their passage to new streams, not by crumbling into dust ‘seeds’, but by dissecting their streams into lots of little streamlets that spread around, eventually joining and infecting new river systems. Some varieties might engineer waterfalls that wear down the rocks faster, and hence speed into solution the raw materials needed to make new clays further downstream. Some varieties of crystal might better themselves by making conditions hard for ‘rival’ varieties that compete for raw materials. Some varieties might become ‘predatory’, breaking up rival varieties and using their elements as raw materials. Keep holding in mind that there is no suggestion of ‘deliberate’ engineering, either here or in modern, DNA-based life. It is just that the world automatically tends to become full of those varieties of clay (or DNA) that happen to have properties that make them persist and spread themselves about.
Now to move on to the next stage of the argument. Some lineages of crystals might happen to catalyse the synthesis of new substances that assist in their passage down the ‘generations’. These secondary substances would not (not at first, anyway) have had their own lineages of ancestry and descent, but would have been manufactured anew by each generation of primary replicators. They could be seen as tools of the replicating crystal lineages, the beginnings of primitive ‘phenotypes’. Cairns-Smith believes that organic molecules were prominent among non-replicating ‘tools’ of his inorganic crystalline replicators. Organic molecules frequently are used in the commercial inorganic chemical industry because of their effects on the flow of fluids, and on the break-up or growth of inorganic particles: just the sorts of effects, in short, that could have influenced the ‘success’ of lineages of replicating crystals. For instance, a clay mineral with the lovely name montmorillonite tends to break up in the presence of small amounts of an organic molecule with the less-lovely name carboxymethyl cellulose. Smaller quantities of carboxymethyl cellulose, on the other hand, have just the opposite effect, helping to stick montmorillonite particles together. Tannins, another kind of organic molecule, are used in the oil industry to make muds easier to drill. If oil-drillers can exploit organic molecules to manipulate the flow and drillability of mud, there is no reason why cumulative selection should not have led to the same kind of exploitation by self-replicating minerals.
At this point Cairns-Smith’s theory gets a sort of free bonus of added plausibility. It so happens that other chemists, supporting more conventional organic ‘primeval soup’
theories, have long accepted that clay minerals would have been a help. To quote one of them (D. M. Anderson), ‘It is widely accepted that some, perhaps many, of the abiotic chemical reactions and processes leading to the origin on Earth of replicating micro-organisms occurred very early in the history of Earth in close proximity to the surfaces of clay minerals and other inorganic substrates.’ This writer goes on to list five ‘functions’ of clay minerals in assisting the origin of organic life, for instance ‘Concentration of chemical reactants by adsorption’. We needn’t spell the five out here, or even understand them. From our point of view, what matters is that each of these five ‘functions’ of clay minerals can be twisted round the other way. It shows the close association that can exist between organic chemical synthesis and clay surfaces. It is therefore a bonus for the theory that clay replicators synthesized organic molecules and used them for their own purposes.
Cairns-Smith discusses, in more detail than I can accommodate here, early uses that his clay-crystal replicators might have had for proteins, sugars and, most important of all, nucleic acids like RNA. He suggests that RNA was first used for purely structural purposes, as oil drillers use tannins or we use soap and detergents. RNA-like molecules, because of their negatively charged backbones, would tend to coat the outsides of clay particles. This is getting us into realms of chemistry that are beyond our scope. For our purposes what matters is that RNA, or something like it, was around for a long time before it became self-replicating. When it finally did become self-replicating, this was a device evolved by the mineral crystal ‘genes’ to improve the efficiency of manufacture of the RNA (or similar molecule). But, once a new self-replicating molecule had come into existence, a new kind of cumulative selection could get going. Originally a side-show, the new replicators turned out to be so much more efficient than the original crystals that they took over. They evolved further, and eventually perfected the DNA code that we know today. The original mineral replicators were cast aside like worn-out scaffolding, and all modern life evolved from a relatively recent common ancestor, with a single, uniform genetic system and a largely uniform biochemistry.
The Blind Watchmaker Page 21
