Mineral surfaces have played a key role in many speculations concerning the origin of life, and for rather simple reasons. One is the potential for the binding and selective entrapment of key molecules, possibly with the added bonus of orientating them in some sort of regular array. Of particular importance would be a prebiotic process of stringing together simpler building blocks, a process known as oligomerization. What might be a demanding and problematic process in a free aqueous environment, with its continuous molecular jostling and recurrent danger of chemical interference by such processes as hydrolysis, might be neatly short-circuited on the surface of some mineral such as a clay or the phosphorus-rich apatite. Mineral surfaces potentially have other advantages. These include an enormous surface area, especially if the mineral grains are small. Thus a cubic centimetre of clay will contain particles whose total surface area, so Christopher Jeans tells me, could, in principle, reach an astonishing 1800 m2, roughly equivalent to nine tennis courts. Moreover, if the mineral grains are not too tightly packed then an interconnecting system of pores exists so that fresh solutions can be swept in to supply a growing molecule. And there is an added bonus if the mineral happens to have some sort of catalytic property, capable of accelerating chemical reactions. It all sounds very promising.
The experimental work is certainly interesting. Although most efforts have been expended in persuading amino acids to link together,29 the necessary step to form first a polypeptide and ultimately a protein, there have also been investigations into the oligomerization of nucleotides,30 which would be analogous to building a strand of DNA. In these experiments typically only a very limited variety of the potential building blocks is used, and quite often it is the oligomerization of only one type of molecule that is attempted.
Moreover, as is so often the case in research on the origin of life, it is difficult to avoid a strong artificiality in the experimental set-up. Thus, on occasion, analogues of 'real' nucleotides are employed, while the chosen substrate is of a chemical purity unlikely to be found in a natural prebiotic environment.31 Given all this, it is perhaps not surprising that the results of oligomerization experiments are variable. Some substrates bind efficiently but others do not. The experimental process typically entails successively feeding the solution across the surface. In this way chains of up to 50 or so monomers can be linked into an oligomer. There is, however, a snag inasmuch as the binding to the mineral surface is typically an irreversible process: the oligomer remains locked in position. It is tempting, of course, to see this as a sort of staging post to life itself, with the mineral-bound strand then reading off multiple copies that then jostle for selective advantage as some sort of protolife. Perhaps so, but the investigators Rihe Liu and Leslie Orgel note that concerning such oligomerizations 'Many more experiments will be needed.'32
It is also worth remarking that while the role of clays in the origin of life has been seen largely in a positive light - be it as catalytic surfaces or in terms of Graham Cairns-Smith's ingenious ideas of a clay-based information system preceding that of DNA33 - these minerals may have unexpected drawbacks. Earlier in the history of the Earth there were many radioactive isotopes that have now disappeared because of their short half-lives. Of particular significance, perhaps, is that the radioisotope potassium-40, which would have been much more abundant at the time life was emerging. Given that potassium is an integral component of many clays, then, as has been pointed out,34 if these minerals 'acted as important nucleation sites, the protobio-logical synthetic reactions had to proceed much more rapidly than deleterious decomposition reactions to avoid the long-term effect of radiolysis and hydrolysis within days.'35
But not all minerals are so radioactive, and the potential role of their surfaces as the key to understanding the origins of life has an almost mesmeric attraction that can be traced back to that uneasy genius J. D. Bernal.36 There has of late been considerable interest in the mineral pyrite, popularly known as 'fool's gold' a compound of iron and sulphur (FeS2). This interest concerns both its chemistry and its surface properties, as well as a possible setting in active hydrothermal vents. For many people this is by far the most encouraging avenue in the pursuit of life's origins, although it comes as little surprise that those who have devoted their research energies in peering into other crucibles adopt a duly critical stance.37 The chief exponent of the possible role of pyrite is Gunter Wachtershauser, of whom the generally (and correctly) sceptical Robert Shapiro38 wrote, 'When I examined his longest paper, I felt as if someone had handed me a biochemistry text from the late twenty-first century. Most of the chemicals were familiar, but they were organized into an evolutionary pattern I had never seen before.'39
The gist of Wachtershauser's approach is to argue that when pyrite is formed from the pre-existing ferrous sulphide (FeS) in combination with hydrogen sulphide energy is made available (by the release of hydrogen). This energy could act to reduce carbon dioxide, so freeing the carbon for involvement in organic reactions. An important corollary to this hypothesis is that the surface of the pyrite crystal also plays a key role in controlling the ensuing chemical reactions that are the first steps in a primitive metabolism.40 It is certainly true that there have been some encouraging experimental results,41 although the experiments were performed under closely controlled conditions, and other research workers have arrived at more discouraging conclusions.42 In addition, attempts to portray a longer series of reactions in this hydrothermal setting look rather more precarious: a number of key steps remain hypothetical.43 Still, these are early days and there is a widespread sense that for all its uncertainties the Wachtershauser school is on to something important. One obvious attraction is the abundance of the main substrate, pyrite. As will be discussed below, the likelihood that the overall setting was in a volcanic hydrothermal system is also attractive, especially as the heat budget of the early Earth was substantially in excess of that of today. Another reason for (cautious) enthusiasm is the possibility of identifying very ancient biochemical traits and pathways: to watch a metabolism grow out of a mineral surface. Even so, it is worth bringing to mind some more general remarks of Andrew Ellington44 when he comments on a disturbing trend in modern thinking about abiogenesis and the evolution of metabolism ... the results of these experiments [on the origin of life] are often interpreted as being proof of a particular scenario, rather than as general support for a more vague view of our origins. While it would be extremely exciting to establish that the RNA world evolved to a particular scenario, it is unfortunately unlikely that sequence minutiae have remained unchanged and unscathed following a several billion year journey through multiple different types of organisms and biochemistries.45
The notion that pyrite (and other metal sulphides, especially nickel) could play the key role in the initiation of life has attracted attention for another potentially important reason. This is because such sulphides are abundant in hydrothermal systems, most famously in the form of the 'black smokers' found on oceanic spreading ridges. As potential sites for the origin of life, these systems are attractive for several reasons. These include their dynamic nature with strong temperature gradients, the possibility of active mineral growth, and a strong flux of both heated water and various chemicals. Such settings are now the focus of investigations into various prebiotic syntheses that might be alternative sources for some of the major building blocks of life.46 Hydrothermal systems have another advantage in that, of all the regions of the early Earth, they would have been the most immune to the searingly powerful destructive forces released by a series of violent impacts early in the history of our planet (see Chapter 5).
The possible association between very hot environments and the earliest life has, in turn, led to a potentially fascinating possibility that the most primitive types of bacterial life are those still found today living in hot springs,47 such as those in both the mid-ocean ridges adjacent to the 'black smokers' and other volcanic regions such as the famous Yellowstone geyser and fumarole pools. This does not necessarily mean that the very first life emerged in such torrid circumstances, although proponents of pyrite-based metabolism and hydrothermal systems will offer their own warm support. The fact that nucleotides require temperatures nearer to freezing for long-term stability (outside, that is, the functioning cell) has already been mentioned (see note 37), and this is not the only difficulty with a hot start to life.48 More interesting is the possibility that it was only those bacteria capable of resisting extreme temperatures, imposed by the series of colossal meteorite impacts (see Chapter 5), that could survive these 'thermal bottlenecks' to repopulate the planet.49 Attractive and popular as this idea might be, there is also a vociferous group50 who reject the notion that bacteria adapted to extreme heat, the hyperthermophiles, represent the common ancestor of all extant life. This group argues that these adaptations to high temperatures must be a secondary feature. Even so, despite this scepticism and more general reservations,51 at the moment the idea that life's origins are to be found in ancient environments nearer to the traditional depictions of Hell - sulphur and boiling temperatures - is probably the most influential idea in town.
Yet even a cursory knowledge of the investigations into the origin of life invites some caution. We have been here before: a new locale, apparently realistic conditions, some intriguing results that indeed confirm the possibility of replicating certain early stages - say stringing amino acids into peptides - of the evolution of living systems. It is all very encouraging, but twin questions remain. First, can we move from the carefully controlled environment of the laboratory bench ('all solutions were prepared from doubly distilled water'52), to a more realistic setting? Second, research on the history of the origin of life may repeat itself. Those first stages certainly require chemical skill and imagination, but the question remains as to whether the further and crucial stages to reach a functioning metabolism are so easily achievable. To put it bluntly, the very success of one system in one context, say pyrite, makes it questionable whether a very different set of reactions will be catalysed with anything like the same efficiency. Thus, while finding much to admire in Wachtershauser's work, Leslie Orgel53 points out that there are difficulties in extrapolation. He writes,
One must expect the results of mineral catalysis to be highly idiosyncratic; most minerals will probably catalyze some reactions and many reactions will no doubt be catalyzed by some minerals ... Nor would the situation be changed if the proposed participants in a complex cyclic reaction scheme were synthesized in situ on a mineral surface. If the products are mobile on the surface, the situation is identical to that for adsorbed molecules. If they are not, one must postulate a series of remarkable coincidences to conclude that all of the reactions are catalyzed on the same mineral and that each intermediate product is formed in the correct position and orientation to become the substrate of the next regiospecific reaction of the cycle. The self-organization of a complex cycle, such as the reductive citric acid cycle, this time on the surface of FeS/FeS2, although logically possible, is very unlikely.'54
In conclusion, it would be most surprising if some, perhaps many, minerals did not play important roles in catalysing in one way (e.g. oligomerization) or another (e.g. a simple cycle) some of the first steps to life, but getting to the next stage seems to be as elusive as ever.
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