Chapter 13 Genesis: a secular version


Carbon’s isotopic tracers

Experimental genesis

Informed guesses

Cometary fertiliser

On the fringe of minerals

Three life-forming environments

Life in the abstract

There have been more contributors to the great question of life’s origins than there are people named in the Old Testament. Once, the focus was on some spontaneous popping into being, either by divine intervention or in the mysterious way of things in general. Oddly, the latter turns out to be not so unlikely, albeit a great deal more complicated than frogs forming from May dew which was the view of Aristotle. You saw in Chapter 3 how the element carbon presents such a wealth of chemical possibilities that it is by far the most likely core of any conceivable type of self-replication that is the essence of life. There we traced the carbon-centred steps involved in building the basic molecules involved in living processes. Chapter 12 put the huge diversity of modern life into the context of chemical processes that permit cells to live and thrive in different ways. The point of departure here returns to carbon itself. Literally extracting that element from the geological record provides the key to judging when life on Earth appeared.

Carbon’s isotopic tracers

Carbon, at the centre of life and (in CO2) pivotal to climatic temperature, does not come in a single atomic form. How elements are assembled in stars and how protons and neutrons hold together in the nucleus makes a range of isotopes possible for most elements; some stable, some prone to disintegration. Carbon has two stable isotopes, one with 6 protons and 6 neutrons, signified by 12C the other  13C with an extra neutron. There is a third, 14C, which forms by cosmic-rays that knock a proton from the most common isotope of nitrogen 14N. Carbon-14 decays radioactively quite quickly and forms a means of dating very recent events back to around 50 ka, so it does not reside for long in fossil organic matter. Although having the same valency and only slightly differing relative atomic mass, carbon’s two stable isotopes have subtly different chemical reactivities. That makes little, if any, difference when they enter CO2, but when autotrophs take up the gas to fix carbon, the intricacy of autotrophic chemistry does exploit the isotopic differences. Specifically, the heavier 13CO2 molecule diffuses less easily into cells than the lighter form of the gas, and is consequently fixed more slowly. The relative proportion expressed by the mass fraction or isotopic ratio of the the two (13C/12C) changes. That ratio is less in organic molecules than its value in inorganic carbon. Furthermore, because 12C is selectively extracted from the surface environment by life, the ratio increases in the inorganic form that is exposed to living processes. ‘Pristine’ carbon emerges from volcanoes – it comes from the mantle – but is divided eventually between that in life and that in inorganic compounds.

Judging the extent to which such isotopic fractionation has affected a carbon-bearing material depends on measuring the difference between the ratio in the material and that in a universally employed standard material. The difference is expressed as δ13C in parts per thousand or (per mil, ‰, rather than percent %). The standard is itself from the inorganic reservoir, being calcium carbonate secreted from sea water by a squid-like animal whose inner skeleton occurs commonly in rocks about 150 Ma old (the Cretaceous Pee Dee Belemnite or PDB). Carbon isotopes around at that time had themselves fractionated and reached a balance. Compounds formed by ancient life have negative δ13C values relative to the standard, while those in inorganic materials forming at the same time as that life are almost the same as in the standard; their δ13C generally hovers around zero, but there are important exceptions. One is that methane generated by inorganic reduction of carbon dioxide, as in the case of volcanic methane emissions, has δ13C between -22 and -58 ‰. ‘Pristine’ carbon from the mantle (or from extraterrestrial objects) has δ13C values that are slightly negative.

The inorganic materials involved in the isotopic balance are CO2, its various products when dissolved in water, and solid carbonates that form limestones. Of these only limestones are commonly preserved from the distant past. While life has existed there has always been some chance that organic compounds have been trapped in rocks. Of course, the heat and pressure involved in the rock cycle will have changed the structure of those organic compounds. However, the carbon has been locked in them from the moment they were buried as newly dead remains to form waxy carbon-rich material in many sedimentary rocks, which are found back into strata of Archaean age. The δ13C values of these kerogens is a key to whether they represent ancient life or not. Such is the ability of life processes to select isotopically light carbon that there is no mistaking its large negative δ13C signature in sedimentary rocks. On the other hand, limestones that formed while life was around generally deviate only slightly from the standard, because the carbonates in them were made from carbon-bearing ions dissolved in sea water (Figure 13.1).

Carbon isotope fractionation by modern autotrophic organisms, both prokaryote and eukaryote, provides a measure against which δ13C values of ancient kerogens can be checked (Figure 13.1). Unfortunately, they all have wide ranges, so it is difficult to identify which type of organism might be responsible for a kerogen ‘signal’. In particular the range for methanogens blankets the ranges of every other living organism. Life in general has characteristically large negative δ13C values compared with those in carbonates, so if it was present kerogen isotopes would give an unmistakable signal. The graph in Figure 13.1 confirms that the first appearance of oddly shaped stromatolitic masses (Chapter 8) in Archaean sediments dated at ~3.5 Ga are likely to be organic in origin. From rocks of 560 Ma ago, in which tangible fossils appear, back to the early Archaean there is a more or less continuous trend that links through to the present, with a few short-lived blips. The significance of these blips becomes clear in Part VII. Sediments as old as 3.7 Ga from West Greenland have very negative δ13C values, but they are metamorphosed, so we cannot rule out hydrocarbons created abiogenically from inorganic methane produced by reduction of CO2 in the mantle – such isotopically ‘heavy’ methane is found in modern volcanic emissions. Yet nor can we rule out that life existed almost at the beginning of the geological record. This is an irritating outcome. We cannot say for sure when life appeared; it may have done so before 3.7 Ga, but an association of low δ13C values with what may be fossils of bacterial filaments does not appear until several hundred million years later. The first view gives 650 Ma of preceding Hadean rich with possibilities as regards the pace of change and the sorts of conditions under which it formed. But with no sedimentary rocks to go on, only speculation can benefit. An Archaean origin does hold out a slender thread of hope for tying down some direct evidence.

F13_1Figure 13.1 Carbon isotope compositions (δ13C values) for: (left) present-day marine limestones, atmospheric CO2, organic matter in marine sediments, and several autotrophic life forms, all of which, except for methanogens and green sulfur bacteria, fix carbon using the Calvin or C3 cycle; (right) limestones and kerogens throughout geological history.

Stop Press: In August 2016, a team led by Allen Nutman of the University of Wollongong, Australia, announced their having found convincing stromatolites in the 3.7 Ga old Isua metasediments of West Greenland (Chapter 8). Their age is close to the end of the Late Heavy Bombardment (4. 1 to 3.8 Ga; Chapter 10) of the Solar System by errant asteroids and comets. So, if the physical evidence is what it seems to be, life emerged either very quickly after such an energetic episode or conditions at the end of the Hadean were not inimical to living processes and the prebiotic chemistry that led to them. Many scientists have made valiant attempts to shed light on this tremendous issue by trying to create under geologically realistic conditions at least the complex chemicals from which living cells are built, and perhaps even more…

Experimental genesis

There are probably only a few scientists around today who, like Aristotle, reckon that frogs form from May dew and that maggots and rats spring into life spontaneously from refuse. The idea that life emerged from the non-living does, nevertheless, still seem to be the only viable theory. Yet, as early as 1859, Louis Pasteur ruled out sudden, spontaneous means of generation of even microbes from the inorganic world by a simple experiment. He boiled a nutrient liquid in a flask with a drawn-out, S-shaped neck, then left it open to the air to cool. Gas molecules could freely exchange but dust, spores or moulds could not get in. No matter how long the flask lay around, the liquid remained sterile. He concluded that all life today has its origin in living things, and that there is an unbridgeable chasm between life and non-life. He brooked no disagreement, but, of course, some people were not at all happy. Their intervention went completely unnoticed in Pasteur’s day; it had a slow fuse, as you shall see.

Science had also concluded in the 19th century that the Earth had come into being at a finite moment from an already existing universe, albeit without a date being agreed or even conceived of at that time. Hermann von Helmholtz, more renowned for his ventures in physics, suggested in 1879 that life may have colonised the Earth from elsewhere in the universe, by travelling on comets or meteorites. This theory of panspermia (seeds everywhere) has been adopted more recently by some renowned scientists, such as Francis Crick, Fred Hoyle and Chandra Wickramsinghe. Unfortunately, it is a theory that merely passes the buck to some unspecified corner of the cosmos. The idea gains flurries of support now and again, from what appears, at first, to be hard evidence.

In 1864 a large meteorite fell near the French town of Orgeuil. Unusually, it contained about 15% of sticky hydrocarbons. Pondered over as having perhaps carried once-living material from a now atomised planet, but then left in a museum cabinet, Orgeuil achived fame and then notoriety in the early 1960s. George Claus and Bartholemew Nagy reported having found ‘organised elements’ in Orgeuil, and claimed that they were fossils. A maelstrom of research to confirm or refute the claims resulted in George Anders and Frank Fitch from the University of Chicago showing that the mixture causing all the fuss was actually a forged concoction of glue, coal dust and the seeds of a European rush. Claus and Nagy were not to blame, except insofar as they were completely taken in. The forgery was possibly made in response to Pasteur’s experiment that appeared to refute Darwin’s conviction that life must have emerged from the inorganic world. The forger may have felt that evidence for extraterrestrial organic chemistry, undoubtedly present in the Orgeuil meteorite, needed a supplement.

During the week in which I first drafted this chapter (early August 1996), NASA and US president Bill Clinton announced ‘perhaps the greatest scientific achievement of all time’. They unveiled evidence from a meteorite collected from Antarctic ice (and therefore likely to be uncontaminated), which suggested that not only had it come from a large impact on the surface of Mars, but that it contained relics of Martian life forms. This was not a forgery. There were oddly life-like objects in the meteorite (Figure 13.2), and both chemical and isotopic signatures bore some resemblance to what might be expected to result from living processes. However, every single item of evidence, from minute whiskery, egg-shaped and spherical objects to assemblies of polycyclic aromatic hydrocarbons were ambiguous. The hyperbole was probably not unconnected with NASA’s parlous financial situation, a burning ambition by many NASA scientists to bring back the glory days of the Apollo missions, and a looming presidential election. Three months later carbon-isotope evidence from other possible Martian meteorites added support to the notion of life’s emergence on another planet. It came from a group of British scientists, and again was hyped by the UK government of the day, that faced re-election too. Similar claims are likely to follow, such as that in 2014 which suggested a life-like isotopic signature in a martian meteorite newly fallen in Morocco. None of these claims has been substantiated, although NASA and its European counterpart ESA raised funds for a multiplicity of uncrewed missions to Mars, including several robotic vehicles that could rove over the Martian surface. Beagle 2 the only mission with the objective of settling the issue of life on Mars, either dead or still lingering, came to grief on crash landing. NASA has, perhaps wisely, focused on the issue of water, but with the obvious undertone of seeking ‘habitable’ ancient environments. ESA’s latest venture is an orbiter aimed at detecting methane in the thin Martian atmosphere; a possible sign of living methanogens (but equally of abiogenic reduction of CO2 to methane in the martian mantle that may well carry the same simulacrum of life-originated carbon as does terrestrial volcanic methane).

F13_2Figure 13.2 Electron-microscope photograph of supposed primitive lifeforms in a meteorite purported to have come from Mars

But even if Mars did once host life, so what? Would that be a ‘wonder of the Solar System’? Obviously, the Earth did, for sure, and we stem from that early life The Martian meteorite and the detected signs of once abundant water on the surface of Mars, its alteration of igneous rocks and deposition of a great variety of sedimentary rocks including clays are very old (~3.6 Ga). But the Martian dates are about the same as that for the earliest undoubted signs of life on Earth, and there is plenty of promising material here that needs only a few million dollars to study exhaustively, let alone the tens of billions that would be needed for a sample-return mission to Mars, let alone one with a crew. If once upon a time there was life on Mars its microfossils would probably be better preserved there rather than here, but only because it is such a terminally boring place geologically. Maybe they or any preserved organic molecules would answer some questions about general life-forming processes. However, that is being extremely charitable, for it would not answer all the questions. What concerns humans is how our own planet came to be inhabited and how it can remain so. Molecular biology seems likely to be a lot more useful. Ironically, the same week in October 1996 as the politically hype of the British revelation saw isotopic evidence from the Greenlandic sedimentary rocks that life may have been present on Earth at 3.7 Ga – just after the cometary pandemonium that destroyed evidence for the Earth’s first half billion years. Earth’s life has been tough old stuff from its outset. But perhaps not ‘sexy’ enough to attract the undivided attention of governments either side of the Atlantic.

Despite the amazing, indeed bewildering, range of modern life’s strategies, all of it is founded on nucleic acids, the base sequences that they carry, and on the proteins that genetic ‘blueprints’ make from the Lego bricks of the biosphere, amino acids. The route to understanding life’s origin here is paved with this mixture. But, like State Route 1 in California – along the precipitous Pacific coast – it has switchbacks, fearsome bends and the risk of hurtling over a cliff. History reveals halting progress towards resolving the ‘Big Question’.

The fact that chemists can synthesise organic compounds from CHON elements, together with the occurrence of amino acids, carbon-ring compounds and all kinds of simpler CHON molecules in meteorites, comets and even interstellar molecular clouds, show that life is not needed to beget the basic ingredients from which it is built. Research in this vein has always been constrained by the general level of knowledge and the technological sophistication at any one time. Charles Darwin in a letter to his friend Joseph Hooker sighed impatiently in 1871:

‘But if (and Oh, what a big if) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would never have been the case before living creatures were formed’

By the early 20th century speculation extended to the spontaneous, abiogenic formation of enzymes that speed up organic reactions, and that naked genes formed the earliest replicating cells, even though nothing was known about the actual chemical make-up of genes. Experiments directed at the origin of life began with Alexander Oparin’s 1923 observation in the Soviet Union that mixing oily organic compounds with water produced stable mixtures of suspended globules. Oparin suggested that the accumulation of more and more complicated organic molecules in such droplets, enzymes first to catalyse combinations of other materials to form genes and their organisation of self-sustaining metabolism, might be a route to life.J.B.S. (Jack’) Haldane of the University of Cambridge independently proposed a similar model of chemical evolution in 1929, but suggested that the first step was the formation of viruses. Both scientists speculated on a scenario like that dreamed of by Darwin, with a primitive oxygen-free atmosphere containing simple reduced compounds, and with ultraviolet light and lightning powering chemical reactions.

The Oparin-Haldane hypothesis remained untested until the 1950s. At the University of Chicago, Harold Urey and his student Stanley Miller modelled primitive conditions using simple glassware, with separate flasks to represent atmosphere and ocean. The ‘ocean’ was simply 200 ml of water, while C, H, N and S were introduced into the ‘atmosphere’ as methane (CH4), ammonia (NH3) and hydrogen sulfide (H2S). Simmering the ‘ocean’ added water vapour to the reducing atmosphere, and the experiment then depended on continuous electrical discharges through the gas-filled flask. After only a week the Urey-Miller apparatus was stained dark red and was a stinking mess of all manner of organic chemicals. Analysis showed them to include 10 of the 20 amino acids from which proteins are built, together with others that play no role in life. There were fatty acids and sugars, and most exciting of all the purines adenine and guanine – half the bases that make genes in DNA. Miller and Urey’s work was hailed as a stunning, though partial confirmation of the Oparin-Haldane theory. It tied in wonderfully with Crick and Watson’s discoveries of nucleic acid structure. A repeat of the original experiment in 1995 generated the pyrimidines cytosine and thymine too, after adding urea to the starting material to mimic its possible concentration by evaporation in Darwin’s ‘warm little pond’. Re-examination of Miller’s 1995 products by his former student Jeffrey Bada, to whom they were bequeathed found yet more amino acids at concentrations too low for Miller and Urey to have detected.   The Chicago experiments seem to provide the preconditions for natural chemical reactions that, given millions of years, might eventually generate nucleic acids. But research did not follow just one line and others worked in parallel.

More and more evidence accumulated, some from research on the Apollo lunar samples, that at the time of life’s emergence, and probably from the time of the Moon’s formation, the Earth could not have had an atmosphere sufficiently reduced to contain Miller and Urey’s starting gases. They would have had to be emitted by volcanoes, and that implies that magmas then melted from a reducing mantle. All the evidence for Year Zero (Chapter 11) points to a mantle that earlier core formation had depleted in metallic iron, its main reducing agent, before the impact that flung off the Moon. The searing impact would have blasted away any shred of an early, perhaps reduced atmosphere. Moreover, life’s complexity, had it existed beforehand, would have been ripped to shreds. Thereafter, as today, volcanic activity would have been dominated by emission of more oxidised gases (carbon dioxide, water vapour, sulfur-oxygen and nitrogen-oxygen gases), with which highly reduced gases could not have coexisted.

Oh dear! Experiment after experiment on a more realistic early atmospheres fail to produce anything interesting, unless hydrogen is present as a reducing agent. So it seems that despite the excitement encouraged by their soup-kitchen approach to building blocks, Miller and Urey’s work is of marginal relevance to processes on the Earth.

Informed guesses

It is often said in times of perplexing adversity that ‘there are more ways of killing a cat than by drowning it in butter’. Hydrogen is not the only reducing agent. Any element, ion or compound that freely donates electrons could conceivably have encouraged pre-biotic chemical evolution. There are many candidates, but a prerequisite for a believable scenario is that the necessary reducing agent was abundant.

One stab at this is to simulate the synthesis of organic compounds as a result of impact energy, a very abundant commodity early in Earth’s history. The idea has been tested experimentally by a team of Japanese geochemists, led by Yoshiro Furukawa of Tohoku University, using a propellant gun to create high-velocity impacts into a mixture of solid carbon, iron, nickel, water and nitrogen; a highly simplified scenario of ordinary chondrites bombarding atmosphere and ocean, meteoritic iron being the principal reducing agent. The experiments yielded a wealth of organic molecules, including fatty acids, amines and an amino acid (glycene) found in DNA, but somewhat less than yielded by the Miller-Urey experiment. Scaling up the experimental yields to the mass of meteoritic material that probably accreted to the Earth during the Hadean Eon (of the order of 10 21 kg), the authors estimate that at least 1014 kg of organic material would have been present in the surface environment by the time life eventually emerged in the Archaean; only a hundred time less than the estimated mass of the modern biosphere.

Without invoking extraterrestrial reductants, the best bet is ferrous iron in the form of dissolved Fe2+ ions, likely to have pervaded the oceans before the Great Oxidation Event around 2.4 billion years ago. All ancient and modern basaltic lavas that formed by partial melting in the mantle contain abundant ferrous iron in many minerals. While the mantle has not been sufficiently reducing to generate the gases used by Miller and Urey, neither has it been oxidizing enough for all iron to be cast into its oxidised, ferric state. Another possibility for a reductant arises from the weathering and alteration of igneous silicates by water, ultimately to form clay minerals. This involves the donation of hydrogen ions from the various acids involved, principally carbonic acid formed by the solution of CO2 in rain and seawater. Today the bulk of this alteration proceeds to cancel out any reducing conditions, because oxygen, the main acceptor of electrons and therefore the main oxidant in the environment, is so plentiful. However, in some modern environments, particular on the deep ocean floor, oxygen is in short supply. The alteration of ocean-floor basalts by hydrothermal circulation does produce reducing conditions, clays and Fe2+ ions. Much of the ferrous iron combines with sulfur, now through the agency of Sulfobacteria, to form iron sulfide. However, as soon as any remaining Fe2+ ions move beyond the hydrothermal vents they meet oxygen, become Fe3+ and immediately precipitate as insoluble ferric hydroxides. With no oxygen, or very little in the atmosphere ferrous ions would survive. So an alternative scenario for the oxygen-free world at the time of life’s origin is one where clays and Fe2+, both derived from rotted igneous rocks, would have been everywhere.

Take water containing ferrous iron or suspended clay, with some CO2 dissolved in it, and then simply irradiate it with ultraviolet for several weeks and formaldehyde and methanol form. Not the spectacular yields of the Miller-Urey set-up, but simple experiments of this kind do more than hint at realistic, warm little ponds. Equally, they could work in water droplets in the atmosphere nucleated on clays and ferrous salts thrown into the air by storms or impacts and held in suspension by atmosphere-wide giant clouds (Chapter 11). Simple products would have to return to the oceans to complete the building of more complex molecules, since the atmosphere would react with them when the droplets evaporated. Life’s building blocks could have rained into safer and more durable havens.

Life undoubtedly existed at least 3.5 Ga ago, a few hundred million years after the heavy bombardment of the Earth-Moon system, and conceivably even earlier. Impact energies during this storm of asteroids and comets were unimaginably high, with a distinct chance that the biggest collisions may have vapourised at least part of the oceans and so too any of the pre-biological work that quieter conditions between impacts could have achieved. Such conditions imply that earlier self-replicating life would have been extinguished as well. The hidden start of Earth history was a precarious time for any complex CHON compounds, living or not. This is why it is such a surprise to find isotopic evidence for life just after the turmoil ceased, and maybe in its late stages. Life may have taken only a few million years to emerge from the repeated firestorms. It does not stretch imagination too much to suggest that perhaps there had been repeated life-forming and sterilizing events during the Hadean.

There again, we cannot rule out survival of more ancient organisms in some niche protected from the high energy influx, say deep water or wet cracks in rock, to explain life-like carbon isotope signals in the oldest sedimentary rocks. The trouble is, whether we look on Earth, Mars or some odd-looking moon of a giant planet, and even find some amazingly convincing evidence for a life-forming process – a perfectly preserved little pond or antique black smoker – we will never know if other processes had not also gone on. Goodness, we didn’t even know about the lobster-lip creature until 1995, and for the far past finding all life’s little surprises is such a long shot we cannot imagine it.

Deep, dark places or well-lit ponds, it doesn’t matter. Life did form, given an unknown length of time, but certainly in less than 600 to 700 million years. Wherever a chemical environment favours CHON compounds, where liquid water is stable and there is energy available to be stolen and upgraded in complex molecules, life is a possible outcome of multi-element chemistry. First life on Earth could have been photo- or chemo-autotrophic, or even a heterotrophic form that metabolised large inorganic CHON molecules. Self-replication means some sort of digital information built into the molecules. Any chance for changes in the instruction manual for replication, which does not scramble it, would favour molecules that can adapt to other homely environments as the opportunity arises. The bizarre communities that now thrive near deep-ocean hydrothermal vents provide a good analogy – but not an indisputable answer. Metazoans that now live there must have adapted from larval forms that evolved in shallow, well-lit and aerated water – they are eukaryotes after all. To survive, their genes must have included mutations that permitted metabolism linked, perhaps through bacterial intermediaries, to the sulfur-centred chemistry and volcanic energy near volcanic or hydrothermal vents, without the need for light or much oxygen.

Cometary fertiliser

Wandering around the ideas that have occurred to inquisitive and experimentally minded scientists by no means exhausts the possibilities linked to the origin of life. Remember that experiments can last for at most a few years. These ideas all carry deep uncertainties about the early, chemically fragile stages of putting into place the CHON building blocks, such as amino acids. A much simpler alternative remained strangely ignored for more than a century. Meteorites of the carbonaceous chondrite type that survive their fiery passage contain everything that a DNA builder might wish to tinker with – aldehydes, amino acids, purines, pyrimidines and even the polycyclic aromatic hydrocarbons over which NASA, Bill Clinton and Britain’s Tory government almost wet their pants in 1996. Objects in the Kuiper disk, and the long-period comets that swoop from the spherical Oort cloud enveloping the outer limits of the Solar System are equally well endowed. Even the giant molecular clouds between stars are full of simple CHON compounds. The Earth, the comets and most meteorites formed from part of a such a cloud. In Earth’s early days such matter poured out of the sky by the gigatonne, not only in bodies so large that they would break any bond imaginable by their incandescent entry and impact, but in every smaller particle too. There is evidence that even incandescent impacts do not destroy all delicate, complex molecules. The Sudbury impact structure in Ontario contains a layer jumbled with dust and glass that formed 1.8 Ga ago. In it are peculiar spherical molecules made from 60 carbon atoms linked as 12 pentagons joined to 20 hexagons, as in a modern soccer ball or New-Age dwelling. The assemblies have been called buckyballs or fullerenes after the inspirational engineer Buckminster Fuller who invented the geodesic dome. Such bizarre molecules contribute about 10 grams to every tonne in the layer, and given its wide extent and huge mass the layer contains millions of tonnes of carbon in this form. Buckyballs contain helium, whose isotopes indicate that the carbon molecules probably formed and trapped the gas before the Solar System developed. The molecules were probably assembled by helium burning in massive stars that had consumed their hydrogen. Once released from the extreme condition of temperature and pressure where they formed, buckyballs easily lose their content of helium by reheating. That helium is still present in them, even after the Sudbury impact, implies that at least some of the colliding body remained cold, probably because it broke up on entering the atmosphere. Complex CHON compounds in such impactors might therefore survive as well.

In Chapter 9 we saw that extraterrestrial objects contain potentially life-forming compounds formed by reactions at temperatures that never exceeded 200K. Impacts could have delivered them to the early Earth. Such molecules could never have formed life until they reached environments with enough energy and a sufficiently dense packing of matter that polymers might be assembled. For building materials the Solar System’s outer limits are as potentially fertile as a wrecker’s yard for vintage car enthusiasts. Gravitational perturbations continue to ship such goods Earthwards. Pre-biotic chemical evolution is perhaps not some improbable process to be agonised over and debated with furrowed brows. It is the normal cosmic chemistry of C, H, O, N, P, S, and anything else needed from the normal element factories in stars. Preparing the chemical conditions for life seems to be a general property of the universe.

Extraterrestrial organic compounds fall today as lightly as snow flakes, albeit considerably smaller, in the form of fluffy, interplanetary dust particles. They do not burn up, even though the delicate ices that are included in them soon melt and evaporate. Looking to such external influences on the home planet also reveals a rich seam of other possibilities, including metallic iron, a powerful reducing agent that dominates every collection of meteorites. Given an extraterrestrial perspective it would be easy to become wryly amused at how much intellectual effort against a sceptical world went into Oparin and Haldane’s notions of chemical evolution, and at the efforts of those that followed them. Were it not for today’s powerfully oxidizing atmosphere and any number of organisms that might, and probably do consume flakes of exotic biochemicals, the growing consensus for external supplies of building blocks would have emerged long ago. But it is more inspiring than amusing. Were it not for the quality of curiosity that drove Oparin, Haldane, Urey, Miller and dozens of others, questioning the origin of life outside of supernatural creation would never have arisen. Their work forged part of the wealth of modern culture. But for their creative endowment, inspiration, hard work and being just plain lucky, creationism and other barbaric and lazy notions would happily prevail.

The issue in the early 21st century is this: given an abundant supply of useful organic, though abiotic, chemicals how can they be assembled and ‘cooked-up’ to produce chemistry capable of self-replication?

On the fringes of minerals

There can be little doubt that amino acids, the ATUGC bases of DNA and RNA and other fragments of molecules eventually enlisted by life existed on the early Earth, but such supplies do not solve the problem of how life originated here, or anywhere else for that matter. Forming proteins by stringing amino acids together gets us nowhere by itself. Although they do much of the work in cells, in terrestrial life as a whole they have had to be replicated exactly an almost infinite number of times. That has always depended on nucleic acids and genetic coding, which required equally stringent quality control. To form nucleosides (purine or pyrimidine plus ribose sugar); to convert them to nucleotides; their polycondensation to nucleic acids capable of self-replication (Chapter 3), and a final assembly of all this inside a bag porous to the constituents of proteins are hard problems. Without a containing membrane, products of pre-life chemistry would simply dissipate in whatever medium was its host. None of these components exist outside of living cells and there is no obvious ‘test-tube’ route. Several molecular biologists, after reflecting on the problems, somewhat mournfully conclude that a direct path to life-synthesis is impossible. They reject nucleic acids as the start of self-replication and focus on how simpler replicating systems might form and introduce order, thereby opening a pathway to RNA/DNA. This route involves turning to the inorganic world of minerals.

We have already seen that clays are one of several potential electron donors that may have had a role in abiogenic formation of simple organic chemicals. Clays also have complex, but regular molecular structures. Moreover, individual crystals are so tiny that they collect electrostatic charges, which is one reason why muddy water stays cloudy for a very long time. Not only do clays settle slowly because of their minute size, but their charged surfaces repel one another and they dance around. Charged particles can also attract molecules carrying the opposite charge, and we have seen that amino acids, for instance, have different charges at either end (Chapter 3). The British physicist Joseph Bernal introduced these properties into the Origin of Life debate in the 1950s. Bernal’s idea was that clay crystals could bring together diverse organic molecules on their surfaces, long enough for them to have a chance of linking up. This sort of catalytic effect may benefit from another property. The molecular structure of clay could form a template for the larger organic molecules that grew. Taking this a step further, Graham Cairns-Smith of Glasgow University suggested that information in the form of simple coding, akin to that in ACGTU or even using the bases themselves, could be stored by links between the variations in electrical charge on clay surfaces and the assembly of organic compounds.

In 1996 experimental evidence that Bernal and Cairns-Smith’s ideas might well be along the right lines first began to emerge. James Ferris and his co workers at the Rensselaer Polytechnic Institute and the Salk Institute in the USA repeatedly exposed clays to solutions that contained nucleotide building blocks (thereby vaulting over their formation from bases, sugars and phosphates), washing them between each exposure. They found that RNA strands up to 55 nucleotides long eventually adhered to the particles. This is about the minimum size needed for RNA to be capable of simply the cutting and pasting of molecules that is the basis for its function in replication, but far from actual self-replication. Clay-mediated RNA building would not happen naturally today, for two reasons; first because of widespread oxidizing conditions and secondly because one organism or another would probably evolve to consume the products. For this kind of RNA synthesis to work at the dawn of life, the products had to survive the surrounding chemical environment for long periods. Ferris and colleagues calculate that even if one or two new building blocks were added each day, say by tidal washing and re-exposure to a sort of primordial soup, the bonds in the polymers would need to remain stable for several years, even for chains to reach an average length of 30 units. Nucleic acids do have sufficient strength to accumulate in this way, but need peptide bonds to form.

Essentially similar principles should apply to polymerisation on a wide variety of fine-grained mineral particles, perhaps metal sulfides or strange minerals that are found in the bubbles that characterise lava pumice. These last, the zeolites, have a bewildering array of structures, and many of them contain tube-like cavities in their molecules that conceivably might act as templates for helical molecules of RNA and DNA. In the early oceans there would have been a ‘library’ of such mineral templates. Polymerised CHON products might have ‘explored’ and exploited the options until self-replication took over. At that point the replicant polymers would begin to ‘consume’ the available building blocks in replacing and multiplying themselves. That would have been rudimentary life and the beginnings of genetics, but yet to exist within the bag that defines universal cellular life as we know it.

Essentially, this line of thought sees life originally forming as a heterotrophic form, from which the great range of autotrophic metabolisms evolved subsequently. Such a model has one interesting line of support. All life’s molecules rotate polarised light to the left, or counterclockwise, while the basic amino-acid building blocks formed by inorganic process, as in carbonaceous meteorites, do it both ways. If one or the other version starts polymerizing then all others that become attached must have the same symmetry. So, both left- and right-rotating (laevorotatary and dextrorotatary) chains would surely form in equal amounts. What if one variety achieved self-replication first? Would it not then intervene in its surroundings? If it did that successfully, it would multiply rapidly and in so doing would begin to consume all the chemical steps to building amino acids. Here might be the first operation of natural selection, and the opposite alternative (the ‘right-hand path’!) would be starved out of existence before achieving self-replication. That is an even more speculative conjecture than the heterotrophic model itself, and both have determined adversaries. They focus on the nature of what holds biological polymers together, the peptide bond (Chapter 3).

Peptide bonds in living things need proteins to act as condensing agents or enzymes, and they too involve the peptide bond. It is difficult to imagine some multi-process environment bringing together the nucleic acids and proteins that govern what now goes on in cells, even in the most glutinous primordial soup. Moreover, a soup of oceanic dimensions would have been too dilute. To the other main school of thought, some energetic agency outside of life seems essential for polycondensation leading to nucleic acids. A source of electrons is needed too. One of the most common and simple reactions under the conditions of the early Earth, and even today in some environments, is that involving iron and hydrogen sulfide to produce iron sulfide; pyrite or ‘fools gold’. It generates both energy and electrons. The electrons would be available to fix CO2 in organic molecules, which would bind to new pyrite granules as a layer, then to grow, spread and further polymerise on the mineral ‘template’. As in the clay-based model, once self-replication is achieved the molecules involved could abandon the mineral support. All this presupposes that peptide bonds could be formed. The co-originator of this theoretical beginning in chemo-autotrophy, Gunter Wachtershauser of Munich, set out in 1995 with several co-workers to test the hypothesis. They exposed compounds with free -COOH (carboxyl) and -NH2 (amine) groups to the pyrite-forming reaction. Bonds closely related to the peptide linkage did indeed form abundantly. This matched if not stole the thunder of the parallel experimental polymerisation on clays that supported the heterotrophic model.

Support for this pyrite-centred model comes from a number of essential living proteins that involve Fe-S linkages, particularly those associated with several styles of photosynthesis. The most abundant protein on Earth today is Rubisco, the enzyme involved in fixing CO2 through oxygen-forming photosynthesis in plants. An iron-sulfur bond lies at the core of the Rubisco molecule. Is it conceivable that the vast bulk of modern life, both autotrophs and the heterotrophs like us that are sustained by them, carry an Fe-S chemical fossil that points strongly towards an origin on or around ‘fools’ gold’? Many scientists believe that this is the best line to follow. Detailed comparison of the carbon isotope signatures of the oldest kerogens with those in modern autotrophic metabolism suggest strongly that Rubisco and the Calvin (C3) carbon-fixing cycle were around at 3.5 Ga and perhaps all the way back to the first sedimentary rocks. Rubisco seems to have a long pedigree.

A long haul of intricate experimentation seems in prospect, if ever one model or the other for the origin of self-replicating molecules is to gain universal acceptance and that continues in 2016. Mimicking the environmental conditions is difficult, partly because there are many variants on those that we can surmise for the pre-geological period. There are three general candidates: Darwin’s ‘warm little pond’ in a tidal zone subject to both evaporation and continual reflushing; close to submarine volcanic and hydrothermal vents; and, at first sight the most unlikely, in the atmosphere. All three contain fine mineral particles that might support, catalyse or power genesis. The most astonishing is one proposed by Carl Woese, long associated with Wachtershauser and the autotrophic route, and developed by Vern Oberbeck.

Three life-forming environments

As I outlined in Chapter 11, the early Earth probably had strange weather. Strangest of all was limitless convection of water-bearing clouds permitted by the lack of a temperature inversion at the tropopause. Towering clouds rose to the outer limits of the atmosphere. The sky must have blazed with lightning. Such stormy heads would have swept up dust raised by volcanism or impacts. As today, tiny solid particles would have helped condense water droplets. Reactions within them stimulated by light, including ultra-violet radiation, and electrical discharges may have set in play organic chemistry mediated by the templates of mineral molecules. Complex organic molecules coating the particles might then contribute to a layer at the surface of the droplet; a prototype cell membrane within which further reactions might proceed. Droplets in air also form when waves break, which is why modern rainfall is slightly salty. This would carry dissolved inorganic and oily organic compounds formed by reactions in water into a more energetic environment. There, further reactions could build complex molecules that incorporate many of the trace elements essential to life, whose only plausible source is by dissolving from rock. Again, a chemical microcosm in a droplet might just form a stable coating and the first cell envelope. Among all the metaphorical eggs that must be kept in the air by bio-scientists who ponder life’s origin, a crucial one is the bag in which they are carried. To some a gooey coating to a water droplet is implausible, and in any case its formation remains to be demonstrated.

Among the many stable organic molecules that can easily be formed in test tubes are fatty acids called phospholipids with structures that have a double response to water. One end of such an amphiphile has an affinity for water molecules, the other repels them. They also spontaneously assemble to form layered structures. Left in water, such layers curl up to make tiny spheres. As in many instances, a relevant surprise lay in carbonaceous meteorites. David Deamer and his colleagues at the University of California found in one a mix of rare compounds that have this property (Figure 13.3). Moreover, the little bags that form from them efficiently trap simpler biological molecules. The compounds are fatty acids, related to the lipids that dominate living cell membranes, and these last contain another surprise. In the eukarya they are stabilised by cholesterol, the most notorious of its clan. Analysis of ancient kerogens sometimes shows cholestane, to which cholesterol degrades. As you will see, molecular studies show the eukarya to go back almost as far as the separation of the archaea and bacteria. How ironic it would be if the molecule that contributes more to human morbidity and death than any other were found to have arisen as one prerequisite for the origin of life itself.

F13_3Figure 13.3 Cell-like membranes formed by fatty acid amphiphiles found in the Murchison meteorite.

A Darwinian approach, via his ‘warm little pond’, essentially concerns the intertidal environment, bathed in pre-biotic ‘primordial soup’ that came in and then went out with the tides. Here would be abundant energy in ultra-violet form, plus the chance for concentration and washing cycles, and an abundance of clays and other minerals. A ‘warm, big pond’ model, ie the surface layer of the ocean, is a proposition with some supporters, though the catalyzing effect of mineral particles is hard to envisage in open salty water. It would contain many ions to cancel electrostatic charges that help keep fine grains in suspension. There are two main drawbacks governed by early ocean water. The ocean may well have been global with few emergent land masses on which tidal flats might develop. Greater loss of internal heat mainly through submarine volcanic and related processes would have resulted in rapid cycling of ocean water, thereby reducing the chance for pre-biotic molecules to concentrate in it. A ‘soup’ containing building blocks would have been very dilute. Set against this, precursor compounds raining down from processes in the swirling atmosphere and an extraterrestrial source in comets and meteorites could provide sufficient insoluble, oily compounds to form surface slicks. Yet however unlikely in a dilute laboratory solution molecular interactions might be, time of the order of tens of million years is amply sufficient to overcome the dismal statistics.

The third alternative site for life-forming chemistry is in hot water around hydrothermal vents, especially where hot, young oceanic crust reacts with sea water. An avenue of evidence pointing that way is the presence in biological molecules, particularly enzymes, of a variety of elements other than CHON. Phosphorus is, of course essential, as is sulfur. But, interestingly, the metals iron, zinc, molybdenum, copper, nickel and magnesium also occur in higher concentrations than we find normally in ocean water. Their high abundance in biological compounds suggests that such molecules first formed where water was unusually enriched in metals. If those metals are chemical fossils, they point to hydrothermal vents and ‘black smokers’ that emit concentrations of dissolved metal that hot fluids have extracted from the basaltic lavas of oceanic crust. The best odds for creating such trace chemistry are carried by these dark, hot and noxious places. Today they support teeming, though bizarre ecosystems, and you can judge whether they were the birth places for life from evidence in Chapter 14.

We have seen several chemical routes to nucleic acids, proteins and cell-enclosing membranes that are both plausible and exciting to follow experimentally. But they do not provide the answers on their own. Complex as it is, DNA has but one function, providing the genetic template for RNA when it divides. Being tended and rebuilt through complex cell chemistry, DNA has some of the attributes of a queen bee. Carrying the genetic recipe book around and collecting the ingredients is the role of RNA in its three forms (Chapter 3). From this all the necessary proteins are constructed. Yet it is proteins formed as instructed by RNA that perform all the constructional work of a living cell, including the cutting and pasting involved in accurately regenerating the nucleic acids. The main stumbling block to a plausible scenario for the final theoretical step to life has long been that stable proteins depend on nucleic acids and genetic coding. Only when this circular impasse was breached could ideas proceed further. Tom Cech of the University of Colorado and Sydney Altman of Yale found in the 1980s that some naturally occurring RNA could snip apart its own nucleotide sequences, and splice nucleotides to form copies of themselves. They had a similar function to enzymes, so Cech and Altman called them ribozymes. For the notion that RNA ribozymes were the first life-forms, in a primordial RNA world, Cech and Altman received the 1989 Nobel Prize for chemistry.

A world of RNA lifeforms, in retrospect from Cech and Altman’s work, seems obvious, for RNA plays such key roles in cellular life. It is genetic material and RNA nucleotides build those of DNA, which suggests RNA’s primacy. Strange as they are, in the twilight zone between life and the non-living, some viruses use RNA as their replicating code. They may be the oldest molecular fossils, but their sole function now is to enter a living cell and use the metabolism there to make copies of themselves. Gene-bound replication implies mutation, selection and evolution, even in a simple RNA world. A transition or leap to DNA-based life, and the multifarious machinery for translation that entails, requires diversification into at least three kinds of RNA and the adoption of some complexly functional proteins, when most RNA lost its enzyme-like attributes. The appearance of DNA could have been a chemical equivalent of natural selection. Tending to undergo chemical degradation and thus loss of secure replicant ability, RNA in the form of rybozymes might have been outcompeted and relegated to a mere functionary as more stable DNA formed from its close structural and chemical progenitor.

Such is the diversity of modern life, and such a vast range of chemical possibilities exist at the level of giant molecules built around CHON, let alone the fact that direct studies on the origin of life have been underway for only a few decades, that it is hardly surprising that battles rage and less mature though combatant ideas snipe from the sidelines. Lively debate spurs ever more sophisticated and cunning experiment, and more comprehensive investigation of natural things. Among objections to RNA world is that experiments need artificial energy to make the supporting reactions work. In nature energy has to match the requirements of reactions, has to come from somewhere – usually involving some form of chemistry – and must be coupled to production or it disappears as quickly as it was released. Carl Woese, a major player, views the spawning of life not as a consquence of stray RNA molecules but from the appearance of some connectivity between energy and chemistry. He also points out that any immediate precursor to copying via genes must have left a signature in genetic material that survives today. Not only has no such trace been found, but the three divisions of life, archaea, bacteria and eukarya, have too little in common. He believes that RNA and its formation and function comprised ‘work in progress’ at the time of the fundamental splits, and that fully formed they could not have been the trigger. But, then there are the mathematicians …

Life in the abstract

Many of the oddities of complexity and chaos theories have been applied to fundamentals of biology, with some slightly bewildered natural scientists as onlookers. One view, based on computer simulation of chemical complexity in this or that life-generating environment, is that at a critical level of diversity autocatalytic processes snap into place and take on ‘life’. Groups of digital molecules become linked and evolve to ever more complex relations. One view, that of Stuart Kauffman of the Santa Fé Institute, is worth exploring in some depth for it links basic chemistry with aspects of the mathematical theories of complexity.

Most of us understand science in the context of its prevailing method, that of reductionism where the variables in some process are cut down to the bare minimum for experiment’s sake. In chemistry this demands equilibria with two sides, left and right of the equals sign that signifies two alternative directions in which reaction might proceed. That is test-tube science, the artificial world of the closed system of two dimensions. Reality is not like that. Natural chemistry has as many dimensions as there are atoms, ions and compounds that can react. All possible reactions perturb and are perturbed by all other equilibria. In that sense natural systems are open, and there is unlikely to be a steady state. The concentrations of all the participating ‘end members’ oscillate in cycles whose period is limited by other cycles involved in other equilibria that persistently must organise matter and dissipate energy to maintain fleetingly the chemical structures involved. Mixing simple organic molecules that react with one another in a suitable medium often results in oscillations between acid and basic conditions. Rates at which such reactions take place under controlled conditions depend on chemical (pH and Eh) and physical (T and P) variables, but the rates can be speeded-up or slowed by the presence of compounds (catalysts and suppressants) that are not directly involved. For living things enzymes act as catalysts in governing the relationship between genes and cell chemistry.

Kauffman sees the origin of life as order emerging from chaos. Looked at statistically, the chances of assembling the immense chemical complexity of even the simplest life form are beyond belief (of the order of 1 chance in 101000). That is perhaps why some natural scientists throw in the towel, pass the buck through panspermia or become religious. Kauffman suggest a simple game that snaps statistics into a new focus. In days gone by fractious small children could be silenced by getting them to string together granny’s collection of buttons tipped from a vast, inherited repository onto the carpet. Kauffman’s variant is not to be recommended for that, as will become clear. Without looking at the buttons, begin to join them with thread, two at a time. For a while, all you have is isolated links in twos. Then, suddenly, as you lift your chosen button many others dangle from it because you have depleted the stock of solitary buttons. This happens when you have tied about half as many threads as there are buttons, and thereafter there are diminishing additions to the monumental tangle that you have created. This sudden increase in the order and complexity of the system (lonely buttons strewn across the carpet are simple but disorderly!) is called a phase transition. The more buttons that granny and generations past have collected, the faster complexity and order develops once you (eventually) pass the 1:2 ratio of threads to buttons. Modelled mathematically with a near-infinite number of buttons the process jumps to order and complexity instantly, just as in a physical or chemical change in the state of matter, such as water to ice, or mixed solutes to precipitate.

Molten rock involves all chemical elements, but mainly O, Si, Al, Fe, Mg, Mg, Ca, Na, K in that order. Experimenters peering at the structure of such melts using X-rays see submolecular clusters linking the components according to chemical affinities and the laws of combination. This hidden structure, which by the way governs how fluid magmas are, continually breaks down and reforms because of intense heat-driven vibration. At a threshold temperature some clusters suddenly transform into the ordered interconnections of silicate minerals and crystallisation begins, not all the minerals at once, but in a clear order of high- to low-temperature varieties. Silicates are pretty varied, but insignificantly so compared with CHON compounds. Kauffman’s view is that given life’s chemical components in about the right mix, such behaviour leads inevitably and rapidly to self-organisation and self-catalysis. How far that goes depends on the concentration and temperature. Its low values for both limit interstellar gas to the simple compounds observed in them.  Increased concentration and temperature during gravitational accretion to form the nebulae from which new stars ultimately form sets more complex boundaries on phase transitions. Hence the sticky hydrocarbon messes in some meteorites and probably in cometary interiors. Put these components in a dense, warm fluid held by the gravity of a planet and there arises a further step in the complexity of purely inorganic processes. Interwoven equilibria involve ingredients, products and also catalysts and suppressants for other reactions. Rather than a disorganised mess, chemically this is a form of competition, where some ‘species’ are consumed in the generation of others.

At its base, life is chemically simple – CHON plus about 16 other lesser but essential elements. The scaffolding is CHON and these are the commonest elements in the cosmos, because of the well-established laws that govern element manufacture from protons in stars of different kinds (Chapter 9). From a mathematical standpoint the origin of life is no mystery at all, just … emergent complexity that we strive to understand.

As the diversity of compounds in such an open system increases, the ratio of reactions (threads) to compounds (buttons) becomes higher. When the number of reactions equals or exceed the number of chemicals, then a giant web of complexity penetrates the whole CHON world. It snaps into being at a phase transition. Whatever the form and the detail, this is life – self-catalysing and self-renewing. The self-replicating part is the icing on the cake. There is a seeming inevitability to this chemical progression that feeds on itself and its surroundings at ever increasing rates. Inevitable, that is, provided chance opens a window of opportunity in the form of a planet that retains and emits water, which in turn is kept in liquid form because of its ‘right’ distance from the ‘right’ kind of star. Tempering this is inevitability’s interaction with luck; the throw of the chemical dice. By considering only the randomness of things, pure statisticians faced with life can only be glum.

Which came first, self-replication or life as a whole, is really a nonsense conundrum. Similarly chaos and order, neccesity and chance are always bound together, interact and transform, the one to the other. This is the lesson of pure-number mathematics that we may glimpse through fractals such as Mandlebrot’s and others’ strangely real-seeming constructs. It lurks in the discovery of interpenetrating order and chaos in reality as well as in computers, to shed an abstract light on everyday happenings in the universe. It puts reductionism and empirical experimentation into perspective, together with all their ‘conclusions’, ‘proofs’ and ‘falsifications’ of hypotheses. Say any CHON compound has a 1 in a million chance of catalysing any one of millions of reactions, then put 10 such compounds in a jug. Chances of something happening are 1 in a hundred thousand – a jug of dead molecules. Yet developments do take place under such unfavourable circumstances often within days or weeks; the experiments that you met earlier are of that kind. They are a measure of how extraordinarily favourable reality is for the complexity of life. Put a million compounds together, and hey presto – autocatalysis. Life is indeed special, but is an expected property of the universe, and sparks up in windows of opportunity that last long enough for chemical complexity to build slowly; perhaps in an instant.

Put such a system in a bag that connects chemically to the outside, more open system, and self-sustenance has the opportunity to become self-replication. Discrete organisms emerge from a chemical world that itself is in continual motion and change. And what of this bag? Amphiphile mebranes are not hard to imagine as parts of the preceding system, and they might well have been delivered ready-made from the remnants of another, simpler system involved in the formation of carbonaceous meteorites.

The immediate objection to Kauffman’s model is that building complex molecules requires energy to form chemical bonds. In this regard, the downside of multi-dimensional, catalysed equilibria is that all of them are reversible. Take proteins. They link up amino acids using the peptide bond, formed as H2O is ejected from the ends of a reacting pair of amino acids. Immersed in water, the union is broken by water’s re-entry. In these equilibria there is a rough 10 to 1 balance in favour of uncombined and combined building blocks, i.e. 10 amino acids to a single combination of two of them, 10 of the 2-variety to those in threes and so on. For even a 25-chain linkage, the ratio of amino acids to the most complex product of Kauffman’s model is 1025 to 1, about one molecule in a litre jugful of solution. Proteins around 200 long would be more rare than hens with dentures. Kauffman (what a hero!) sees 3 simple ways to destroy this objection. First, it relates to volumes, when the natural world is full of 2-D surfaces. This brings in the clay and sulfide story, and no one sees a problem with their origin and superabundance. There are also 1-D situations, such as the tubular aspects of some minerals structures. The probability of 25-fold combinations falls to 1 in 105 and 1 in 10√5, respectively, by increasing the number of opportunities for building blocks to meet fortuitously. Water a stumbling block? So remove it, says Kauffman, by dehydration. Thirdly, the energy involved in bonds in general is released by their breaking and thus available to build others. That objection becomes support for his model! And what is it that life does? It uses the ATP cycle, continually breaking and remaking bonds in a simple compound, as a way of mediating energy. Energy itself is all-pervading, whether from the Sun or from radioactive decay in the mantle.

The truly controversial thing about Kauffman and his supporters is that their ideas undermine the popular view among biologists and the geochemists whom they influence that genes, DNA and RNA are primary. In the complexity-theory model they are what transcends and survives from the preceding complexity of self-catalysis and self-sustenance, in the manner of chicken and eggs surviving from preceding creatures that laid eggs, and ultimately from the origin of eukaryote meiosis and gametes, of which the vertebrates’ egg is one. Such ideas are dangerous, as was that of Alfred Wegener. Whether they are or are not cyberfantasy rests on mathematicians convincing biologists to devise and run experimental tests. That promises to be a fascinating dialogue. But, as always, there is another way of sneaking up on the issue of life’s origin. The molecular chemistry of modern life forms gives an opportunity to backtrack.

Check out Further Reading for Part IV to see source material and links to articles

If you enjoyed reading this chapter online and want to learn more, Stepping Stones is available to download as an eBook


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