6

Almost a Miracle

‘How remarkable is life? The answer is: very. Those of us who deal in networks of chemical reactions know of nothing like it.’

– George Whitesides¹

The universe abounds in complexity, from everyday systems such as turbulent streams and snowflakes to grand cosmic structures like nebulae and spiral galaxies. However, one class of complex systems – life – stands out as especially remarkable. In his Dublin lectures Schrödinger identified life’s ability to buck the trend of the second law of thermodynamics as a defining quality. Living organisms achieve this entropy-defying feat by garnering and processing information and directing it into purposeful activity. By coupling patterns of information to patterns of chemical reactions, using demons to achieve a very high degree of thermodynamic efficiency, life conjures coherence and organization from molecular chaos. One of the greatest outstanding questions of science is how this unique arrangement came about in the first place.

How did life begin? Because living matter has both a hardware and a software aspect – chemistry and information – the problem of origins is doubly difficult. In a curious historical coincidence, just three weeks after Crick and Watson’s famous paper on the double-helix structure of DNA appeared in Nature, the 15 May 1953 edition of Science carried an article by a little-known chemist named Stanley Miller. Entitled ‘A production of amino acids under possible primitive Earth conditions’, it was subsequently hailed as a trailblazer for attempts to re-create life in the laboratory.² Miller put a mixture of common gases and some water in a flask and sparked electricity through it for a week, producing a brown sludge. Chemical analysis showed that this simple procedure had succeeded in making some of the amino acids life uses. It looked as if Miller had taken the first step on the long road to life with little more than a bottle of gas and a pair of electrodes. The conjunction of these two papers – one on life’s giant informational molecule, the other on its simple chemical building blocks – aptly symbolizes the central problem of biology: what came first, complex organic chemistry or complex information patterns? Or did they somehow bootstrap each other into existence in lockstep? What is clear is that chemistry alone falls short of explaining life. We must also account for the origin of organized information patterns. And not just information: we also need to know how logical operations emerged from molecules, including digital information storage and mathematically coded instructions, implying as they do semantic content. Semantic information is a higher-level concept that is simply meaningless at the level of molecules. Chemistry alone, however complex, can never produce the genetic code or contextual instructions. Asking chemistry to explain coded information is like expecting computer hardware to write its own software. What is needed to fully explain life’s origin is a better understanding of the organizational principles of information flow and storage and the manner in which it couples to chemical networks, defined broadly enough to encompass both the living and non-living realms. And the overriding question is this: can such principles be derived from known physics or do they require something fundamentally new?

IN THE BEGINNING …

Francis Crick once described the origin of life as ‘almost a miracle, so many are the conditions which would have had to have been satisfied to get it going’.³ And it’s true that the more ‘miraculous’ life appears to be, the harder it is to figure out how it can have started. In 1859 Charles Darwin’s magnum opus On the Origin of Species first appeared. In it he presented a marvellous account of how life has evolved over billions of years from simple microbes to the richness and complexity of Earth’s biosphere today. But he pointedly left out of his account the question of how life got started in the first place. ‘One might as well speculate about the origin of matter,’ he quipped in a letter to a friend.⁴ Today, we are not much further forward. (Except we do more or less understand the origin of matter in the Big Bang.) My earlier chapters will have convinced the reader, I hope, that life is not just any old phenomenon but something truly special and peculiar. How, then, can we account for the transition from non-life to life?

The enigma of life’s origin is actually three problems rolled into one: when, where and how did life begin? Let me first deal with when. The fossil record can be traced back about 3.5 billion years, to a geological epoch known as the Archaean. It’s hard to find many rocks this old, let alone spot any fossils therein. One outcrop of Archaean chert has, however, been intensively studied. It is located in the Pilbara region of Western Australia, about a four-hour drive into the bush from the town of Port Headland. The terrain is rugged, sparsely vegetated and scoured by mostly dry riverbeds prone to flash flooding. The hills here are a rich red hue and rocky outcrops harbour important traces of ancient microbial activity. Dating these rocks indicates that our planet was already hosting a primitive form of life within a billion years of its formation. The most persuasive evidence comes from curious geological features known as stromatolites. They appear as ranks of wavy lines or little humps decorating the exposed rock surfaces. If the interpretation is correct, these features are remains of what, 3.5 billion years ago, were microbe-covered mounds, created by successive microbial colonies depositing mats of grainy material on the exposed surfaces, layer by layer. There’s just a handful of places on Earth where one may today see similar stromatolite structures complete with their living microbial residents. Most geologists are confident that the Pilbara stromatolites (and others in younger rocks around the world) are the fossil relics of something similar, dating from the far past. The same Pilbara geological formation contains additional signs of life in remnants of an ancient reef system and a few putative individual fossilized microbes. It’s hard to tell from the shapes alone that the ‘fossils’ are more than merely marks in a rock; any organic material has long gone. However, very recently the biogenic interpretation received a boost.⁵ About 1 per cent of the carbon on Earth is in the form of the lighter isotope C¹². Life favours this lighter form so fossils usually possess a slight additional abundance of it. An analysis of the Pilbara rocks shows that the carbon isotope ratio is correlated with the physical shapes of the marks, as it would be if these were fossils of different microbial species. The results are hard to explain non-biologically.

The evidence of the Pilbara tells us that life was established on Earth by 3.5 billion years ago, but it gives little clue as to when life may have actually started. It’s possible that all older traces of biological activity have been obliterated by normal geological processes, and by the bombardment of our planet by large asteroids that occurred until about 3.8 billion years ago – the same bombardment that cratered the moon so thoroughly. The problem is a lack of older rocks. Greenland has some dating back more than 3.8 billion years, with hints of biological modification, but they aren’t decisive. Nevertheless, Earth itself is only 4.5 billion years old so life has been present here for at least 80 per cent of its history.

WHERE DID LIFE BEGIN?

Although the when part of the origin question can at least be bounded, it is much harder to guess where life first appeared. I don’t mean the latitude and longitude as such but the geological and chemical setting.

The first thing to say is that there is no compelling evidence that terrestrial life started on Earth. It may have got going elsewhere and come to Earth ready-made. For example, it may have begun on Mars, which before about 3.5 billion years ago was warmer and wetter than today, and more Earth-like. In some respects, Mars offered a more favourable environment for pre-biotic chemistry. For example, the effects of the asteroid bombardment may have been less severe and the chemical make-up of the red planet was better for driving metabolism. Obviously, there would have to be a way for life to spread from Mars to Earth, and there is. The bombardment by asteroids and comets, which was severe in the early history of the solar system (but has never entirely ceased), is capable of blasting vast amounts of rock into space, much of which goes into solar orbit. A fraction of ejected Mars rocks will eventually fall to Earth (and vice versa – terrestrial rocks go to Mars). Rocks from Mars, which fall as meteorites, have been collected from all over the world; my university has several. Over the history of our planet, trillions of tons of Martian material have come here. Ensconced in a chunk of rock, a microbe could withstand the harsh conditions of outer space. The greatest hazard in crossing the interplanetary void is radiation, but even a moderately sized rock would screen most of that out. It has been estimated that some hardy radiation-resilient microbes could survive for millions of years inside space rocks, easily long enough to reach Earth and seed it with Martian life. The same scenario works in reverse: viable terrestrial microbes can reach Mars. What this means is that Earth and Mars are not quarantined from each other. Cross-contamination by microbial life could have been going on throughout history. This makes it hard to be sure that life on Earth began here and not there. It is possible, but less likely, that life reached Earth from Venus, which is now very hostile to life but may have been more congenial billions of years ago. Another possibility, taken seriously in some quarters, is that life was originally incubated in a comet and delivered to Earth either by a direct impact or, more probably, from cometary dust that filtered down after a near-miss.

Shifting the cradle of life from Earth to somewhere else doesn’t much advance the more important question of what geological setting would be conducive to producing life. Many scenarios have been touted: deep ocean volcanic vents, drying lagoons, pores in sub-ocean rocks … the list is long. About the only thing everyone agrees on is that oxygen gas would have been a frustrating factor. Today, complex organisms require oxygen for their metabolism, but this was a late development. On Earth, there was very little free oxygen in the atmosphere before about 2 billion years ago, and present levels were not attained until within the last billion years. Oxygen may feel good to breathe, but it is a highly reactive substance that attacks and breaks down organic molecules. Aerobic life has evolved all sorts of mechanisms to cope with it (such as anti-oxidants). Even so, reactive oxygenic molecules regularly damage DNA and cause cancer. When it comes to the origin of life, free oxygen is a menace.

The elements essential to life do include oxygen, of course, but also hydrogen, nitrogen, carbon, phosphorus and sulphur. The truly essential element is carbon, the basis of all organic chemistry, and an ideal choice because of the limitless variety of complex molecules it can form. Chemists envisage the first steps towards life to have taken place where there was a good supply of carbon (for example, from carbon dioxide) and also hydrogen, perhaps free, or as a constituent of methane or hydrogen sulphide. A popular suggestion of locale is in the vicinity of volcanic vents under the ocean, where sulphur is also in good supply and the rocky surfaces offer all sorts of possible catalysts. Scientists have focused on such places because of the discovery of rich ecosystems clustering near deep subsea vents, perilously close to the scalding high-pressure effluent spewing from the volcanic depths. The primary producers at the base of the food chain are heat-loving microbes known as ‘hyperthermophiles’; some of these dare-devil organisms have been found thriving in water above 120oC. (The water doesn’t boil at these temperatures because of the intense pressure.) Nobody expected to find life in the dark depths, and certainly not in the pressure-cooker conditions near volcanoes. But the surprise didn’t end there. One of the most astonishing discoveries in biology in recent decades is that life is not restricted to the Earth’s surface or the oceans but extends deep underground, both on land and beneath the sea bed. The full extent of this subterranean biosphere is still being mapped, but microbes have been found living several kilometres down, inside rock (the South African extremophiles I mentioned in Chapter 2 are one such example).fn1

There has been a lively debate about whether life started deep inside the Earth’s crust or whether it infiltrated the subsurface after first establishing itself above (or having arrived from Mars perhaps). Genetic sequencing has shown that hyperthermophiles occupy the deepest and, by implication, oldest branches on the tree of life, suggesting that heat resilience is a very ancient feature of terrestrial biology, but that does not necessarily mean the first living things were hyperthermophiles. Life may have started somewhere cooler and then diversified, with some microbes evolving the necessary heat-damage-repair mechanisms to enable them to colonize the hot subsurface or the sea bed near ocean vents. Because the early bombardment probably included impacts by objects big enough to heat-sterilize large areas of the surface (if not the whole planet), then only the heat-loving subterranean microbes would have survived. They would thus represent a genetic bottleneck rather than representatives of the very first life forms. At this stage, it’s impossible to know.

Armed with a basic notion of the chemical setting (no oxygen!), scientists have spent decades trying to re-create conditions in the laboratory that might illuminate the first chemical steps on the long pathway to life, following Miller’s pioneering efforts in 1953. Many subsequent pre-biotic synthesis experiments have been done but, to be honest, they don’t get very far, in spite of the dedication and ingenuity of the scientists. By the standards of biological molecular complexity, these attempts barely make it to first base.

There is a more fundamental reason why efforts to cook up life in the lab are unlikely to solve the mystery of life’s origin. As I have stressed in this book, the distinctive character of life is its ability to store and process information in an organized manner. Of course, life also requires complex chemistry; organic molecules form the substrate in which life performs its software feats. But it’s only half the story – the hardware half. Obviously, there was a chemical pathway from non-life to life, even if we have scant idea what it was, but the actual chemical steps may not have been as important as the really critical transition: the one from inchoate molecular mayhem to organized information management. How did that happen?

HOW DID LIFE BEGIN?

I’ve left the hardest problem – how life began – to last. The short answer is, nobody knows how life began! It’s worse: nobody even knows how to go about estimating the odds for it to happen. But a lot hinges on the answer. If life starts easily, the universe should be teeming with it. Furthermore, if terrestrial life is the product of a universe that embeds some form of life principle in its basic laws, then the place of human beings in the great cosmic scheme would be profoundly different than if we were the products of a freak chemical accident.

As I have mentioned, a basic unknown about the pathway from non-life to life is whether it was a long, steady slog up a pre-biotic version of Mount Improbable, or whether it took place in fits and starts, with long periods of stasis interrupted by great leaps forward (or upward, in this metaphor). Given that Mount Improbable is so incredibly high, it won’t do for a chemical mixture to attain a toehold in the foothills only to slide back down again. There has to be some sort of ratcheting effect to lock in the gains and limit the losses while the system hangs out for the next step. But ideas like this, which seem sensible enough, run into the problem of teleology. A chemical soup doesn’t know it’s trying to make life – a chemical soup doesn’t know anything at all – so it won’t act to protect its hard-won complexity from the ravages of the second law of thermodynamics. Scenarios in which chemistry ‘strives’ towards life are patently absurd. The same problem doesn’t occur once life gets going, because natural selection can ratchet up the gains and DNA storage can lock them in. But chemistry without natural selection has no recourse to such mechanisms.fn2

The backsliding problem afflicts almost all studies of the complexification pathway to life. There are many clever experiments and theoretical analyses demonstrating the spontaneous formation of complexity in a chemical mixture, but they all hit the same issue: what happens next? How does a chemical broth build on some spontaneously emerging complexity to then ramp up to something even more complex? And on and on, until the summit of the pre-biotic Mount Improbable is reached? The most promising break-out from this straitjacket comes from work on ‘autocatalytic’ chemical cycles. The idea here is that certain molecules, say A and B, react to make other molecules, C, that happen to serve as catalysts to accelerate the production of A and B. There is thus a feedback loop: groups of molecules catalyse their own production. Scaling this up, there could be a vast network of organic molecules forming a quasi-stable system of autocatalysis, with many interlocking feedback loops, combining in a tangled web of reactions that is self-sustaining and robust.⁶ All this is easy to state in words, but are there such chemical systems out there? Yes, there are. They are called living organisms and they deploy all the aforementioned features. But now we are going round in circles, because we want to ascertain how all this marvellous chemistry can take place before life. We can’t put the solution by hand into the problem we are trying to solve and then claim to have solved it.

And the problem is more severe than I have stated. One of the informational hallmarks of life is the way it manages digital information using a mathematical code. Recall that triplets of letters (A, G, C, T) stand for specific amino acids from among the toolkit of twenty or so used to make proteins. The coded instructions transported from DNA to the protein assembly machinery (ribosomes, tRNA, and so on) are a prime example of Shannon’s information theory at work, with the instructions playing the role of the message, the communication channel being the watery innards of the cell and the noise being thermal or chemical mutational damage to the mRNA en route.

An explanation for the origin of life as we know it has to include an explanation for the origin of such digital information management and – especially – the origin of the code. (It doesn’t have to be the actual code known life uses, but the origin of some sort of code needs an explanation.) This is a tough, tough problem. Biochemists Eugene Koonin and Artem Novozhilov call it ‘the most formidable problem of all evolutionary biology’, a problem that ‘will remain vacuous if not combined with understanding of the origin of the coding principle itself and the translation system that embodies it’. They don’t think it will be solved any time soon:

Summarizing the state of the art in the study of the code evolution, we cannot escape considerable skepticism. It seems that the two-pronged fundamental question: ‘why is the genetic code the way it is and how did it come to be?’, that was asked over 50 years ago, at the dawn of molecular biology, might remain pertinent even in another 50 years. Our consolation is that we cannot think of a more fundamental problem in biology.⁷

It’s certainly correct that biologists have puzzled over the origin of the code for a long time. A popular proposed solution is that primitive life didn’t use a code, that what we have today represents a sort of software upgrade which evolved later once natural selection kicked in. The so-called RNA world theory has developed along these lines. Since it was discovered in 1982 that RNA can both store information and catalyse RNA chemical reactions (not as well as proteins, but maybe well enough to pass muster) biochemists have wondered whether an RNA soup could ‘discover’ replication with variation and selection all on its own, with proteins coming later. Even if this explanation is along the right lines, however, it is all but impossible to estimate the odds of such a scenario being played out on a planet. It’s easy to imagine those odds being exceedingly adverse.

Fifty years ago the prevailing view among biologists was that the origin of life was a chemical fluke, involving a sequence of events that collectively was so low in probability that it would be unlikely to recur anywhere else in the observable universe. I have already quoted Crick. His French contemporary Jacques Monod criticized the idea that life is somehow ‘waiting in the wings’ ready to burst forth whenever conditions permit. He summarized the prevailing view among scientists as follows: ‘the universe is not pregnant with life’, and therefore ‘Man knows at last that he is alone in the indifferent immensity of the universe, whence which he has emerged by chance.’⁸ George Simpson, one of the great neo-Darwinists of the postwar years, dismissed SETI, the search for intelligent life beyond Earth, as ‘a gamble at the most adverse odds with history’.⁹ Biologists such as Monod and Simpson based their pessimistic conclusions on the fact that the machinery of life is so stupendously complex in so many specific ways that it is inconceivable it would emerge more than once as a result of chance chemical reactions. In the 1960s to profess belief in extraterrestrial life of any sort, let alone intelligent life, was tantamount to scientific suicide. One might as well have expressed a belief in fairies. Yet by the 1990s sentiment had swung. The Nobel prizewinning biologist Christian de Duve, for example, described the universe as ‘a hotbed of life’. He was so convinced that life would emerge wherever it had a chance he called it ‘a cosmic imperative’.¹⁰ And that seems to be the fashionable view today, where appeal is often made to the huge number of habitable planets deemed to be out there. Consider, for example, the sentiments expressed by Mary Voytek, former head of NASA’s Astrobiology Institute: ‘With all the other planets around all the other stars, it’s impossible to imagine that life would not have arisen somewhere else.’¹¹ Well, it’s not only possible, it’s actually rather easy to imagine. Suppose, for example, the transition from non-life to life involved a sequence of a hundred chemical reactions, each requiring a particular temperature range (for example, 5–10°C for the first, 20–30°C for the second, and so on). Perhaps the transition also demanded tightly constrained pressure, salinity and acidity ranges, not to mention the presence of a host of catalysts. There might be only one planet in the observable universe where the necessary dream run of conditions occurred. My conclusion: habitability does not imply inhabited.

Why is it now scientifically respectable to search for life beyond Earth, whereas it was taboo even to talk about it half a century ago? There is no doubt that the discovery of so many extra-solar planets has provided astrobiology with a huge fillip. However, though no planets outside the solar system had been detected in the sixties, most astronomers nevertheless supposed they were there. A further point astrobiologists now make is the discovery of organic molecules in space, providing evidence that abundant ‘raw material’ for life is scattered throughout the universe. That may be so, but there is a vast complexity gulf separating simple building blocks such as amino acids from a metabolizing, replicating cell. The fact that the first small step across that chasm might have already been taken in space is almost irrelevant. Yet another reason given for the current optimism about life beyond Earth is the recognition that some types of organisms can survive in a much wider range of physical conditions than was recognized in the past, opening up the prospect for life on Mars, for example, and generally extending the definition of what constitutes an ‘Earth-like’ planet. But this at most amounts to a factor of two or three in favour of the odds for life. Set against that is the exponentially small probability that any given complex molecule will form by random assembly from a soup of building blocks. In my opinion, we remain almost completely in the dark about how life began, so attempts to estimate the odds of it happening are futile. You cannot determine the probability of an unknown process! We cannot put any level of confidence – none at all – on whether a search for life beyond Earth will prove successful.

There is one argument for the ubiquity of life that does carry some force. Carl Sagan once wrote, ‘The origin of life must be a highly probable affair; as soon as conditions permit, up it pops!’¹² It is true that life was here on Earth very soon (in geological terms) after our planet became congenial for it. Therefore, reasoned Sagan, it must start readily. Unfortunately, the conclusion doesn’t necessarily follow. Why? Well, had life not started quickly, there wouldn’t have been time for it to evolve as far as intelligence before Earth became uninhabitable, fried to a crisp by the steadily increasing heat of the sun. (In about 800 million years the sun will be so hot it will boil the oceans.) Put simply, unless life was quick off the mark, we wouldn’t be here today discussing it. So, given that our own existence on this planet depends on life forming here, it’s entirely possible that the origin of terrestrial life was an extreme outlier, an immense fluke.

MAKING LIFE IN THE LAB

Sometimes it is suggested that, if we could only make life in the laboratory, it would demonstrate clearly that it isn’t a fluke but can start up easily. Media reports often give the misleading impression that life has already been created in the lab, often with the moral subtext that ‘playing God’ in this manner might invite Frankenstein-like comeuppance. For example, on 20 May 2010 Britain’s Daily Telegraph featured a headline ‘Scientist Craig Venter creates life for first time in laboratory sparking debate about “playing God” ’. This is deeply misleading. The misunderstanding comes down to the ambiguous term ‘create’. In one sense, humans have been creating life for centuries, the most obvious example being dogs. Dogs are artificial animals produced from wolves by generations of cross-breeding and careful selection. Twenty thousand years ago there were wolves but no Great Danes or chihuahuas. In more recent years genetic engineering techniques such as gene transplantation have enabled many novel organisms to be created, including a variety of GM foods. New technology known as CRISPR enables genomes to be rewritten more or less to order. What Venter and his colleagues did was brilliant and deservedly attention-grabbing. He took a simple bacterium (mycoplasma genitalium) and replaced its DNA with a customized version. In other words, Venter kept almost all the hardware (the cell) and just switched the software (the DNA). The mycoplasma obligingly booted up the new software and ran the re-engineered genetic instructions; the new organism was dubbed mycoplasma laboratorium. The computer equivalent would be like buying a PC and reinstalling your own version of the operating system with a few designer embellishments added. Would that amount to creating a computer? Not really. Loose talk of creating life in the lab conflates chemistry with information, hardware with software. The main point is that re-engineering existing life, which is what Venter did, is very far indeed from making life from scratch.

Occasionally, there are media reports suggesting that even that more ambitious goal is close at hand. On 27 July 2011 The New York Times reported, beneath the dramatic headline ‘It’s alive! It’s alive! Maybe right here on Earth’, that ‘a handful of chemists and biologists … are using the tools of modern genetics to try to generate the Frankensteinian spark that will jump the gap separating the inanimate and the animate. The day is coming, they say, when chemicals in a test tube will come to life.’ The reporting is accurate enough. However, the definition of life being employed in the said experiments is extremely loose: a mixture of molecules that can make copies of themselves with occasional errors (mutations). In terms of chemistry, this work is without doubt an outstanding accomplishment and provides a helpful piece in the jigsaw puzzle of life. But, as the experimenters would be the first to admit, their molecular replication system is a far cry from a living cell with an autonomous existence.

The fundamental problem is not the simplicity of the components in these experiments. It is something far deeper. To attain even the modest successes announced so far requires special equipment and technicians, purified and refined substances, high-fidelity control over physical conditions – and a big budget. But above all, it needs an intelligent designer (aka a clever scientist). The organic chemist must have a preconceived notion of the entity to be manufactured. I’m not denigrating the scientists involved or the glittering promise of the field of synthetic biology, only its relevance to the natural origin of life. Astrobiologists want to know how life began without fancy equipment, purification procedures, environment-stabilizing systems and – most of all – without an intelligent designer. It may turn out that life is indeed easy to make in the lab but would still be exceedingly unlikely to happen spontaneously in the grubby and uncertain conditions available to Mother Nature. After all, organic chemists can readily make plastics, but we don’t find them occurring naturally. Even something as simple as a bow and arrow is straightforward for a child to make but would never be created by an inanimate process. So just because we might (one day) find life easy to create does not of itself demonstrate a cosmic imperative.

What would swing the debate is if, by synthesizing life many times and in many different ways, scientists uncovered certain common principles which could then be applied to real-world conditions. And that would open up the profound question of whether such principles already lurk within the corpus of scientific knowledge or require something entirely new. Schrödinger was open-minded on this matter: ‘We must therefore not be discouraged by the difficulty of interpreting life by the ordinary laws of physics. For that is just what is to be expected from the knowledge we have gained of the structure of living matter. We must also be prepared to find a new type of physical law prevailing in it,’ he wrote.¹³ I agree with Schrödinger. I believe there are new laws and principles that emerge in information-processing systems of sufficiently great complexity, and that a full explanation for life’s origin will come from a detailed study of such systems. I shall return to this speculative theme in the Epilogue.

Meanwhile, all is not hopeless on the observational front.

Box 12: Is life a planetary phenomenon?

Life as we know it has three fundamental features: genes, metabolism and cells. Clearly, they didn’t all spring into existence at once, and one of the challenges in origin-of-life research is to decide what came first. Among the three, cells are the easiest to form. There are many substances that spontaneously produce cellular structures, so an early speculation is that legions of small vesicles were available on the early Earth to serve as natural ‘test tubes’ in which nature might experiment with complex organic chemistry. Cells also fulfil another critical function. Darwinian evolution needs a unit to select on, and cells fit the bill. Even a non-living blob can reproduce after a fashion by fissioning into two smaller blobs, opening the way for a population of similar entities to serve an evolutionary role. Without the existence of individuals the original version of Darwinism is meaningless.

Recently, an opposing view has gained attention. Perhaps cells came later, after complex chemistry had already established something like metabolic cycles and networks. This chemical self-organization could occur in ‘the bulk’ – in the open oceans, say – on a large scale. Once the metabolic processes became self-sustaining and self-reinforcing, the way would have been open for fragmentation into individual units, culminating in what we would today recognize as living cells. It would be a top-down approach to the origin of life. The pre-cellular phase may have been restricted to thermodynamically favourable environments, such as deep ocean volcanic vents, or it may have encompassed the entire planet. Eric Smith and the late Harold Morowitz paint a picture of life as an essentially geological or planetary phenomenon, in which the geochemistry of the early Earth co-evolves with pre-life. Eventually, what we call life emerges, they conjecture, from a sort of planetary phase transition.¹⁴ It is an intriguing hypothesis.

A SHADOW BIOSPHERE

Suppose chance played only a subordinate role in incubating life and that the process was more ‘law-like’, more of an imperative, as de Duve expressed it. Is it possible that the blueprint for life is somehow embedded in the laws of physics and is thus an expected product of an intrinsically bio-friendly universe? Perhaps. The trouble is, these musings are philosophical, not scientific. What sort of law would imply that life arises more or less automatically wherever conditions permit? There is nothing in the laws of physics that singles out ‘life’ as a favoured state or destination. All the laws of physics and chemistry discovered so far are ‘life blind’ – they are universal laws that care nothing for biological states of matter, as opposed to non-biological states. If there is a ‘life principle’ at work in nature, then it has yet to be discovered.

For the sake of argument, let me join the ranks of the optimists who say that life starts easily and is widespread in the cosmos. If life is inevitable and common, how might we obtain evidence for it? If we found a second sample of life (on another planet, a moon, a comet) that we could be sure had arisen from scratch independently of known life, the case for de Duve’s cosmic imperative would be instantly and dramatically confirmed. In my view, the most promising place to search for a second genesis is right here on our own planet. If life does indeed get going easily, as so many scientists fervently believe, then surely it should have started many times on Earth. Well, how do we know it didn’t? Has anybody actually looked?

Consider this scenario: life emerges on planet Earth 4 billion years ago. Ten million years later a huge asteroid strikes, releasing so much heat that the oceans boil and the surface of the planet is sterilized. The massive blow would not, however, destroy all life. Vast quantities of rock would be spewed into space, some of it containing Earth’s first tiny inhabitants. The microbial cargo could survive for many millions of years, orbiting the sun. Eventually, some of this material would find its way back to Earth and fall as meteorites, bringing life home. But meanwhile, in the few million years since the cataclysmic impact, life has got going a second time (it starts easily, remember), so when the ejected material returns there are now two forms of life on our planet. Because the barrage of huge objects continued for 200 million years, this same scenario could have played out many times, so that when the bombardment finally abated there may have been dozens of independently formed organisms cohabiting our planet. The fascinating question is, might at least one of these examples of life-as-we-don’t-know-it have survived to the present day? Almost all life on Earth is microbial, and you can’t tell by looking what makes a microbe tick. You have to delve into its molecular innards. So might there be, intermingled with the microbes representing ‘our’ form of life, representatives of this ‘other’ life – it would be truly alien life, in the sense of being descended from an independent genesis. The existence of an alien microbial population has been dubbed a ‘shadow biosphere’, and it carries the intriguing possibility that there might be alien life right under our noses – or even in our noses – overlooked so far by microbiologists.¹⁵

Identifying shadow life would be a challenge. My colleagues and I have come up with some broad strategies, which I explained in The Eerie Silence. For example, we could search in places where conditions are so extreme they lie beyond the reach of all known life – even of the extremophile kind – such as near volcanic vents beneath the sea in regions of the effluent where the temperature exceeds 130oC. On the other hand, if shadow life is intermingled with known life, the task of identifying it would be harder. A chemical agent that killed or slowed the metabolism of all known life might enable a minority population of shadow-life microbes to flourish and so stand out. A few scientists have made a start along these lines but, considering the momentous consequences of such a discovery, it is surprising how little attention it has attracted. All it would take to settle this question is the discovery of a single microbe – just one – which represents life, but not as we know it. If we had in our hands (or rather under our microscopes) an organism whose biochemistry was sufficiently unlike our own that an independent genesis was unavoidable, the case for a fecund universe would be made. If life can happen twice, it can surely happen a zillion times. And that single alien microbe doesn’t have to be on some far-flung planet; it could be here on Earth. It could be discovered tomorrow, upending our vision of the cosmos and mankind’s place within it and greatly boosting the prospect that intelligent life may be out there somewhere.

Looking back over the past 3.5 billion years, the origin of life was the first, and most momentous, transformation. However, the history of evolution contains other major transitions, critical steps without which further advance would be impossible.¹⁶ It took a billion years or so after life began for the next major transition: the arrival of eukaryotes. Another big step was sex. Later came the leap from unicellularity to multicellularity. What prompted these further transformations to occur? Are there any common underlying features? Eukaryogenesis, sex and multicellularity: all involved marked physical alterations. But the true significance lay not with changes in form or complexity but with the concomitant reorganization of informational architecture. Each step represented a mammoth ‘software upgrade’. And the biggest upgrade of all began about 500 million years ago with the appearance of a primitive central nervous system. Fast-forward to today, and the human brain is the most complex information-processing system known. From that system stems what is undoubtedly the most astonishing phenomenon of all in life’s magic puzzle box – consciousness.