Lynn Margulis, who is largely responsible for promoting the idea now all but universally accepted that mitochondria and chloroplasts are symbiotic bacteria, has tried to do the same thing with cilia. Inspired by possible re-enactments such as we saw in the Mixotrich's Tale, she traces cilia back to spirochaete bacteria. Unfortunately, in view of the beauty and persuasiveness of the mixotrich parallel, the evidence that cilia (undulipodia) are symbiotic bacteria is found unpersuasive by almost everybody who was persuaded by Margulis's evidence in the case of mitochondria and chloroplasts.

Because the Great Historic Rendezvous is a true rendezvous in the forward historical direction, our pilgrimage, from now on, should strictly be a split pilgrimage. We should follow the separate backward pilgrimages of the various partic.i.p.ants in the eukaryotic compact until they are finally reunited in the deep past, but I think that would make for a gratuitously complicated journey. Both chloroplasts and mitochondria have their affinities with the eubacteria, not the other prokaryotic group, the Archaea. But our nuclear genes are slightly closer to Archaea, and the next rendezvous in our backwards story is with them.

1 There are two main processes by which energy is extracted from food fuel: anaerobic (without oxygen) and aerobic (with oxygen). Both are chemical sequences in which fuel, rather than being burned, is coaxed into trickling out its energy in a way that can be efficiently used. The most common anaerobic sequence yields pyruvate as its major product, and this is the starting point of the most common aerobic cascade. Termites go out of their way to deprive their guts of free oxygen, thereby forcing their microbes to use only the anaerobic process, using wood fuel to produce pyruvate that the termite can then use for aerobic energy release. There are two main processes by which energy is extracted from food fuel: anaerobic (without oxygen) and aerobic (with oxygen). Both are chemical sequences in which fuel, rather than being burned, is coaxed into trickling out its energy in a way that can be efficiently used. The most common anaerobic sequence yields pyruvate as its major product, and this is the starting point of the most common aerobic cascade. Termites go out of their way to deprive their guts of free oxygen, thereby forcing their microbes to use only the anaerobic process, using wood fuel to produce pyruvate that the termite can then use for aerobic energy release.

2 Bacteria (including Archaea) also have a monopoly (apart from lightning strikes and human industrial chemists) on nitrogen fixation. Bacteria (including Archaea) also have a monopoly (apart from lightning strikes and human industrial chemists) on nitrogen fixation.

Rendezvous 38 ARCHAEA.

After the uncertainty about what happened at Rendezvous 37 Rendezvous 37, and indeed about how many rendezvous are concealed behind that figleaf of a heading, it is a relief to return to a rendezvous about which most people now agree. All the eukaryotic pilgrims at least their nuclear genes are next joined by the archaeans, formerly called Archae-bacteria. Whether it is Rendezvous 38 Rendezvous 38, 39 39, 40 40 or or 41 41 might be up for grabs (or, rather, up for research in the next couple of years). But it is agreed that the prokaryotes or, as some would still call them, bacteria, are of two very different kinds the eubacteria and the archaeans. And the prevailing view is that the Archaea are closer cousins to us than they are to the Eubacteria, which is why I have placed the two rendezvous in the order I have. But it has to be remembered that, owing to the odd circ.u.mstances of the Great Historic Rendezvous, bits of our cells are closer to the eubacteria, even if our nuclei are closer to the archaeans. might be up for grabs (or, rather, up for research in the next couple of years). But it is agreed that the prokaryotes or, as some would still call them, bacteria, are of two very different kinds the eubacteria and the archaeans. And the prevailing view is that the Archaea are closer cousins to us than they are to the Eubacteria, which is why I have placed the two rendezvous in the order I have. But it has to be remembered that, owing to the odd circ.u.mstances of the Great Historic Rendezvous, bits of our cells are closer to the eubacteria, even if our nuclei are closer to the archaeans.

My Oxford colleague Tom Cavalier-Smith, whose view of the early evolution of life is informed by his great knowledge of microbial diversity, has coined the name Neomura to embrace both archaeans and eukaryotes but exclude the Eubacteria. He also uses 'bacteria' to embrace eubacteria and archaeans but not not eukaryotes. Bacteria is for him, therefore, a 'grade' name whereas Neomura is a clade. The clade to which the eubacteria belong is simply life, since it includes the Archaea and the eukaryotes too. eukaryotes. Bacteria is for him, therefore, a 'grade' name whereas Neomura is a clade. The clade to which the eubacteria belong is simply life, since it includes the Archaea and the eukaryotes too.

Cavalier-Smith believes that the Neomura arose only 850 million years ago, which is a more recent dating than I have dared to contemplate. He thinks the archaeans evolved their peculiar biochemical features within the bacteria as an adaptation to thermophily. Thermophily comes from the Greek for 'love of heat', which in practice usually means living in hot springs. He believes that these heat-loving bacteria 'thermophiles' then split into two. Some became hyperthermophiles (liking it very hot indeed) and gave rise to the modern Archaea. Others left the hot springs and, under cooler conditions, became the eukaryotes by absorbing other prokaryotes and making use of them, in the manner of the Mixotrich's Tale. If he is right, we know the conditions in which Rendezvous 38 Rendezvous 38 takes place: in a hot spring, or perhaps in a volcanic upwelling from the bottom of the sea. But of course he may not be right, and it has to be said that his view is far from the consensus. takes place: in a hot spring, or perhaps in a volcanic upwelling from the bottom of the sea. But of course he may not be right, and it has to be said that his view is far from the consensus.

[image]

Archaeans join. Most experts believe the archaeans are the sister group to the eukaryotes, on the basis of nuclear DNA, as well as certain details of biochemistry and cellular morphology. However, if mitochondrial DNA were used, the closest relatives would be the -proteobacteria because that is what mitochondria themselves once were (see the Great Historic Rendezvous). Archaeans are usually recognised as falling into two groups: the crenarchaeotes and the euryarchaeotes. DNA sequences from hot springs suggest one other, early-diverging branch, the korarchaeotes, but none have actually been seen. No species numbers are given: it isn't clear what 'species' means in as.e.xual organisms. Most experts believe the archaeans are the sister group to the eukaryotes, on the basis of nuclear DNA, as well as certain details of biochemistry and cellular morphology. However, if mitochondrial DNA were used, the closest relatives would be the -proteobacteria because that is what mitochondria themselves once were (see the Great Historic Rendezvous). Archaeans are usually recognised as falling into two groups: the crenarchaeotes and the euryarchaeotes. DNA sequences from hot springs suggest one other, early-diverging branch, the korarchaeotes, but none have actually been seen. No species numbers are given: it isn't clear what 'species' means in as.e.xual organisms.

Images, left to right: Desulfurococcus mobilis; Methanococcoides burtonii Desulfurococcus mobilis; Methanococcoides burtonii.

It was the great American microbiologist Carl Woese of the University of Illinois who discovered and defined the Archaea (then called Archaebacteria) in the late 1970s. The deep separation from other bacteria was controversial at first because it was so different from previous ideas. But it is now very widely accepted, and Woese has been justly honoured with prizes and medals, including the highly prestigious Crafoord Prize and the Leeuwenhoek Medal.

The Archaea include species that thrive in different kinds of extreme conditions, whether it is very high temperatures, or very acid, alkaline or salty water. The archaeans as a group seem to 'push the envelope' of what life can tolerate. n.o.body knows whether Concestor 38 was such an extremophile, but it is an intriguing possibility.

Rendezvous 39 EUBACTERIA.

When the pilgrimage began, our time machine ground away in bottom gear and we thought in terms of tens of thousands of years. We changed up through the gears, upgrading our imaginations to cope with millions, then hundreds of millions of years as we accelerated back to the Cambrian, picking up animal pilgrims along the way. But the Cambrian is alarmingly recent. For the great majority of its career on this planet life has been nothing but prokaryotic life. We animals are a recent afterthought. For the home stretch to Canterbury, our time machine has to go into hyperdrive to save the book from intolerable longueur longueur. With what may seem almost indecent haste, our pilgrims, now including the eukaryotes and the archaeans, speed backwards to what I am a.s.suming is one last rendezvous Rendezvous 39 Rendezvous 39 with the Eubacteria. But it might be more than one, and we might be closer to some eubacteria than others. Such uncertainty is why the tree on the opposite page is drawn unrooted. with the Eubacteria. But it might be more than one, and we might be closer to some eubacteria than others. Such uncertainty is why the tree on the opposite page is drawn unrooted.

Bacteria, as we have already seen and as Taq's Tale will agree, are supremely versatile chemists. They are also the only non-human creatures known to me who have developed that icon of human civilisation, the wheel. Rhizobium Rhizobium tells the tale. tells the tale.

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Eubacteria join. An unrooted tree (see the Gibbon's Tale), with crosses marking two tentative positions for the true root. The tip of each branch represents the present day. Traditionally, the eubacteria have been considered the sister group to the rest of life, equivalent to dangling the root from Concestor 39 (cross A). However, with no outgroups, there is no firm evidence to support this. Another possibility is that the root lies within the eubacteria (e.g. cross B), which would mean more rendezvous points. Within the eubacteria, there is considerable disagreement about phylogenetic relationships. The groups used here are generally accepted; their interrelationships are not. This particularly applies to the cyan.o.bacteria. An unrooted tree (see the Gibbon's Tale), with crosses marking two tentative positions for the true root. The tip of each branch represents the present day. Traditionally, the eubacteria have been considered the sister group to the rest of life, equivalent to dangling the root from Concestor 39 (cross A). However, with no outgroups, there is no firm evidence to support this. Another possibility is that the root lies within the eubacteria (e.g. cross B), which would mean more rendezvous points. Within the eubacteria, there is considerable disagreement about phylogenetic relationships. The groups used here are generally accepted; their interrelationships are not. This particularly applies to the cyan.o.bacteria.

Images, clockwise from top: Escherichia coli Escherichia coli 0111; 0111; Chlamydia Chlamydia sp.; sp.; Leptospira interrogans; Leptospira interrogans; chloroplast from unknown plant; chloroplast from unknown plant; Thermus aquaticus; Staphylococcus aureus Thermus aquaticus; Staphylococcus aureus.

THE RHIZOBIUM RHIZOBIUM'S TALE The wheel is the proverbial human invention. Take apart any machine of more than rudimentary complexity and you'll find wheels. Ship and aeroplane propellors, spinning drills, lathes, potters' wheels our technology runs on the wheel and would seize up without it. The wheel may have been invented in Mesopotamia during the fourth millennium BC BC. We know it was elusive enough to need need inventing because the New World civilisations still lacked it by the time of the Spanish conquest. The alleged exception there children's toys seems so bizarre as to prompt suspicion. Could it be one of those myths that spreads purely because it is so memorable, like the Inuit having 50 words for snow? inventing because the New World civilisations still lacked it by the time of the Spanish conquest. The alleged exception there children's toys seems so bizarre as to prompt suspicion. Could it be one of those myths that spreads purely because it is so memorable, like the Inuit having 50 words for snow?

Whenever humans have a good idea, zoologists have grown accustomed to finding it antic.i.p.ated in the animal kingdom. Examples pervade this book, including echo-ranging (bats), electrolocation (the Duckbill's Tale), the dam (the Beaver's Tale), the parabolic reflector (limpets), the infrared heat-seeking sensor (some snakes), the hypodermic syringe (wasps, snakes and scorpions), the harpoon (cnidarians) and jet propulsion (squids). Why not the wheel?

It is possible that the wheel impresses us only by contrast with our rather undistinguished legs. Before we had engines driven by fuels (fossilised solar energy), we were easily outpaced by animal legs. No wonder Richard III offered his kingdom for four-footed transportation out of his predicament. Perhaps most animals wouldn't benefit from wheels because they can already run so fast on legs. After all, until very recently, all our wheeled vehicles have been pulled by leg power. We developed the wheel not to go faster than a horse, but to enable a horse to transport us at its own pace or a bit less. To a horse, a wheel is something that slows you down.

Here's another way in which we risk overrating the wheel. It is dependent for maximum efficiency on a prior invention the road (or other smooth, hard surface). A car's powerful engine enables it to beat a horse or a dog or a cheetah on a hard, flat road. But run the race over wild country or ploughed fields, perhaps with hedges or ditches in the way, and it is a rout: the horse will leave the car wallowing.

Well then, perhaps we should change our question. Why haven't animals developed the road? There is no great technical difficulty. The road should be child's play compared with the beaver dam or the bower-bird's ornamented arena. There are even some digger wasps that tamp soil hard, picking up a stone tool to do so. Presumably the same skills could be used by larger animals to flatten a road.

But it raises an unexpected problem. Even if roadbuilding is technically feasible, it is a dangerously altruistic altruistic activity. If I as an individual build a good road from A to B, you may benefit from the road just as much as I do. Why should this matter? Because Darwinism is a selfish game. Building a road that might help others will be penalised by natural selection. A rival individual benefits from my road just as much as I do, but he does not pay the cost of building. Freeloaders, who use my road and don't bother to build their own, will be free to concentrate their energy on out-reproducing me, while I slave away on the road. Unless special measures are taken, genetic tendencies towards lazy, selfish exploitation will thrive at the expense of industrious roadbuilding. The upshot will be that no roads get built. With the benefit of foresight, we can see that everybody will be worse off. But natural selection, unlike us humans with our big, recently evolved brains, has no foresight. activity. If I as an individual build a good road from A to B, you may benefit from the road just as much as I do. Why should this matter? Because Darwinism is a selfish game. Building a road that might help others will be penalised by natural selection. A rival individual benefits from my road just as much as I do, but he does not pay the cost of building. Freeloaders, who use my road and don't bother to build their own, will be free to concentrate their energy on out-reproducing me, while I slave away on the road. Unless special measures are taken, genetic tendencies towards lazy, selfish exploitation will thrive at the expense of industrious roadbuilding. The upshot will be that no roads get built. With the benefit of foresight, we can see that everybody will be worse off. But natural selection, unlike us humans with our big, recently evolved brains, has no foresight.

What is so special about humans that we have managed to overcome our antisocial instincts and build roads that we all share? Oh, there is so much. No other species comes remotely close to a welfare state, to an organisation that takes care of the old, that looks after the sick and the orphaned, that gives to charity. On the face of it these things present a challenge to Darwinism, but this is not the place to go into that. We have governments, police, taxation, public works to which we all subscribe whether we like it or not. The man who wrote, 'Sir, You are very kind, but I'd prefer not to join your Income Tax Scheme', heard back, we may be sure, from the Inland Revenue. Unfortunately, no other species has invented the tax. They have, however, invented the (virtual) fence. An individual can secure his exclusive use of a resource if he actively defends it against rivals.

Many species of animals are territorial, not just birds and mammals, but fish and insects too. They defend an area against rivals of the same species, often so as to sequester a private feeding ground, or a private courtship bower or nesting area. An animal with a large territory might benefit by building a network of good, flat roads across the territory from which rivals were excluded. This is not impossible, but such animal roads would be too local for long-distance, high-speed travelling. Roads of any quality would be limited to the small area that an individual can defend against genetic rivals. Not an auspicious beginning for the evolution of the wheel.

But now, finally, we come to the teller of this tale. There is one revealing exception to my premise. Some very small creatures have have evolved the wheel in the fullest sense of the word. The wheel may even have been the first locomotor device ever evolved, given that for most of its first 2 billion years, life consisted of nothing but bacteria. Many bacteria, of which evolved the wheel in the fullest sense of the word. The wheel may even have been the first locomotor device ever evolved, given that for most of its first 2 billion years, life consisted of nothing but bacteria. Many bacteria, of which Rhizobium Rhizobium is typical, swim using thread-like spiral propellors, each driven by its own continuously rotating propellor shaft. It used to be thought that these 'flagella' were wagged like tails, the appearance of spiral rotation resulting from a wave of motion pa.s.sing along the length of the flagellum, as in a wriggling snake. The truth is much more remarkable. The bacterial flagellum is typical, swim using thread-like spiral propellors, each driven by its own continuously rotating propellor shaft. It used to be thought that these 'flagella' were wagged like tails, the appearance of spiral rotation resulting from a wave of motion pa.s.sing along the length of the flagellum, as in a wriggling snake. The truth is much more remarkable. The bacterial flagellum1 is attached to a shaft that rotates freely and indefinitely in a hole that runs through the cell wall. This is a true axle, a freely rotating hub. It is driven by a tiny molecular motor which uses the same biophysical principles as a muscle. But a muscle is a reciprocating engine, which, after contracting, has to lengthen again to prepare for a new power stroke. The bacterial motor just keeps on going in the same direction: a molecular turbine. is attached to a shaft that rotates freely and indefinitely in a hole that runs through the cell wall. This is a true axle, a freely rotating hub. It is driven by a tiny molecular motor which uses the same biophysical principles as a muscle. But a muscle is a reciprocating engine, which, after contracting, has to lengthen again to prepare for a new power stroke. The bacterial motor just keeps on going in the same direction: a molecular turbine.

The fact that only very small creatures have evolved the wheel suggests what may be the most plausible reason why larger creatures have not. It's a rather mundane, practical reason, but nonetheless important. A large creature would need big wheels which, unlike man-made ones, would have to grow in situ in situ rather than being separately fashioned out of dead materials and then mounted. For a large, living organ, growth rather than being separately fashioned out of dead materials and then mounted. For a large, living organ, growth in situ in situ demands blood or something equivalent, and probably something equivalent to nerves too. The problem of supplying a freely rotating organ with blood vessels (not to mention nerves) that don't tie themselves in knots is too vivid to need spelling out. There might be a solution, but we need not be surprised that it has not been found. demands blood or something equivalent, and probably something equivalent to nerves too. The problem of supplying a freely rotating organ with blood vessels (not to mention nerves) that don't tie themselves in knots is too vivid to need spelling out. There might be a solution, but we need not be surprised that it has not been found.

Human engineers might suggest running concentric ducts to carry blood through the middle of the axle into the middle of the wheel. But what would the evolutionary intermediates have looked like? Evolutionary improvement is like climbing a mountain. You can't jump from the bottom of a cliff to the top in a single leap. Sudden, precipitous change is an option for engineers, but in nature the summit of the evolutionary mountain can be reached only via a gradual ramp upwards from the starting point. The wheel may be one of those cases where the engineering solution can be seen in plain view, yet be unattainable in evolution because it lies on the other side of a deep valley: unevolvable by large animals but within the reach of bacteria because of their small size.

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A true axle, a freely rotating hub ... driven by a tiny molecular motor.

In an imaginative piece of lateral thinking, Philip Pullman, in his epic of childhood fiction His Dark Materials His Dark Materials, solves the problem for big animals in a completely unexpected but very biological way. He invents a species of benevolent, trunked animal, the mulefa, who have evolved symbiotically with a species of gigantic tree that sheds hard, circular, wheel-like seed pods. The feet of the mulefa have a h.o.r.n.y, polished spur which fits into a hole in the centre of a seed pod, which then works as a wheel. The trees gain from the arrangement because whenever as eventually must happen a wheel wears out and has to be discarded, the mulefa disperse the seeds inside. The trees have evolved to return the favour by making the pods perfectly circular, with a suitable hole for the mulefan axle right in the centre, into which they secrete a high-grade lubricating oil. The mulefa's four legs are placed in a diamond pattern. The fore and aft legs are in the midline, and they are the ones that slot into the wheels. The other two legs, halfway along the body and to the sides, have no wheels and are used to punt the animal along like an old-fashioned boneshaker bicycle without pedals. Pullman cleverly notes that the whole system is made possible only by a geological peculiarity of the world on which these creatures live. Basalt happens to form in long, ribbon-like lines over the savannah, which serve as unmade but hard roads.

Short of Pullman's ingenious symbiosis, we may provisionally accept the wheel as one of those inventions that, even if it were a good idea in the first place, cannot evolve in large animals: either because of the prior need for a road, or because the problem of the twisted blood vessels could never be solved, or because the intermediates to a final solution would never be good for anything. Bacteria were able to evolve the wheel because the world of the very small is so very different and presents such different technical problems.

As it happens, the bacterial flagellar motor itself has recently, in the hands of a species of creationists who call themselves 'Intelligent Design Theorists', been elevated to the status of icon of alleged unevolvability. Since it manifestly exists, the conclusion of their argument is different. Whereas I proposed unevolvability as an explanation for why large animals like mammals don't grow wheels, creationists have seized upon the bacterial flagellar wheel as something that cannot exist and yet does so it must have come about by supernatural means!

This is the ancient 'Argument from Design', also called the 'Argument from Paley's Watchmaker', or the 'Argument from Irreducible Complexity'. I have less kindly called it the 'Argument from Personal Incredulity' because it always has the form: 'I personally cannot imagine a natural sequence of events whereby X could have come about. Therefore it must have come about by supernatural means.' Time and again scientists have retorted that if you make this argument, it says less about nature than about the poverty of your imagination. The 'Argument from Personal Incredulity' would lead us to invoke the supernatural every time we see a good conjuror whose tricks we cannot fathom.

It is perfectly legitimate to propose the argument from irreducible complexity as a possible explanation for the lack of something that doesn't exist, as I did for the absence of wheeled mammals. That is very different from evading the scientist's responsibility to explain something that does does exist, such as wheeled bacteria. Nevertheless, to be fair, it is possible to imagine validly using some version of the argument from design, or the argument from irreducible complexity. Future visitors from outer s.p.a.ce, who mount archaeological digs of our planet, will surely find ways to distinguish designed machines such as planes and microphones, from evolved machines such as bat wings and ears. It is an interesting exercise to think about how they will make the distinction. They may face some tricky judgements in the messy overlap between natural evolution and human design. If the alien scientists can study living specimens, not just archaeological relics, what will they make of fragile, highly strung racehorses and greyhounds, of snuffling bulldogs who can scarcely breathe and can't be born without Caesarian a.s.sistance, of blear-eyed Pekinese baby surrogates, of walking udders such as Friesian cows, walking rashers such as Landrace pigs, or walking woolly jumpers such as Merino sheep? Molecular machines nanotechnology crafted for human benefit on the same scale as the bacterial flagellar motor, may pose the alien scientists even harder problems. exist, such as wheeled bacteria. Nevertheless, to be fair, it is possible to imagine validly using some version of the argument from design, or the argument from irreducible complexity. Future visitors from outer s.p.a.ce, who mount archaeological digs of our planet, will surely find ways to distinguish designed machines such as planes and microphones, from evolved machines such as bat wings and ears. It is an interesting exercise to think about how they will make the distinction. They may face some tricky judgements in the messy overlap between natural evolution and human design. If the alien scientists can study living specimens, not just archaeological relics, what will they make of fragile, highly strung racehorses and greyhounds, of snuffling bulldogs who can scarcely breathe and can't be born without Caesarian a.s.sistance, of blear-eyed Pekinese baby surrogates, of walking udders such as Friesian cows, walking rashers such as Landrace pigs, or walking woolly jumpers such as Merino sheep? Molecular machines nanotechnology crafted for human benefit on the same scale as the bacterial flagellar motor, may pose the alien scientists even harder problems.

Francis Crick, no less, has speculated semi-seriously in Life Itself Life Itself that bacteria might not have originated on this planet but been seeded from elsewhere. In Crick's fantasy, they were sent in the nose-cone of a rocket by alien beings, who wanted to propagate their form of life, but shrank from the technically harder problem of transporting themselves and relied, instead, upon natural evolution to finish the job once the bacterial infection had taken root. Crick, and his colleague Leslie Orgel, who originally suggested the idea with him, supposed that the bacteria had originally evolved by natural processes on the home planet, but they could equally, while in the mood for science fiction, have added a touch of nanotechnological artifice to the mix, perhaps a molecular gearwheel like the flagellar motor which we see in that bacteria might not have originated on this planet but been seeded from elsewhere. In Crick's fantasy, they were sent in the nose-cone of a rocket by alien beings, who wanted to propagate their form of life, but shrank from the technically harder problem of transporting themselves and relied, instead, upon natural evolution to finish the job once the bacterial infection had taken root. Crick, and his colleague Leslie Orgel, who originally suggested the idea with him, supposed that the bacteria had originally evolved by natural processes on the home planet, but they could equally, while in the mood for science fiction, have added a touch of nanotechnological artifice to the mix, perhaps a molecular gearwheel like the flagellar motor which we see in Rhizobium Rhizobium and many other bacteria. and many other bacteria.

Crick himself whether with regret or relief it is hard to say finds little good evidence to support his own theory of Directed Panspermia. But the hinterland between science and science fiction const.i.tutes a useful mental gymnasium in which to wrestle with a genuinely important question. Given that the illusion of design conjured by Darwinian natural selection is so breathtakingly powerful, how do we, in practice, distinguish its products from deliberately designed artefacts? Another great molecular biologist, Jacques Monod, began his Chance and Necessity Chance and Necessity in similar terms. Could there be genuinely persuasive examples of irreducible complexity in nature: complex organisation made of many parts, the loss of any one of which would be fatal to the whole? If so, might this suggest genuine design by a superior intelligence, say from an older and more highly evolved civilisation on another planet? in similar terms. Could there be genuinely persuasive examples of irreducible complexity in nature: complex organisation made of many parts, the loss of any one of which would be fatal to the whole? If so, might this suggest genuine design by a superior intelligence, say from an older and more highly evolved civilisation on another planet?

It is possible that an example of such a thing might eventually be discovered. But the bacterial flagellar motor, alas, is not it. Like so many previous allegations of irreducible complexity, from the eye on, the bacterial flagellum turns out to be eminently reducible. Kenneth Miller of Brown University deals with the whole question in a tour de force tour de force of clear exposition. As Miller shows, the allegation that the component parts of the flagellar motor have no other functions is simply false. As one example, many parasitic bacteria have a mechanism for injecting chemicals into host cells called the TTSS (Type Three Secretory System). The TTSS makes use of a subset of the very same proteins that are used in the flagellar motor. In this case they are used not for providing rotatory motion of a circular hub, but for making a circular hole in a host's cell wall. Miller summarises: of clear exposition. As Miller shows, the allegation that the component parts of the flagellar motor have no other functions is simply false. As one example, many parasitic bacteria have a mechanism for injecting chemicals into host cells called the TTSS (Type Three Secretory System). The TTSS makes use of a subset of the very same proteins that are used in the flagellar motor. In this case they are used not for providing rotatory motion of a circular hub, but for making a circular hole in a host's cell wall. Miller summarises: Stated directly, the TTSS does its dirty work using a handful of proteins from the base of the flagellum. From the evolutionary point of view, this relationship is hardly surprising. In fact, it's to be expected that the opportunism of evolutionary processes would mix and match proteins to produce new and novel functions. According to the doctrine of irreducible complexity, however, this should not be possible. If the flagellum is indeed irreducibly complex, then removing just one part, let alone 10 or 15, should render what remains 'by definition nonfunctional.' Yet the TTSS is indeed fully functional, even though it is missing most of the parts of the flagellum. The TTSS may be bad news for us, but for the bacteria that possess it, it is a truly valuable biochemical machine.The existence of the TTSS in a wide variety of bacteria demonstrates that a small portion of the 'irreducibly complex' flagellum can indeed carry out an important biological function. Since such a function is clearly favored by natural selection, the contention that the flagellum must be fully a.s.sembled before any of its component parts can be useful is obviously incorrect. What this means is that the argument for intelligent design of the flagellum has failed.

Miller's indignation at 'Intelligent Design Theory' receives a boost from an interesting source: his deep religious convictions, which are more fully articulated in Finding Darwin's G.o.d Finding Darwin's G.o.d. Miller's G.o.d (if not Darwin's) is the G.o.d revealed in or perhaps synonymous with the deep lawfulness of nature. The creationist quest to demonstrate G.o.d through the negative route of the Argument from Personal Incredulity turns out, as Miller shows, to a.s.sume that G.o.d capriciously violates violates his own laws. And this, to those like Miller of a thoughtfully religious disposition, is a cheap and demeaning sacrilege. his own laws. And this, to those like Miller of a thoughtfully religious disposition, is a cheap and demeaning sacrilege.

As a non-religious person I can sympathetically b.u.t.tress Miller's argument with a parallel one of my own. If not sacrilegious, the intelligent design style of argument from personal incredulity is lazy lazy. I have satirised it in an imagined conversation between Sir Andrew Huxley and Sir Alan Hodgkin, both sometime presidents of the Royal Society, who shared the n.o.bel Prize for working out the molecular biophysics of the nerve impulse.

'I say, Huxley, this is a terribly difficult problem. I can't see how the nerve impulse works, can you?''No, Hodgkin, I can't, and these differential equations are fiendishly hard to solve. Why don't we just give up and say that the nerve impulse propagates by nervous energy?''Excellent idea, Huxley, let's write the letter to Nature Nature now: it'll only take one line, then we can turn to something easier.' now: it'll only take one line, then we can turn to something easier.'

Andrew Huxley's elder brother Julian made a similar point when, long ago, he satirised vitalism, then usually epitomised by Henri Bergson's name of elan vital elan vital, as tantamount to explaining that a railway engine was propelled by elan locomotif elan locomotif.2 My censure of laziness, and Miller's of sacrilege, do not apply to the hypothesis of directed panspermia. Crick was talking about superhuman, not supernatural, design. The difference really matters. On Crick's world view, superhuman designers of bacteria, or of the means to seed Earth with them, would themselves have originally evolved by some local equivalent of Darwinian selection on their own planet. Crucially, Crick would always seek what Daniel Dennett calls a 'crane': would never resort as Henri Bergson would to a 'skyhook'. My censure of laziness, and Miller's of sacrilege, do not apply to the hypothesis of directed panspermia. Crick was talking about superhuman, not supernatural, design. The difference really matters. On Crick's world view, superhuman designers of bacteria, or of the means to seed Earth with them, would themselves have originally evolved by some local equivalent of Darwinian selection on their own planet. Crucially, Crick would always seek what Daniel Dennett calls a 'crane': would never resort as Henri Bergson would to a 'skyhook'.

The main objection to the irreducible complexity argument amounts to a demonstration that the allegedly irreducible complex ent.i.ty, the flagellar motor, the blood-clotting cascade, the Krebs cycle, or whatever it might be, is actually reducible. The personal incredulity was simply wrong. To this we add the reminder that, even if we can't yet yet think of a step-by-step pathway by which the complexity might have evolved, the eager slide to a.s.suming that it is therefore supernatural is either sacrilegious or lazy, according to taste. think of a step-by-step pathway by which the complexity might have evolved, the eager slide to a.s.suming that it is therefore supernatural is either sacrilegious or lazy, according to taste.

But there is another objection that needs to be mentioned: the 'arch and scaffolding' of Graham Cairns-Smith. Cairns-Smith was writing in a different context, but his point works here too. An arch is irreducible in the sense that if you remove part of it, the whole collapses. Yet it is possible to build it gradually by means of scaffolding. The subsequent removal of the scaffolding, so that it no longer appears in the visible picture, does not ent.i.tle us to a mystified and obscurantist attribution of supernatural powers to the masons.

The flagellar motor is common among bacteria. Rhizobium Rhizobium was chosen to tell the tale because of a second claim to impress us with the versatility of bacteria. Farmers sow plants of the pea family, Leguminosae, as a part of most good crop rotation schemes for one very good reason. Leguminous plants can use raw nitrogen straight out of the air (it is by far the most abundant gas in our atmosphere) rather than having to suck up nitrogen compounds from the soil. But it isn't the plants themselves that fix atmospheric nitrogen and turn it into usable compounds. It is symbiotic bacteria specifically was chosen to tell the tale because of a second claim to impress us with the versatility of bacteria. Farmers sow plants of the pea family, Leguminosae, as a part of most good crop rotation schemes for one very good reason. Leguminous plants can use raw nitrogen straight out of the air (it is by far the most abundant gas in our atmosphere) rather than having to suck up nitrogen compounds from the soil. But it isn't the plants themselves that fix atmospheric nitrogen and turn it into usable compounds. It is symbiotic bacteria specifically Rhizobium Rhizobium housed for the purpose in special nodules provided for them, with every indication of inadvertent solicitude, on the roots of the plants. housed for the purpose in special nodules provided for them, with every indication of inadvertent solicitude, on the roots of the plants.

Such contracting out of ingenious chemical tricks to chemically much more versatile bacteria is an extremely common pattern throughout animals and plants. It is the main message of Taq's Tale.

TAQ'S TALE Written with Yan Wong Having reached our most ancient rendezvous, having gathered into our pilgrimage all of life as we know it, we are in a position to survey its diversity. At the deepest level, the diversity of life is chemical. The trades plied by our fellow pilgrims span a range of skills in the arts of chemistry. And, as we have seen, it is the bacteria, including archaeans, who display the fullest spread of chemical skills. Bacteria taken as a group are the master chemists of this planet. Even the chemistry of our own cells is largely borrowed from bacterial guest workers, and it represents a fraction of what bacteria are capable of. Chemically, we are more similar to some bacteria than some bacteria are to other bacteria. At least as a chemist would see it, if you wiped out all life except bacteria, you'd still be left with the greater part of life's range.

The particular bacterium that I choose to tell the tale is Thermus aquaticus Thermus aquaticus, known fondly to molecular biologists as Taq. Different bacteria seem alien to us for different particular reasons. Thermus aquaticus Thermus aquaticus, as its name suggests, likes to be in hot water. Very Very hot water. As we saw at hot water. As we saw at Rendezvous 38 Rendezvous 38, many of the archaeans are thermophiles and hyperthermophiles, but the archaeans don't have a monopoly on this way of life. Thermophiles and hyperthermophiles are not taxonomic categories, but something more like trades or guilds, like Chaucer's Clerk, Miller and Physician. They make their living in places where nothing else can: the scalding-hot springs of Rotorua and Yellowstone Park, or the volcanic vents on mid-ocean ridges. Thermus Thermus is a eubacterial hyperthermophile. It can survive with little problem in near-boiling water although it prefers a more balmy 70C, or so. It doesn't quite hold the world temperature record there are deep-sea archaeans that thrive at up to 115C, well above the normal boiling point of water. is a eubacterial hyperthermophile. It can survive with little problem in near-boiling water although it prefers a more balmy 70C, or so. It doesn't quite hold the world temperature record there are deep-sea archaeans that thrive at up to 115C, well above the normal boiling point of water.3 Thermus is famous in molecular biology circles for being the source of the DNA duplication enzyme known as Taq polymerase. Of course, all organisms have enzymes to duplicate DNA, but is famous in molecular biology circles for being the source of the DNA duplication enzyme known as Taq polymerase. Of course, all organisms have enzymes to duplicate DNA, but Thermus Thermus has had to evolve one that can withstand near-boiling temperatures. This is useful for molecular biologists because the easiest way to ready DNA for duplication is to boil it, separating it into its two const.i.tuent strands. Repeated boiling and cooling of a solution containing both DNA and Taq polymerase duplicates or 'amplifies' even the most minute quant.i.ties of original DNA. The method is called the 'polymerase chain reaction', or PCR, and it is brilliantly clever. has had to evolve one that can withstand near-boiling temperatures. This is useful for molecular biologists because the easiest way to ready DNA for duplication is to boil it, separating it into its two const.i.tuent strands. Repeated boiling and cooling of a solution containing both DNA and Taq polymerase duplicates or 'amplifies' even the most minute quant.i.ties of original DNA. The method is called the 'polymerase chain reaction', or PCR, and it is brilliantly clever.

Thermus's fame as a wizard of the biochemistry laboratory is justification enough to let it tell this tale. But, as it happens, there may be another reason that Thermus Thermus is particularly well placed to present the instructively alien perspective of bacteria. is particularly well placed to present the instructively alien perspective of bacteria. Thermus Thermus belongs to the small group of bacteria known as the Hadobacteria. In his taxonomic scheme mentioned at belongs to the small group of bacteria known as the Hadobacteria. In his taxonomic scheme mentioned at Rendezvous 39 Rendezvous 39, Tom Cavalier-Smith suggests that the Hadobacteria, together with their cousins the green non-sulphur bacteria, may be the earliest branching bacterial group. If so, their group is as distant a cousin of the rest of life as it is possible to be.

According to this view, Thermus Thermus and its relatives are out on a limb. All the rest of the bacteria share an ancestor with each other and with the rest of life, which and its relatives are out on a limb. All the rest of the bacteria share an ancestor with each other and with the rest of life, which Thermus Thermus doesn't share. If upheld, this means the following. Just as any bacterium might lump 'the rest of life' into one 'cadet branch' of the family of life, so, within the bacteria, doesn't share. If upheld, this means the following. Just as any bacterium might lump 'the rest of life' into one 'cadet branch' of the family of life, so, within the bacteria, Thermus Thermus can lump 'the rest of the bacteria' into one branch of the bacteria. This, together with its penchant for being boiled, was my reason for singling out can lump 'the rest of the bacteria' into one branch of the bacteria. This, together with its penchant for being boiled, was my reason for singling out Thermus Thermus to tell a tale of life's diversity. But whereas the evidence for the special status of to tell a tale of life's diversity. But whereas the evidence for the special status of Thermus Thermus is not particularly secure, there is no doubt that the great majority of life's diversity at the fundamental level of chemistry is microbial, and a substantial majority of it is bacterial. The tale of life's diversity, insofar as it is mostly chemical diversity, is rightfully told by a bacterium, and it might as well be Taq. is not particularly secure, there is no doubt that the great majority of life's diversity at the fundamental level of chemistry is microbial, and a substantial majority of it is bacterial. The tale of life's diversity, insofar as it is mostly chemical diversity, is rightfully told by a bacterium, and it might as well be Taq.

Traditionally, and understandably, the tale was told from the point of view of big animals us. Life was divided into the animal kingdom and the vegetable kingdom, and the difference seemed pretty clear. The fungi counted as plants because the more familiar of them are rooted to the spot and don't walk away while you try to study them. We didn't even know about bacteria until the nineteenth century, and when they were first seen through powerful microscopes people didn't know where to put them in the scheme of things. Some thought of them as miniature plants, others as miniature animals. Yet others put the light-trapping bacteria in the plants (as 'blue-green algae') and the rest in the animals. Much the same was done with the 'protists' single-celled eukaryotes that are not bacteria and are much larger than bacteria. The green ones were called Protophyta and the rest Protozoa. A familiar example of a protozoan is Amoeba Amoeba, once thought to be close to the grand ancestor of all life how wrong we were, for an Amoeba Amoeba is scarcely distinguishable from a human when viewed through the 'eyes' of bacteria. is scarcely distinguishable from a human when viewed through the 'eyes' of bacteria.

All that was in the days when living organisms were cla.s.sified by their visible anatomy, in which bacteria are much less diverse than animals or plants and it was pardonable to put them down as primitive animals and plants. It was another matter entirely when we began to cla.s.sify creatures using the much richer information provided by their molecules, and when we looked at the range of chemical 'trades' perfected by microbes. Here's approximately how things look today.

[image]

The deepest divisions of life. Tree of life, showing division into three major domains, based on recent molecular work. Adapted from Gribaldo and Philippe [ Tree of life, showing division into three major domains, based on recent molecular work. Adapted from Gribaldo and Philippe [113].

If animals and plants are treated as a pair of kingdoms, by the same standards there are dozens of microbial 'kingdoms', whose uniqueness ent.i.tles them to the same status as animals and plants. The diagram above shows the tip of the iceberg. Not only have some deep-rooted branches been omitted, but I've shown only those that live in accessible places and can be cultured in the laboratory. Indeed, simply trawling new locations for DNA and not bothering to inquire which organisms they come from can find entire new microbial kingdoms. The ever-resourceful Craig Venter and his team claim to have found at least 1,800 new species of microbes by a shotgun a.n.a.lysis of DNA floating around in the Sarga.s.so Sea. Animals, plants and fungi const.i.tute just three small branches of the tree of life. What distinguishes these three familiar kingdoms from the others is that the organisms in them are large, being built of many cells. The other kingdoms are almost entirely microbial. Why do we not unite them into one microbial kingdom, on a par with the three great multicellular kingdoms? One reason, and a sound one, is that, at the biochemical level, many of the microbial kingdoms are as different from each other, and from the big three, as the three familiar kingdoms are from each other.

It would be worthless to argue in detail whether there 'really' are, say, 20 kingdoms on this scale of difference, or 25 or 100. What is clear from the diagram is that these dozens fall into three main super-kingdoms 'domains' in the terminology of Carl Woese, already mentioned as the originator of this new view of life. The three domains are first our own, the eukaryotes, in whose company we have been travelling for most of our journey. Second, the Archaea the microbes we met at Rendezvous 38 Rendezvous 38 who, on the old view of life, would be lumped in with the third domain, the true (or Eu-) bacteria. It is the members of this third, eubacterial domain who have joined us for the last leg of our pilgrimage. It is a privilege to share these final steps with the most ubiquitous and efficient DNA-propagators that have ever existed. who, on the old view of life, would be lumped in with the third domain, the true (or Eu-) bacteria. It is the members of this third, eubacterial domain who have joined us for the last leg of our pilgrimage. It is a privilege to share these final steps with the most ubiquitous and efficient DNA-propagators that have ever existed.

The star diagram itself is not, of course, based upon the sorts of features we can see and touch. If you want to compare organisms, you have to choose features that they all approximately share. You can't compare legs if most species don't have legs. Legs, heads, leaves, collar bones, roots, hearts, mitochondria each is restricted to a subset of creatures. But DNA is universal, and there's a handful of particular genes that all living creatures share with each other, with only minor, countable differences. These are what we must use for large-scale comparison. Perhaps the best example is provided by the codes that go to make ribosomes.

Ribosomes are cellular machines that read RNA messages (themselves transcribed from DNA genes) and churn out proteins. Ribosomes are vital to all cells, and are universally present. They are themselves largely made of RNA called rRNA and completely separate from the RNA message 'tapes' that the ribosomes read and translate into protein. rRNA is itself originally specified by DNA genes. The sequence of rRNA may be read directly, or as the DNA genes that code it: rDNA. Either way, I shall call it rDNA. rDNA is particularly useful for direct comparison between any creature and any other because they all have it. rDNA is used not only because of its ubiquity. Just as important, it shows the right amount of genetic variation sufficiently similar between all living species that there is something to compare, yet not so extremely similar as to leave no differences to count. Using the methods of the Gibbon's Tale, we can use rDNA to piece together the whole tree of life, and work out the vast evolutionary distances within, and even between, the major domains. We must take care. rDNA is fully vulnerable to 'long branch attraction' and other such pitfalls. But with the a.s.sistance of other genes too, and the use of rare genomic changes insertions and deletions of large chunks of DNA a tentative tree can be drawn. That is what we have on page 569. Certainly, some branches in this tentative tree are uncertain, particularly within the Eubacteria, and this may reflect their tendency to exchange DNA between themselves a problem we have not met in any eukaryotes. Nevertheless, researchers have found a core group of bacterial genes that are rarely swapped, so it is conceivable that we may one day agree upon an unimpeachable order of branching within the tree of life. I look forward to it.

Taxonomic distance, as measured by comparing genomes, is one way of looking at diversity. Another is to look at the range of ways of life, the range of 'trades' that our pilgrims ply. At first sight, different bacteria might seem more similar in this regard than, say, a lion is to a buffalo, or a mole to a koala. To big animals like us, burrowing underground for worms seems very different, as a way of life, from chewing leaves up a gum tree. But from the chemical point of view of our bacterial storyteller, all moles, koalas, lions and buffalos are doing much the same thing. All are deriving their energy by breaking down complex molecules ultimately put together by energy from the sun captured by plants. Koalas and buffalos eat the plants directly. Lions and moles get their solar energy at one remove, by eating other animals that (ultimately) eat plants.

The primary source of outside energy is the sun. The sun, through symbiotic green bacteria inside plant cells, is the only begetter of energy for all the life we can see with the unaided eye. Its energy is trapped by green solar panels (leaves) and used to drive uphill the synthesis of organic compounds, such as sugar and starch in plants. In a series of energy-coupled downhill and uphill chemical reactions, the rest of life is then powered by the energy originally trapped from the sun by plants. Energy flows through the economy of life, from the sun to plants to herbivores to carnivores to scavengers. At every step of the way, not only between creatures but within them, every transaction in the energy economy is wasteful. Inevitably, some of it is dissipated as heat and never recovered. Without the ma.s.sive inflow of energy from the sun, life would, or so the textbooks used to say, grind to a halt.

That is still mostly true. But those textbooks reckoned without the bacteria and archaeans. If you are a sufficiently ingenious chemist, it is possible to dream up alternative schemes of energy flow on this planet, which do not start with the sun. And if a useful piece of chemistry can be dreamed up, the chances are that a bacterium got there first: maybe even before they discovered the solar energy trick, and that was more than 3 billion years ago. There has to be some kind of external source of energy, but it doesn't have to be the sun. There is chemical energy locked up in lots of substances, energy that can be released by the right chemical reactions. Sources economically worth mining by living creatures include hydrogen, hydrogen sulphide, and some iron compounds. We will revisit the mining way of life at Canterbury.

Although our tales are not, in the main, told in the first person, let us make an exception for the last word of all our tales, and give it to Thermus aquaticus: Thermus aquaticus: Look at life from our perspective, and you eukaryotes will soon cease giving yourselves such airs. You bipedal apes, you stump-tailed tree-shrews, you desiccated lobe-fins, you vertebrated worms, you Hoxedup sponges, you newcomers on the block, you eukaryotes, you barely distinguishable congregations of a monotonously narrow parish, you are little more than fancy froth on the surface of bacterial life. Why, the very cells that build you are themselves colonies of bacteria, replaying the same old tricks we bacteria discovered a billion years ago. We were here before you arrived, and we shall be here after you are gone.

1 The bacterial flagellum, as we have seen, is completely different in structure from the eukaryotic (or protozoan) flagellum or 'undulipodium' that we met in the Mixotrich's Tale. Unlike the eukaryotic arrangement of 9+2 microtubules, the bacterial flagellum is a hollow tube made of the protein flagellin. The bacterial flagellum, as we have seen, is completely different in structure from the eukaryotic (or protozoan) flagellum or 'undulipodium' that we met in the Mixotrich's Tale. Unlike the eukaryotic arrangement of 9+2 microtubules, the bacterial flagellum is a hollow tube made of the protein flagellin.

2 It is depressing to reflect that Henri Bergson a vitalist represents the nearest approach to a scientist in the entire list of 100 winners of the n.o.bel Prize for Literature. The nearest compet.i.tor is Bertrand Russell, but he won it for his humanitarian writings. It is depressing to reflect that Henri Bergson a vitalist represents the nearest approach to a scientist in the entire list of 100 winners of the n.o.bel Prize for Literature. The nearest compet.i.tor is Bertrand Russell, but he won it for his humanitarian writings.

3 Again, if it seems surprising that water can be found so far above its normal boiling point, remember that water boils hotter at high pressure. Again, if it seems surprising that water can be found so far above its normal boiling point, remember that water boils hotter at high pressure.

CANTERBURY.

As befits the destination of a 4-billion-year pilgrimage, our Canterbury has a patina of mystery. It is the singularity known as the origin of life, but we could better call it the origin of heredity. Life itself is not clearly defined, a fact that contradicts intuition and traditional wisdom. Ezekiel, Chapter 37, in which the prophet was commanded down into the valley of the bones, identifies life with breath. I cannot resist quoting the pa.s.sage ('bone to his bone' such wonderful economy of language).

So I prophesied as I was commanded: and as I prophesied, there was a noise, and behold a shaking, and the bones came together, bone to his bone.And when I beheld, lo, the sinews and the flesh came up upon them, and the skin covered them above: but there was no breath in them.Then said he unto me, Prophesy unto the wind, prophesy, son of man, and say to the wind, Thus saith the Lord G.o.d: Come from the four winds, O breath, and breathe upon these slain, that they may live.

And, of course, the winds did. A great army breathed and stood up. Breath, for Ezekiel, defines the difference between dead and alive. Darwin himself implied the same in one of his more eloquent pa.s.sages, the concluding words of The Origin of Species The Origin of Species (emphasis added): (emphasis added): Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved. by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

Darwin rightly reversed Ezekiel's order of events. The breath of life came first and created the conditions under which bones and sinews, flesh and skin would eventually evolve. Incidentally, the phrase 'by the Creator' is not present in the first edition of the Origin Origin. It was added in the second edition, probably as a sop to the religious lobby. Darwin later regretted this in a letter to his friend Hooker: I have long since regretted that I truckled to public opinion and used the Pentateuchal term of creation, by which I really meant 'appeared' by some wholly unknown process. It is mere rubbish thinking at present of the origin of life; one might as well think of the origin of matter.

Darwin probably (and in my view rightly) saw the origin of primitive life as a relatively (and I stress relatively) easy problem compared with the one he solved: how life, once begun, developed its amazing diversity, complexity and powerful illusion of good design. Nevertheless, Darwin did later (in another letter to Hooker) venture a guess about the 'wholly unknown process' that started it all. He was led to it through wondering why we don't see life originating again and again.

It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. 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, &c., present, that a proteine compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly absorbed, which would not have been the case before living creatures were found.

The doctrine of spontaneous generation had only lately been experimentally attacked by Pasteur. It had long been believed that rotting meat spontaneously generated maggots, that goose barnacles spontaneously generated goslings and even that dirty laundry placed with wheat generated mice. Perversely, the spontaneous generation theory was supported by the Church (following Aristotle in this as in so much else). I say perversely because, at least with hindsight, spontaneous generation was as direct a challenge to divine creation as evolution would ever be. The idea that flies or mice could spring spontaneously into existence hugely underestimates the stupendous achievement that the creation of flies or mice would be: an insult to the Creator, one might have thought. But the science-free mindset fails to grasp how complex and inherently improbable a fly or a mouse is. Darwin was perhaps the first to appreciate the full magnitude of this mistake.

As late as 1872 in a letter to Wallace, the co-discoverer of natural selection, Darwin could still find it necessary to express his scepticism about 'Rotifers and Tardigrades being spontaneously generated', as had been suggested in a book, The Beginnings of Life The Beginnings of Life, which he otherwise admired. His scepticism was on target as usual. Rotifers and tardigrades are complex life forms beautifully fitted to their respective ways of life. For them to be spontaneously generated would imply that they became fit and complex 'by a happy accident, and this I cannot believe'. Happy accidents of such magnitude were anathema to Darwin, as they should have been to the Church for a different reason. The whole rationale of Darwin's theory was, and is, that adaptive complexity comes about by slow and gradual degrees, step by step, no single step making too large a demand on blind chance as explanation. The Darwinian theory, by rationing chance to the small steps needed to supply variation for selection, provides the only realistic escape escape from sheer luck as the explanation of life. If rotifers could spring into existence just like that, Darwin's life-work was unnecessary. from sheer luck as the explanation of life. If rotifers could spring into existence just like that, Darwin's life-work was unnecessary.

But natural selection itself had to have a beginning. In this sense alone, some kind of spontaneous generation must have happened, if only once. The beauty of Darwin's contribution was that the single spontaneous generation we must postulate did not have to synthesise anything complicated like a maggot or a mouse. It only had to make ... well, now we approach the heart of the problem. If not breath, what was the vital ingredient that first enabled natural selection to get going and lead eventually, after epics of c.u.mulative ev