Tuning in on Darwin

The Wizard in Our Genes?

The key word is flexibility, the ability to adapt constantly. Darwin said it clearly. People thought that he mainly talked about survival of the fittest. What he said was that the species that survive are usually not the smartest or the strongest, but the ones most responsive to change.

- Philippe Kahn

Evolution Rising from the Grave

Reactivation of a dormant message signals the dawn of a new humanity

Darwin's Radio is a book by Greg Bear published by Del Ray 1999
reviewed by Michael A Goldman

It is the dawn of the third millennium. It may be the dawn of a new humanity. It is the greatest scare since the bubonic plague. It manifests itself as cold-like symptoms in pregnant women, and as devastating malformations in their fœtuses. It shakes the very sequence of the human genome. A cataclysm of biblical proportions in the making, it is dubbed Herod's influenza. A hitherto-silent human endogenous retrovirus has excised itself from the genome and become mobile and infectious once again.

SHEVA - scattered human endogenous retrovirus activation - has been seen in the past. In mass graves, where men and pregnant women had been slaughtered, their remains tested positive for SHEVA. It was also present in a well-preserved Neanderthal couple found in an ice cave - the woman's unborn child had met a violent end. The dread disease spreading across the face of the Earth today had broken out æons before, at the dawn of Homo sapiens sapiens. Renegade scientists would show that SHEVA was not the awful disease Herod's influenza implied. It was, rather, Darwin's radio, conveying the message that would catapult mankind to its next step in evolution.

I am not a reader of mainstream science fiction, but for years I have told incredulous students, colleagues and friends that good, and even bad, science fiction can spark public interest in and understanding of science and its implications for our society. Jurassic Park has taught a generation about the possibilities and impossibilities of genetic engineering. Gattaca makes us think about where we're going with human genetic screening and genetic enhancement. But Greg Bear's novel Darwin's Radio goes one step further. It not only gets the non-scientific reader to think about details of molecular biology such as retroviruses, it challenges busy scientists to think freely about what the disjointed discoveries of the past few decades might really mean. And it reminds us just how closed to new ideas we can sometimes be. Bear restates the idea that science moves forward when orthodoxy is challenged. But the orthodoxy challenged must be last week's dogma; tampering with today's can be hazardous to one's career. This the scientists in Darwin's Radio learn. The pioneering thinkers of yesterday are the devoted traditionalists of today. New ideas enter science grudgingly. New paradigms are resisted with a vengeance.

In writing Darwin's Radio, Bear treads on the toes of the scientific elite: "Ernst Mayr's kids are sweating ice cubes ...Dawkins is beside himself." He has mustered a cadre of facts, loosely connected and ill understood. There are little happenings at the periphery of Mendelian genetics, at the edge of neo-Darwinian theory, and what used to be beyond the realm of molecular biology. We know that bacteria contain extrachromosomal elements that transmit traits, such as drug resistance, between individuals and between species. We know that elements exist which insert in and excise from the genome, sometimes in response to environmental signals or stresses.

Such extraordinary magic isn't limited to single-celled organisms. Even before bacterial transposons, plasmids and phages were understood, Barbara McClintock had seen evidence for movable elements in maize, with all the prescience of Kaye Lang, Bear's heroine. We are faced with our growing knowledge of the human genome. It is littered with the footprints of ancient retrotransposons, ancient retroviral infections and shocking, widespread rearrangements. Stephen Jay Gould and others have advocated the notion that evolution occurs by punctuated equilibria. The fossil record shows bursts of speciation and extinction cutting across entire fauna and flora. Yet we seldom make the connection that vertical transmission of genetic information could occur within and among higher eukaryotic species, and that clusters of transposition events could occur in response to environmental signals. Bear the science fiction writer makes the connection for us.

Most of us believe that simple, incremental changes in allele frequencies, driven by the forces of genetic drift, mutation, recombination, migration and natural selection, are enough to explain evolution - from adaptation to speciation, to the origin of higher taxa. There is no compelling evidence to the contrary, but neither is there compelling evidence in favour of the idea; we simply haven't observed or catalogued the forces and changes that create new species. Bear fills this void in our understanding with the notion that radical changes in the genome, brought about by mobilisation of transposable elements such as human endogenous retroviruses, result in rapid change at the subspecies or species level.

I'm not afraid of this concept. Bear goes a little further in suggesting that such change can occur over about a generation, an idea that might be a little too radical at the moment. However, he does mention data suggesting that fruitflies can adapt to a new environment in just a few generations of selection. He further suggests that speciation (or subspeciation) can occur in response to environmental stimuli, such as stress. This doesn't seem so incredible in view of the way in which prophages become excised from bacterial chromosomes in response to stress. Hard scientific evidence tells us that the scenario is possible. Although not every step in evolution must be preceded by a rampant retroviral infection, the possibility that such a sudden shake-up of the genome can be the fuel for evolutionary change may soon become dogma. Richard Goldschmidt's idea of the "hopeful monster" seemed strange in his day, but monsters produced by homeotic mutations are now on the cover of every textbook in genetics and developmental biology. Only the "hopeful" is missing at the moment.

One of Bear's wilder speculations is about the rapidity and directedness of evolutionary change. Mobile elements "hopping around like bugs on a hot griddle" can surely produce genetic change, but the likelihood of beneficial genetic change would be nil. Bear concedes in his novel that there isn't a simple, direct path to a new species - terrifying malformations, straight from an episode of The X-Files, characterise the aborted fœtuses in the first three-quarters of the book.

But there is some basis for speculating that something other than a grossly random mess would result from a massive mobilisation of transposable elements. First, preferred sites of integration can occur, and might ensure that a majority of elements land where they will do little harm. There is evidence that some repetitive-sequence elements in the human genome harbour specific regulatory elements, like retinoic-acid response elements, which respond to developmental signals. Thus, it is possible that simple insertion of a retroviral element in the vicinity of a gene could result in an alteration in the timing or positioning of its expression in early development. The result could be a different, yet perfectly viable, organism.

As an established science fiction writer who takes pride in keeping ahead of scientific developments, Bear exploits the latest in nanotechnology in his grand scheme of evolution. It's hard for me to imagine Bear's "mighty Wizard in our genes", the consummate computer in our genomes, responding to input from the environment, carrying out crosstalk with other individuals and species, and playing back several different scenarios for ushering the human species to new levels of achievement. But computer scientists say that the next generation of data processors will depend not on incremental improvements in the silicon microchips of today, but on a quantum leap into the realm of molecules whose sequence of chemical bonds encodes information on a nanometric scale. If we can use the DNA molecule to carry out calculations in ways that were unimaginable just 10 years ago, is it impossible to believe that nature has used the same molecules to encode instructions about instructions we do not yet understand?

Darwin's Radio, no matter how preposterous or prophetic one thinks the science, is superb "hard" science fiction, speculating about the connections among well-known facts. It is not a textbook of conventional ideas. It is an entertaining, even riveting story, delivered poetically. It portrays scientists as real people, responding to the intense politics of the biomedical world, the funding imperative in public and private sector alike, and the terrifying challenge of a disease that threatens to decimate the human species. It takes a hard look at the challenges faced by a woman scientist with radical ideas, and the excitement of discovering a totally new way of looking at biological evolution. Whether you read it to pass a cold, snowy night by the fire, or to free your mind for the new paradigms that will emerge in the next millennium, I promise you an engaging journey.

Michael A Goldman is in the Department of Biology, San Francisco State University, San Francisco, California 94132-1722, USA

Source: Nature Vol 404 2 March 2000

Fractal Awareness

I prefer non-fiction books because life provides only a limited time for reading. I've found non-fiction usually gives me a more lasting tangible benefit. However, I made an exception for this book after reading the above review. Though I found the writing a bit melodramatic, the science was good and the philosophy I couldn't fault because it rather closely matched my own.

Personally, I think of cells as aware - sentient, perhaps. I think they have the equivalent of culture, rules, purpose. I think I'm an emergent product - a by-product, even - of their complex interactions. Likewise I think lives and types of lives braid together to create meta-lives. I see nothing mystical about all this. I just feel there is no reason for life, and even complexity, not to be fractal, to regress and progress. I am a matrix and am in one.

I accept that the flow of humanity can bend me in a certain direction and change the course of my existence merely by virtue of the place and time in which I live. But if I'm not one of the "connections" life prunes early, I specialise, plug in, and do my "thing" - my life's work.

Some people collect power, others knowledge or money or whatever. Collectors become the framework for bigger patterns to emerge that involve more living things. It's all so impersonal.

We're quite small, really, staking out our path and trajectory.

I think we can choose what level to live on - at least on (rare?) occasions. But mostly, I see myself buffeted, grasping for whatever's in reach, and occasionally able to build an artful structure from unrelated bits. Novel connections - that, I see is the value. I do think instinct directs us more of the time than most of us would (care to) admit.

The Cell: Its Secret Life

Where it's all at

The building-blocks of the human body are an early fount of information technology.

In the 17th century, the newest plaything for the aspiring naturalist was a microscope. Robert Hooke, an English scientist, was an early user. One of the first objects he placed under its beady, magnifying lens was a thin slice of cork. Bottle-stoppers, he discovered, are light and spongy because they are full of holes. They are made of regular, repeating boxes. These reminded Hooke of the cloistered apartments in which the monks of the cork-oak's Mediterranean homeland lived out their solitary existences. Cells, that is.

Hooke had stumbled upon the first indication of one of the fundamental facts of biology: living creatures are made exclusively of cells and substances made by cells. People may like to think of themselves as individuals but it is just as accurate to describe them as clones. Everyone alive today is a clone of several thousand billion cells, each descended from a single fertilised egg and thus united by the closest biological bond of all-genetic identity. The cells in a body support one another, talk to one another, feed one another and even, occasionally, lay down their lives for one another. With the possible exception of brains, cells are the most complicated things around - more complicated in many ways than the bodies they are part of. Fewer than 300 different types of cell, each specialised for a particular task, labour on behalf of a human body. A cell, whose labour is divided mainly among its constituent protein molecules, requires 30 times that number of different sorts of protein to run the intricate, co-ordinated web of chemical reactions that is generally referred to as "life". And each protein needs an extensive supporting team of other chemicals if it is to function properly. All told, an average human cell has about 25,000 sorts of molecule floating around inside it, all but a handful of which it, or its ancestors, have manufactured. Indeed, during the period when the ability of cells to turn out such a variety of substances was first being worked out, it was a popular metaphor to describe them as factories. They take in raw materials and convert them into finished products.

Metaphors change, and in the age of the desk-top computer a new one seems more apt. To compare a cell's complexity with that of a brain is suggestive. Like a brain, a cell is a system for processing information. To the modern way of thinking its internal design is less reminiscent of a set of reaction vessels than of an array of switches. Not so much a factory, more the smallest microprocessor.

There are differences, of course. Microprocessors, like most man-made computers, can handle only one instruction at a time. Cells, again like brains, can handle lots. But the similarities are striking. The biochemical network within a cell is a machine for shuffling data as much as it is one for manipulating molecules. Every step it takes is dictated by chemical messages from within and without - from the genes in its DNA, from hormones interacting with its surface receptors and from its chemical "memory" (substances it has recently made to remind it what to do next). And instead of electrical energy to power this machine, there is more chemistry: adenosine tri-phosphate (ATP), a molecule that charges proteins with the pzazz they need to perform their chemical and informational transformations.

Divide and rule

The origin of this intricate piece of wetware is obscure. Cells come in two very different types. The great divide in biology is not between plants and animals, or even between microscopic unicellular blobs like amoebae and sophisticated multicelled creatures such as squirrels and sequoias. It is between bacteria and the rest. Bacteria are the oldest known form of life. They were certainly around 3.5 billion years ago, and the earth is only 4.5 billion years old. But old also means primitive. Bacteria have about as much in common with human-type cells as a pre-feudal city-state does with a modern industrial democracy. They are, in fact, little more than membrane-bound bags of molecules. Though they still function as chemical computers, they are back in the valve age in terms of sophistication. The molecules inside them slosh around haphazardly. Chemical interactions happen by chance. Even their DNA - the molecule that contains their ultimate instruction set - is tacked on almost as an afterthought to the outer membrane.

"Real" cells - the sort that make up human bodies - are, in contrast, highly organised. They have many more membranes than just the one that holds the outside world at bay. The space inside them is divided by these membranes into complicated structures known as organelles, which are specialised for different tasks. One of these organelles - the nucleus - contains the cell's DNA (of which it has several hundred times as much as the average bacterium). The nucleus is so prominent that 19th-century microscopists felt it was the defining feature of a "real" cell. After the fashion of the time, they invented a Greek word to give substance to this feeling: eukaryotic (good nucleus). Bacteria were relegated to the lowlier class of "prokaryotic" creatures.

Eukaryotic cells need their organelles be cause they are huge: about 1,000 times bigger than an average bacterium. A molecule drifting around inside a bacterium cannot travel very far. Sooner, rather than later, it is going to meet something suitable to react with. In a eukaryotic cell it could wander forever. By walling off compartments of bacterial dimensions a cell keeps control over its contents. And there is a bonus. Membranes are made of fatty molecules that spread out into thin sheets in the otherwise watery interior of the cell in a way reminiscent of a drop of oil on a puddle. Some proteins would rather float in these fatty layers than in the watery part - providing yet another way of controlling them.

Organelles are the things that give eukaryotic cells their edge over their bacterial forebears. It is therefore ironic that two of the main types of organelle are widely believed to be bacterial turn-coats.

Unite and conquer

Mitochondria and chloroplasts are the power-packs of cells - the sources of their ATP. Mitochondria, which are found in almost every cell, use sugar and oxygen to make this fuel. Chloroplasts, which are found only in plants, use sunlight. Mitochondria and chloroplasts have some important chemical differences from the rest of the cells they inhabit. They also reproduce independently, and they have their own DNA - not divided into elongated chromosomes like the DNA of a nucleus, but joined up in a circle in the bacterial fashion. The ancestors of each are reckoned to have been bacteria similar to particular species alive today (the curious chemistry is the clue that gives away which ones). These bacterial entrepreneurs must have made mutually profitable alliances with early, energy-starved eukaryotes - an indefinite supply of ATP in exchange for shelter from a hostile world. The details of how the bargains were struck are obscure, although it is clear that mitochondria made theirs first, and it seems probable that the link-up (which happened more than 1.7 billion years ago) was the event that permitted the rise of the eukaryotes. Viewed this way, mitochondria can be thought of as the most successful group of organisms on earth.

Chloroplasts, being confined to plants, are less numerous. But they are even more important for life as a whole because they provide most of its energy. Mitochondrial ATP is made by breaking sugar molecules down one step at a time in a chemical sequence known as the Krebs cycle, and using the power released by each step to form the critical, energy-rich chemical bond in an ATP molecule. Chloroplasts are able to form this bond with the energy from sunlight. A lot of their ATP is used to power the Calvin cycle - a sort of Krebs cycle in reverse that builds up sugar instead of destroying it. So, since the output of the Calvin cycle provides the building blocks for a plant's biochemical needs, it also provides for the needs of anything which eats that plant or eats the thing that ate the plant.

The colonisation of eukaryotes by mitochondria and chloroplasts, and the internal division of labour it allowed, was a portent of things to come. The first eukaryotic cells had to do everything by themselves, of course. An amoeba is both a cell and a complete organism. But the complexity of eukaryotic cells has permitted them to perform a trick that bacteria find extremely difficult - to come together in the mutually dependent colonies known today as animals, plants and fungi. To do this, they had to learn to specialise and communicate.

Less is more

Specialisation means two things: doing some things very well, and doing others not at all. It is controlled through the genes, which are embodied in the DNA of the nucleus. Each gene contains the information needed to make a particular molecule of a substance called RNA (similar to DNA but more flexible). Most RNA molecules, in turn, contain the information to make a protein; and a large group of proteins, known as enzymes, does most of the work of converting one sort of molecule in the biochemical network into another. No cell, not even the simplest bacterium, needs to make all of its available repertoire of RNA all of the time. If a particular type of sugar is not around, for example, there is not much point turning out an enzyme to digest it. The key differences about eukaryotic cells - those which allow them to specialise and co-operate - are that they can start or stop producing particular sorts of RNA in response to signals from other cells, and that they can stop producing particular sorts of RNA permanently - a permanence that is transmitted to their offspring.

One of the most familiar specialist cells is the red blood cell. It carries oxygen around the body. It can do nothing else. In the final stages of its development, all its biochemistry is devoted to making hæmoglobin - a protein with an especial liking for oxygen. When it has made enough, it ejects its nucleus, having no further use for it. This is extreme. A more moderate example of specialisation is a muscle cell. Unlike red blood cells, muscle cells keep their nuclei, their mitochondria and all their other organelles. But they overdose on two proteins called actin and myosin. Most cells contain some fibres of actin and myosin, fibres that can contract and relax. They hold a cell in shape, move things around inside it and can even move the whole cell around if necessary.

Muscle cells, though, contain so much actin and myosin that, in co-operation, they can move hundred-weight sacks of coal around. A normal feature of a cell has been exaggerated, and others concomitantly repressed, in the name of division of labour. A muscle cell will never, itself, reproduce. But it is helping its brethren in the gonads, whose task is to pass the clone's genes on to the next generation, to do what they are good at.

The medium is the message

Communication means more chemistry. Nothing needs to communicate with a red blood cell. In effect it is already dead - the only chemical reactions it performs are with the oxygen and carbon dioxide it carries to and from the lungs. Muscle cells, however, must be told what to do. In their case, the information is conveyed from the tips of nerve cells by chemicals known as neurotransmitters. Nerves also use neurotransmitters to talk among themselves. Longer-distance communication, however, is done with another class of chemicals: hormones.

Signal molecules such as hormones and neurotransmitters are picked up by specialised proteins that straddle the cell's outer membrane. These proteins act like the modems that translate telephone signals to a computer: they interpret the hormone's message to the cellular processor that they are part of. Stimulating a "modem" protein starts a molecular relay-race known as transduction. The information is passed from one sort of molecule to another like a baton. This baton, though, is intangible. In the case of a muscle cell being told to contract, for example, several batons are set moving. Some pass to other proteins in the membrane, which then act as channels for the simplest imaginable chemical messengers - charged atoms (known as ions) of sodium and potassium. The passage of these ions through the membrane spreads the message to contract to the whole cell. Other batons order the release of calciurn ions from stores within the cell. Calcium is the signal that tells the actin and myosin fibres to start moving against one another.

If a transduction pathway reaches into the nucleus, the baton it is passing can have longer-lasting effects: reprogramming the cell by switching genes on or off according to the signal received. Some reprogramming is temporary, some permanent.

The most drastic order that a cell can receive is to kill itself. Cells infected with viruses are often told to commit suicide. So are tumour cells. Less terminal, but almost as far-reaching, are instructions to switch particular "master genes" on or off. These genes are themselves involved in signalling - the proteins copied from their RNA find their way back to the nucleus. Some act as spanners in the works of the delicately ticking mechanisms that turn out RNA. Others are stimulants to activity. Because what a cell does, and therefore what it is, is governed by the proteins it contains, throwing the "master gene" switches means specialising. The cell is on its way to whichever of the 300 destinations its orders have prescribed.

The least drastic reprogramming is to switch the mixture of workaday proteins such as enzymes that it is producing. By this stage in its life, though, its card has been marked. It is a specialist. It cannot go back.

Getting in shape

Proteins are able to do the many different jobs required of them because, at the molecular scale, chemistry is geometry. Geometry is something that proteins have lots of. They are composed of small molecules called amino acids that have the atomic equivalents of a hook at one end and an eye at the other, allowing them to link into chains of indefinite length. A protein may be made of up to 20 different sorts of amino acid which, apart from the hook-and-eye arrangements, are chemically very different from one another. Some like to snuggle up together, while others repel one another. The result is that most protein chains fold into a roughly ball-shaped structure composed of a series of convoluted loops, spirals and corrugations. This leaves all sorts of nooks and crannies into which lesser molecules can fit, somewhat as a key fits a lock.

Enzymes, for instance, work by changing their shape once they have their targets in a firm grip. Those that assemble things have adjacent locks that bring reluctant chemical partners so close together that, with a little encouragement from some ATP, they can be made to embrace each other. Enzymes that break things apart simply snap them by their change of shape.

The geometries of actin and myosin, the proteins that are over-expressed in muscle cells, have evolved to match one another. Unlike most proteins, these two form long filaments that fit together like ratchets and, with a little ATP to energise them, move up or down each other, lengthening or shortening the fibre they are part of.

The "modem" proteins and ion channels that inhabit a cell's outer membrane rely on their shapes as well. The modems have slots the correct shape to fit their target messenger - or a drug chosen to imitate its shape. Channels are hollow cylinders with bores the right size to pass the appropriate substance. Gene regulators also rely on shape. Their bumps fit neatly into the critical bits of the double helix of DNA. That helix, too, consists of long strings of interchangeable units - the nucleosides. Like amino acids, nucleosides have a hook-and-eye arrangement that allows them to link together, and a variable element that gives them their specific properties. The variable element in a nucleoside is known as a base.

The main role of the bases is not to create a shape (although they do this as well) but to carry yet more information - the most fundamental information in the cellular computer. The order in which the bases are strung out along a molecule of DNA is the message of life. The four types of base are four molecular "letters" conventionally known by the initials of their full chemical names: A, C, G and T. "A", for instance, stands for the adenosine also found in ATP. Bases are able to impose shape on a molecule of DNA because, like amino acids, they are fussy about who they team up with. They like to travel in pairs - but A will only fit with T, and G with C. No other combinations work. Therefore, two parallel strings of nucleosides can link up only if their bases complement each other. The result, with a twist, is the famous double helix. This helix is itself coiled, the coils are coiled again, like a light-bulb filament, and the whole lot is packaged into a chromosome. Unzip it and each half of a DNA molecule can act as a template to recreate the other - the fundamental basis of cell division that allows each daughter cell to receive a complete set of DNA from its parent. Unzip a short length and an RNA copy of part of one strand can be made and taken away to the ribosomes: the place where proteins are made.

Most ribosomes float in the membrane of yet another organelle, the most convoluted found in a cell. It is known as the endoplasmic reticulum. Each ribosome is like a miniature paper-tape reader with the RNA messenger acting as the tape. The message is read off three bases at a time; each group of three is the code for one of the 20 amino acids. Another sort of RNA fetches the appropriate amino acid and, with the injection of a little energy from some ATP, this is joined to the end of the protein. When the tape runs out, the new protein floats free.

Or almost free. Some proteins simply sit in the membrane of the endoplasmic reticulum because that is where they are destined to live. Some are sent on to another stack of membranes known as the Golgi apparatus, to be fitted with molecular bells and whistles such as sugar molecules, that help them to work more efficiently. Some of these are then exported, for one of the Golgi's other roles is to package hormones for release at the cell's surface. Some (usually made by the few free-floating ribosomes) even descend back into the depths of the nucleus to help regulate its goings-on.

And so the cycle is complete, the messengers dispatched and the biochemical network sustained. Except for one thing. In many cells, the amount of new material produced exceeds that used up or exported, so the cell grows until it is big enough to perform a trick that no man-made computer can manage: it divides itself into two. But that, best beloved, is another story.

Source: The Economist 25 December 1993 - 7 January 1994

Live Universes

Sir

In their recent Commentary article (Nature 384, 107; 1996), John Maynard Smith and Eors Szathmary consider the idea (proposed by L Smolin and J A Wheeler) that universes are "alive" in the sense that they can pass on hereditary information, show variability from one generation to the next, and are subject to selective pressures. It seems ironic that evolutionary biologists should take such an idea seriously enough to comment (even sceptically) on it, while at the same time treating with derision the much less radical proposals that ecosystems or planets (in particular, Earth) may also be alive.

The possibility that life forms exist at levels above that of individual organisms is generally denied because these higher-order systems fail one or more of the criteria established for the recognition of life. But this seems tautological; there is no reason to expect that super-organisms would meet criteria based on observations of individual organisms. Isn't it time to consider the possibility that the boundary between life and non-life may be diffuse, non-stationary over time, and dependent upon scale?

M G Bjornerud
Geology Department,
Lawrence University,
Appleton, Wisconsin 54912, USA
e-mail: bjornerm@lawrerx:e.edu

Source: Nature vol 385 9 January 1997

The Oort Crowd

The gods truly are moving in mysterious ways

by Ken MacLeod

As we enter the first year of the 23rd century (or the last year of the 22nd - some arguments never go away) we look back with satisfaction at the triumphs of science and technology in the first two centuries of the third millennium. The advances in medicine, in biotechnology, in communications, in atmospheric engineering have been more than adequately celebrated elsewhere. They are familiar to the most isolated farmer on the barest rocks of Antarctica. But in long-term significance for the human prospect, nothing can compare to the discovery of the gods.

The word "gods" is used advisedly. Humanity's earliest speculations about the nature of any superhuman intelligences with which it might share the Universe are, paradoxically, more relevant to our real situation than the predictions of alien contact in the once-popular genre of science fiction. It must be admitted, however, that some of its practitioners reached part of the truth.

That truth, as we all know, is that a large, undetermined and (for good reason) indeterminable fraction of the bodies in the asteroid belt, the Kuiper Belt and the Oort Cloud are the sites of complex intelligent life. The precise evolutionary route(s) from extremophilic microorganisms to intelligence, apparently bypassing multicellular organisation, remain unknown and perhaps (again, for good reason) unknowable. Computer simulations have yielded interesting, if inconclusive, results (Chang-Hoskins, 2197, provides a useful overview).

Those of our readers who have benefited from advances in medicine may recollect, and our younger readers can easily retrieve, the excitement that greeted the initial, accidental discovery of an ET intelligence in 2031. The first downloads from the Gates Foundation asteroid prospector revealed, not the potential wealth of resources expected in a carbonaceous chondrite, but a complex interior structure variously described as "crystalline", "fractal" and "organic". Fortunately for scientific openness, the drilling operation was webcast live, and as the pictures slowly scrolled down the screens of a few hundred thousand space enthusiasts, the news spread across the net faster than a virus.

In those first hasty, misspelled e-mails and postings we can see - from references to the structure as "the alien computer" or "Asteroid City" or even as "the starship" - the depth of initial misapprehension. Far from having been built by beings broadly similar to ourselves, the structure itself was the alien, or the civilisation - the nature and number of centres of consciousness within it remain controversial. And it was neither alone nor isolated.

Billions of years of evolutionary "tuning" have given the cometary minds an exquisite sensitivity to the electromagnetic output of each other's internal chemical and physical processes. Their communications are, once looked for, as detectable as they are incomprehensible. Some of the larger bodies in the Oort seem to act as relays, extending the communications net across solar and possibly interstellar distances. (As is known, the tenuous outer reaches of the Oort Cloud intersect those of their Centaurian equivalent.)

Despite strenuous efforts, no human communication with the extraterrestrial minds has been established (the results claimed by Lunan, 2049, are at best ambiguous). They are, to us, in precisely the position of the gods postulated by Epicurus, serene in the spaces between the worlds.

These gods, while indifferent, are not passive. Subtle control over their outgassings results, over very long periods, in orbital changes. More rapid processes occur within the asteroids. Careful study of recent and historical Near-Earth Objects suggests that the orbits of at least some NEOs have been the result of conscious intent.

In view of the above, it appears in retrospect unfortunate that the first probe to the Oort Cloud and beyond, launched in 2030, should have used as its initial means of propulsion a plasma sail consisting of ionised gas within a "magnetic bubble" thousands of kilometres across, and as its secondary means a prototype "electromagnetic ramscoop" sucking in vast quantities of interstellar and cometary matter. Subsequent changes in the volume and intensity of intercometary communication, and in the orbits of numerous comets and asteroids, cannot be accounted for by the physical effects of its passage. They can only be considered a response.

The effect on human society of the discovery of the gods has been positive. Excluded from many of the space-based resources once thought unoccupied, we turn to a less profligate use of our planet's own. The expectations of John Stuart Mill, in his famous chapters on the "stationary state" and "the probable futurity of the laboring classes", have been largely realised.

But, as our astronomical and space-defence workers' cooperatives continue their urgent sky-watching, there may be some risk of overlooking a danger closer to home. There is no reason to suppose that extremophilic consciousness is confined to minor interplanetary bodies. Perhaps the majority of the Earth's biomass consists of subterranean extremophiles.

Watch the ground.

Ken MacLeod is the author of The Star Fraction, The Stone Canal, The Cassini Division and The Sky Road. His next novel, Cosmonaut Keep, expands on some of the ideas in this article.

Source: Nature Volume 406 13 July 2000 from the "futures" column