Climbing Mount Improbable

  DNA, building its robot vehicles to ride around in, has the tool of exponential growth at its command. Exponential growth puts great power in the hands of naturally selected genes. It means that a tiny adjustment to a detail of embryonic growth control can have the most dramatic effect on the outcome. A mutation that tells a particular sub-lineage of cells to go on dividing just one more time — say go on for twenty-five cell generations instead of twenty-four — can in principle have the effect of doubling the size of a particular bit of the body. The same trick, of changing numbers of cell generations, or rates of cell division, can be used by genes during embryology to change the shape of a bit of the body. Modern humans have a prominent chin compared with our fairly recent ancestor Homo erectus. All that it takes to change the shape of the chin is small adjustments in the numbers of cell generations in particular regions of the embryonic skull.

  In a way, the remarkable thing is that cell lineages stop dividing when they are supposed to, in such a way that all our bits are well proportioned relative to one another. In some cases, of course, cell lineages notoriously do not stop dividing when they should. When that happens we call it cancer. Randolph Nesse and George Williams (in their brilliant book which they wrote under the excellent title Darwinian Medicine, but which the publishers then saddled with an assortment of unmemorable and geographically variable titles) make a wise point about cancer. Before we wonder why we get cancer, we should wonder why we don't get it all the time.

  Who knows whether humans will ever attempt to build things by gigatechnology? But people are already talking about nanotechnology. Just as ‘giga’ means a billion, ‘nano’ means a billionth. Nanotechnology means engineering things that are a billionth of the size of the builder.

  There are people — and the leading ones are not all New Agers or {293}

  Figure 9.5 A fantasy in nanotechnology. Robot devices sent in to repair red blood corpuscles.

  cult fanatics — who are now saying that something like Figure 9.5 will be a reality in the not too distant future. If they are right, there is hardly an area of human life that will not be dramatically affected. Doctoring is an example. Modern surgeons are highly skilled people, with delicate, precision instruments. To remove the lens of an eye when it is clouded with cataract and replace it with a substitute lens, as modern surgeons do, is an amazing feat of skill. The instruments that they use are impressively fine and precise. But, compared with the scale of nanotechnology, they are still immensely crude. Listen to Eric Drexler, the American scientist who is emerging as the high priest of nanotechnology, as he invokes a nano-scale view of present-day scalpels and surgical stitching.

  Modern scalpels and sutures are simply too coarse for repairing capillaries, cells, and molecules. Consider ‘delicate’ surgery from a cell's {294} perspective: a huge blade sweeps down, chopping blindly past and through the molecular machinery of a crowd of cells, slaughtering thousands. Later, a great obelisk plunges through the divided crowd, dragging a cable as wide as a freight tram behind it to rope the crowd together again. From a cell's perspective, even the most delicate surgery, performed with exquisite knives and great skill, is still a butcher job. Only the ability of cells to abandon their dead, regroup, and multiply makes healing possible.

  The ‘obelisk’, of course, is a delicate surgical needle, and the cable as wide as a goods train is the finest surgical thread. Nanotechnology holds out the dream of constructing surgical instruments small enough to work on the same scale as the cells themselves. Such instruments would be far too tiny to be controlled by the fingers of a surgeon. If a piece of thread is the width of a goods train on the cells’ scale, think how wide a surgeon's fingers would be. There would have to be little automatic machines, tiny robots, not unlike miniature versions of the industrial robots we met earlier in this chapter.

  Now a robot this small might be wonderful at repairing, say, a diseased red blood cell. But there is a daunting army of red blood cells for the robot to get round, about 30 billion in each one of us. So, how on earth can the little nanotechnology robot cope? You will already have guessed the answer: exponential multiplication. The hope is that the nanotechnology robot would use the same self-multiplying technique as the blood cells themselves. The robot would clone itself, replicate itself. By using the power of exponential growth, the population of robots is supposed to soar up into the billions in just the same way as the population of red blood cells went up into the billions.

  Nanotechnology of this kind is all in the future, and it may never come to anything. The reason the scientists who are proposing nanotechnology think it is worth a try is this. They know that, however strange and alien it may seem to us, the equivalent of it already does work in our cells. The world of DNA and protein molecules is a world that really works on a scale that, if we did it, would be called nanotechnology. When a doctor inoculates you with immunoglobu-lins to stop you getting hepatitis, the doctor is loading your {295} bloodstream with the natural equivalent of nanotechnology tools. Each immunoglobulin molecule is a complicated object which, like any other protein molecule, relies upon its shape to do its job (Figure 9.6). These little medical instruments work only because there are millions of them. They have been mass-produced — cloned up — using exponential population-growth techniques. In this case they are biological techniques: they are often grown up in the blood of a horse, for instance. Other vaccines prompt the body's own tendency to clone up antibodies like the horse immunoglobulins. The hope is that nanotechnology tools, looking pretty much like miniature industrial robots, might be cloned up too by cunningly designed artificial procedures.

  Figure 9.6 Real life nanotechnology: an immunoglobulin molecule. {296}

  Nanotechnology seems to us very strange, scarcely believable. The world of machines down there at the level of the atoms seems an alarmingly alien world, more strange than life on other planets as imagined by science-fiction writers. Nanotechnology is, for us, something that may come about in the future. It is something exciting, perhaps a bit scary and apparently new. But, far from nanotechnology being really new and alien, it is old. It is we big things that are new, alien, strange. We are products of a flashy new (only a few hundred million years) gigatechnology (giga from the point of view of our genes). Fundamentally, life is based in the nanoworld of the very small (nano from our point of view, a world of protein molecules, made to the coded specifications of DNA molecules and controlling the interactions of other molecules.

  Nanotechnology is for the future. Let us return to the main message of this chapter and of the previous one. The genes of an elephant or a human, like the genes of a virus, can be seen as a Duplicate Me computer program. Virus genes are coded instructions that say (if they happen to be parasitizing an elephant): ‘Elephant cells, duplicate me.’ Elephant genes say: ‘Elephant cells, work together to make a new elephant, which must be programmed in its turn to grow and make more elephants, all programmed to duplicate me.’ The principle is the same. It is just that some Duplicate Me programs are more indirect and longwinded than others. Only parasitic programs can afford to be shortwinded, because they use ready-made machinery to obey their instructions. Elephant genes are not so much non-parasitic programs as mutually parasitic sub-routines. An elephant's genes are like a gigantic colony of mutually supportive viruses. Each elephant gene plays a part that is no larger than the role played by a virus gene. Each one plays its own small part in the cooperative building of the machinery that they all need for their program execution. Each one flourishes in the presence of the others. Virus genes also flourish in the presence of the set of cooperating elephant genes, but they do not contribute anything positive in return. If they did, we should probably not call them virus genes but elephant genes. To put it another way, every body contains social and antisocial genes. The {297} antisocial ones we call virus genes (and other kinds of parasite genes). The social ones we call elephant (human, kangaroo, sycamore, etc.) genes. But the genes themselves, whether social or antisocial,
whether virus genes or ‘own genes, are all just DNA instructions, and they all say, in one way or another, by fair means or foul, briefly or longwindedly, ‘Duplicate me.’ {298}

 
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  CHAPTER 10

  WE HAVE COME A LONG WAY AND ARE FINALLY READY to return to the most difficult and complicated of all my stories, that of the fig. Let's begin with the following which sounds, at first hearing, like just another literary tease worthy of the unfortunate lecturer whom I lampooned in my opening paragraphs. A fig is not a fruit but a flower garden turned inside out. It looks like a fruit. It tastes like a fruit. It occupies a fruit-shaped niche in our mental menus and in the deep structures recognized by anthropologists. Yet it is not a fruit; it is an enclosed garden, a hanging garden and one of the wonders of the world. I am not going to leave this statement dangling as a self-indulgent profundity to be plucked by the ‘sensitive’ and baffle everyone else. Here is what it means.

  The meaning is rooted in evolution. Figs are descended, via a chain of infinitesimally graded intermediates, from ancestors that were superficially very different from modern figs. Imagine a time-lapse film built up as follows. The first frame is a modern fig, picked today from the tree, sectioned down the middle, laid on a sheet of card and photographed. Frame two is a similar fig from a century ago. Carry on through the centuries, fig on fig, frame by frame, through a fig that might have been eaten by Jesus, or plucked by a slave for Nebuchadnezzar in the Hanging Gardens of Babylon, a fig from the land of Nod, East of Eden, figs that sweetened the short, sugar-starved lives of Homo erectus, Homo habilis and little Lucy of the Afar; {299} back before the time of cultivation, back to the wild figs of the forest and beyond. Now run the film and watch the modern fig transform itself into its remote ancestor. What changes shall we see?

  Undoubtedly there'll be some shrinking as we go backwards, for cultivated figs have been plumped up over the centuries from smaller, harder, wild ancestors. But this is a superficial change and, interesting as it is, it'll all be over within the first few millennia of our backward journey. More radical and startling is the change that we'll see as we run the film further back through millions of years. The ‘fruit’ will open out. The tiny, almost invisible hole at the apex of the fig will pout, gape, yawn until it is no longer a hole but a cup. Look carefully at the inner surface of the cup and you'll see that it is lined with tiny flowers. First the cup is a deep one then, as we reel the film back in time, it becomes steadily shallower. Perhaps it goes through a flat stage like a sunflower, for a single sunflower too is in truth hundreds of small flowers, packed into a mass bed. Pressing on beyond the sunflower stage, our fig cup turns inside out until the florets are on the outside, as in a mulberry (the fig is a member of the mulberry family). Further back, beyond the mulberry stage, the florets separate and become more recognizably distinct flowers as in a hyacinth (although hyacinths are not closely related to figs).

  Is it perhaps a little contrived, pretentious even, to describe a single fig as ‘a garden inclosed’? After all, you'd hardly describe a hyacinth or a mulberry as a garden exposed. My defence is a good one and goes beyond being haunted by a phrase from the Song of Solomon. Look at a garden through the eyes of the insects that pollinate its flowers. A garden, on the human scale, is a population of flowers covering many square yards. The pollinators of figs are so tiny that, to them, the whole interior of a single fig might seem like a garden, though admittedly a small, cottage garden. It is planted with hundreds of miniature flowers, both male and female, each with its own diminutive parts. Moreover the fig really is an enclosed and largely self-sufficient world for the minuscule pollinators.

  The pollinators are technically wasps belonging to one family, the Agaonidae, and they are tiny, too small to be seen clearly without a lens. By ‘technically’ wasps I mean that, although you might notice no great {300} resemblance to the yellow-and-black ‘yellowjackets’ that menace the summer jam jar, fig wasps share a wasp ancestor with them. Fig flowers are pollinated only by these tiny wasplets (Figure 10.1). Almost every species of fig (and there are more than 900 of them) has its own private species of wasp which has been its lone genetic companion through evolutionary time since the two of them split off together from their respective predecessors. The wasps depend totally on the fig for their food and the fig depends utterly on the wasps to carry its pollen. Each species would promptly go extinct without the other. It is only the female wasps, who travel outside their natal fig, that carry the pollen. They are shaped as you might imagine highly miniaturized wasps to be. The males by contrast have no wings, for they are born and die within the closed, dark world of a single fig, and it is hard to believe that they are wasps at all, let alone wasps of the same species as their own females.

  The problem with telling the life story of the fig wasps is that it is a cycle and it isn't obvious where our description should break in. This can't be helped and I'll start with the hatching of new wasp grubs, each one curled up in its tiny capsule at the base of one of the

  Figure 10.1 Interior of fig with male and female fig wasps. {301}

  female flowers deep inside the enclosed garden. Feeding on the developing seed, it grows into an adult and chews its way out of its capsule, emerging into the comparative freedom of the fig's dark interior. The males and females then follow somewhat different life stories. The males hatch first and each one scours the fig searching for the capsule of an unborn female. When he finds one, he chews his way through the ovule wall and mates with the still unborn virgin. She then emerges from her birth capsule and sets off on her own journey through the miniature hanging garden. The details of what happens next vary a little from species to species. The following is typical. The female looks for male flowers, which are often to be found near the entrance to the fig. Using custom-built pollen-brushes on her front legs, she works away in the dark, systematically shovelling pollen into special pollen pockets in the recesses of her breast.

  It is revealing that she takes such deliberate pains to stock up with pollen and that she has special pollen-carrying receptacles. Most pollinating insects simply find themselves dusted, willy-nilly, with the stuff. They don't have dedicated pollen-carrying apparatus, nor pollen-loading instincts. Bees do. They have pollen baskets on their legs which bulge yellow or brown with pollen stuffed into them. But bees, unlike fig wasps, are transporting the pollen to feed to their larvae. Fig wasps don't transport pollen for food. They deliberately take it on board, using special pollen-carrying pockets, for the sole purpose of fertilizing figs (which benefits the wasps only in a more indirect way). We'll return to the whole matter of the apparently amicable cooperation between figs and their pollinators.

  Laden with precious pollen, the female leaves the fig for the airy openness of the outer world. Exactly how she gets out varies from species to species. In some, she crawls through the ‘garden gate’, the little hole at the end of the fig (Figure 10.2). In other species, it is the males’ task to cut a hole through the wall of the fig through which the females leave, and they do so in collaboration, with dozens of males working together. The male's role in life is now over but the female has her big moment still before her. She flies off in the unaccustomed air, searching, probably by smell, for another fig of her own single right species. The individual fig that she seeks must also be in {302}

  Figure 10.2 The garden gate: exterior of fig to show the entrance.

  the right phase of its life, the phase in which female flowers are ripe.

  Having found the right kind of fig, the female locates the minute hole at the tip of it and crawls through into the dark interior. The door is so narrow that she is likely to tear her wings out by the roots as she squeezes through. Investigators who have examined fig pores {303} have found them clogged with disembodied wings, antennae and other wasp fragments. From the fig's point of view, the advantage of having such an uncomfortably narrow entrance is that it keeps out unwanted parasites. The wing-tearing gau
ntlet run by the female probably also serves to wipe her clean of bacteria and harmful dirt. From the wasp's point of view, even if it is painful to have her wings ripped out by the roots, she is never going to need them again and they would probably have hampered her movements in the confines of the enclosed garden. Recall that queen ants often bite off their own wings when they have finished their mating flight and have reached the stage where wings would get in the way underground.

  Inside the fig, the female wasp sets about her final mission before she dies, and it is a dual one. She pollinates all the female flowers that she visits inside the fig, and she lays eggs in some of them. Not in all. If she laid eggs in all the flowers of a fig, that fig would have failed as an organ of reproduction for the tree — all its seeds would have been eaten by wasp grubs. Does this sparing of some flowers betoken altruistic restraint on the part of the wasp? That's a question that needs careful handling. There are certain Darwinistically respectable ways in which a kind of restraint on the part of wasps could evolve into existence. But there are at least some species in which the fig tree looks after its own interests by rationing the number of flowers in which the wasp is allowed to lay eggs. The techniques are ingenious and I'll turn aside from my account of the normal life cycle just long enough to describe two of them.