The Greatest Show on Earth Page 40

  but what might that mean? The harder we look at the border between life and non-life, the more elusive does the distinction become. Life, the animate, was supposed to have some sort of vibrant, throbbing quality, some vital essence – made to sound yet more mysterious when dropped into French: élan vital.† Life, it seemed, was made of a special living substance, a witch’s brew called ‘protoplasm’. Conan Doyle’s Professor Challenger, a fictional character even more preposterous than Sherlock Holmes, discovered that the Earth was living, a kind of giant sea urchin whose shell was the crust that we see, and whose core consisted of pure protoplasm. Right up to the middle of the twentieth century, life was thought to be qualitatively beyond physics and chemistry. No longer. The difference between life and non-life is a matter not of substance but of information. Living things contain prodigious quantities of information. Most of the information is digitally coded in DNA, and there is also a substantial quantity coded in other ways, as we shall see presently.

  In the case of DNA, we understand pretty well how the information content builds up over geological time. Darwin called it natural selection, and we can put it more precisely: the non-random survival of information that encodes embryological recipes for that survival. Self-evidently it is to be expected that recipes for their own survival will tend to survive. What is special about DNA is that it survives not in its material self but in the form of an indefinite series of copies. Because there are occasional errors in the copying, new variants may survive even better than their predecessors, so the database of information encoding recipes for survival will improve as time goes by. Such improvements will be manifest in the form of better bodies and other contrivances and devices for the preservation and propagation of the coded information. On the ground, the preservation and propagation of DNA information will normally mean the survival and reproduction of bodies containing it. It was at the level of bodies, their survival and reproduction, that Darwin himself worked. The coded information within them was implicit in his world-view, but not made explicit until the twentieth century.

  The genetic database will become a storehouse of information about the environments of the past, environments in which ancestors survived and passed on the genes that helped them to do so. To the extent that present and future environments resemble those of the past (and mostly they do), this ‘genetic book of the dead’ will turn out to be a useful manual for survival in the present and future. The repository of that information will, at any one moment, reside in individual bodies, but in the longer term, where reproduction is sexual and DNA is shuffled from body to body, the database of survival instructions will be the gene pool of a species.

  Each individual’s genome, in any one generation, will be a sample from the species database. Different species will have different databases because of their different ancestral worlds. The database in the gene pool of camels will encode information about deserts and how to survive in them. The DNA in mole gene pools will contain instructions and hints for survival in dark, moist soil. The DNA in predator gene pools will increasingly contain information about prey animals, their evasive tricks and how to outsmart them. The DNA in prey gene pools will come to contain information about predators and how to dodge and outrun them. The DNA in all gene pools contains information about parasites and how to resist their pernicious invasions.

  Information on how to handle the present so as to survive into the future is necessarily gleaned from the past. Non-random survival of DNA in ancestral bodies is the obvious way in which information from the past is recorded for future use, and this is the route by which the primary database of DNA is built up. But there are three further ways in which information about the past is archived in such a way that it can be used to improve future chances of survival. These are the immune system, the nervous system, and culture. Along with wings, lungs and all the other apparatus for survival, each of the three secondary information-gathering systems was ultimately prefigured by the primary one: natural selection of DNA. We could together call them the four ‘memories’.

  The first memory is the DNA repository of ancestral survival techniques, written on the moving scroll that is the gene pool of the species. Just as the inherited database of DNA records the recurrent details of ancestral environments and how to survive them, the immune system, the ‘second memory’, does the same thing for diseases and other insults to the body during the individual’s own lifetime. This database of past diseases and how to survive them is unique to each individual and is written in the repertoire of proteins that we call antibodies – one population of antibodies for each pathogen (disease-causing organism), precisely tailored by past ‘experience’ with the proteins that characterize the pathogen. Like many children of my generation, I had measles and chickenpox. My body ‘remembers’ the ‘experience’, the memories being embodied in antibody proteins, along with the rest of my personal database of previously vanquished invaders. I have fortunately never had polio, but medical science has cleverly devised the technique of vaccination for planting false memories of diseases never suffered. I shall never contract polio, because my body ‘thinks’ it has done so in the past, and my immune system database is equipped with the appropriate antibodies, ‘fooled’ into making them by the injection of a harmless version of the virus. Fascinatingly, as the work of various Nobel Prize-winning medical scientists has shown, the immune system’s database is itself built up by a quasi-Darwinian process of random variation and non-random selection. But in this case the non-random selection is selection not of bodies for their capacity to survive, but of proteins within the body for their capacity to envelop or otherwise neutralize invading proteins.

  The third memory is the one we ordinarily think of when we use the word: the memory that resides in the nervous system. By mechanisms that we don’t yet fully understand, our brains retain a store of past experiences to parallel the antibody ‘memory’ of past diseases and the DNA ‘memory’ (for so we can regard it) of ancestral deaths and successes. At its simplest, the third memory works by a trial-and-error process that can be seen as yet another analogy to natural selection. When searching for food, an animal may ‘try’ various actions. Though not strictly random, this trial stage is a reasonable analogy to genetic mutation. The analogy to natural selection is ‘reinforcement’, the system of rewards (positive reinforcement) and punishments (negative reinforcement). An action such as turning over dead leaves (trial) turns out to yield beetle larvae and woodlice hiding under the leaves (reward). The nervous system has a rule that says, ‘Any trial action that is followed by reward should be repeated. Any trial action that is followed by nothing, or, worse, followed by punishment, for example pain, should not be repeated.’

  But the brain’s memory goes much further than this quasi-Darwinian process of non-random survival of rewarded actions, and elimination of punished actions, in the animal’s repertoire. The brain’s memory (no need for inverted commas here, because it is the primary meaning of the word) is, at least in the case of human brains, both vast and vivid. It contains detailed scenes, represented in an internal simulacrum of all five senses. It contains lists of faces, places, tunes, social customs, rules, words. You know it well from the inside, so there is no need for me to spend my words evoking it, except to note the remarkable fact that the lexicon of words at my disposal for writing, and the identical, or at least heavily overlapping, dictionary at your disposal for reading, all reside in the same vast neuronal database, along with the syntactic apparatus for arranging them into sentences and deciphering them.

  Furthermore, the third memory, the one in the brain, has spawned a fourth. The database in my brain contains more than just a record of the happenings and sensations of my personal life – although that was the limit when brains originally evolved. Your brain includes collective memories inherited non-genetically from past generations, handed down by word of mouth, or in books or, nowadays, on the internet. The world in which you and I live is richer by far because of those who
went before us and inscribed their impacts on the database of human culture: Newton and Marconi, Shakespeare and Steinbeck, Bach and the Beatles, Stephenson and the Wright brothers, Jenner and Salk, Curie and Einstein, von Neumann and Berners-Lee. And, of course, Darwin.

  All four memories are part of, or manifestations of, the vast super-structure of apparatus for survival which was originally, and primarily, built up by the Darwinian process of non-random DNA survival.

  ‘INTO A FEW FORMS OR INTO ONE’

  Darwin was right to hedge his bets, but today we are pretty certain that all living creatures on this planet are descended from a single ancestor. The evidence, as we saw in Chapter 10, is that the genetic code is universal, all but identical across animals, plants, fungi, bacteria, archaea and viruses. The 64-word dictionary, by which three-letter DNA words are translated into twenty amino acids and one punctuation mark, which means ‘start reading here’ or ‘stop reading here’, is the same 64-word dictionary wherever you look in the living kingdoms (with one or two exceptions too minor to undermine the generalization). If, say, some weird, anomalous microbes called the harumscaryotes were discovered, which didn’t use DNA at all, or didn’t use proteins, or used proteins but strung them together from a different set of amino acids from the familiar twenty, or which used DNA but not a triplet code, or a triplet code but not the same 64-word dictionary – if any of these conditions were met, we might suggest that life had originated twice: once for the harumscaryotes and once for the rest of life. For all Darwin knew – indeed, for all anyone knew before the discovery of DNA – some existing creatures might have had the properties I have here attributed to the harumscaryotes, in which case his ‘into a few forms’ would have been justified.

  Is it possible that two independent origins of life could both have hit upon the same 64-word code? Very unlikely. For that to be plausible, the existing code would have to have strong advantages over alternative codes, and there would have to be a gradual ramp of improvement towards it, a ramp for natural selection to climb up. Both these conditions are improbable. Francis Crick early suggested that the genetic code is a ‘frozen accident’, which, once in place, was difficult or impossible to change. The reasoning is interesting. Any mutation in the genetic code itself (as opposed to mutations in the genes that it encodes) would have an instantly catastrophic effect, not just in one place but throughout the whole organism. If any word in the 64-word dictionary changed its meaning, so that it came to specify a different amino acid, just about every protein in the body would instantaneously change, probably in many places along its length. Unlike an ordinary mutation, which might, say, slightly lengthen a leg, shorten a wing or darken an eye, a change in the genetic code would change everything at once, all over the body, and this would spell disaster. Various theorists have come up with ingenious suggestions for special ways in which the genetic code might evolve: ways in which, to quote one of their papers, the frozen accident might be ‘thawed’. Interesting as these are, I think it is all but certain that every living creature whose genetic code has been looked at is descended from one common ancestor. No matter how elaborate and different the high-level programs that underlie the various life forms, all are, at bottom, written in the same machine language.

  Of course we cannot rule out the possibility that other machine languages may have arisen in yet other creatures that are now extinct – the equivalent of my harumscaryotes. And the physicist Paul Davies has made the reasonable point that we haven’t actually looked very hard to see if there are any harumscaryotes (he doesn’t use the word, of course) that are not extinct but still lurking in some extreme redoubt of our planet. He admits that it is not very likely, but argues – somewhat along the lines of the man who searches for his keys under a street lamp rather than where he lost them – that it is a lot easier and cheaper to look thoroughly on our planet than to travel to other planets and look there. Meanwhile, I don’t mind recording my private expectation that Professor Davies won’t find anything, and that all surviving life forms on this planet use the same machine code and are all descended from a single ancestor.

  ‘WHILST THIS PLANET HAS GONE CYCLING ON ACCORDING TO THE FIXED LAW OF GRAVITY’

  Humans were aware of the cycles that govern our lives long before we understood them. The most obvious cycle is the day/night cycle. Objects floating in space, or orbiting other objects under the law of gravity, have a natural tendency to spin on their own axis. There are exceptions, but our planet is not one of them. Its period of rotation is now twenty-four hours (it used to spin faster) and we experience it, of course, as night follows day.

  Because we live on a relatively massive body, we think of gravity primarily as a force that pulls everything towards the centre of that body, which we experience as ‘down’. But gravity, as Newton was the first to understand, has a ubiquitous effect, which is to keep bodies throughout the universe in semi-permanent orbit around other bodies. We experience this as the yearly cycle of seasons, as our planet orbits the sun.* Because the axis on which our planet spins is tilted relative to the axis of rotation around the sun, we experience longer days and shorter nights during the half of the year when the hemisphere on which we happen to live is tilted sunwards, the period that climaxes in summer. And we experience shorter days and longer nights during the other half of the year, the period that, at its extreme, we call winter. During our hemisphere’s winter, the sun’s rays, when they strike us at all, do so at a shallower angle. The glancing angle spreads a winter sunbeam more thinly over a wider area than the same beam would cover in summer. On the receiving end of fewer photons per square inch, it feels colder. Fewer photons per green leaf means less photosynthesis. Shorter days and longer nights have the same effect. Winter and summer, day and night, our lives are governed by cycles, just as Darwin said – and Genesis before him: ‘While the earth remaineth, seedtime and harvest, and cold and heat, and summer and winter, and day and night shall not cease.’

  Gravity mediates other cycles that also matter to life, although they are less obvious. Unlike other planets that have many satellites, often relatively small, Earth happens to have a single large satellite, which we call the moon. It is large enough to exert a significant gravitational effect in its own right. We experience this principally in the cycle of tides: not just the relatively fast cycle as tides come in and out daily, but the slower monthly cycle of spring tides and neap tides, which is caused by interactions between the sun’s gravitational effect and that of the monthly orbiting moon. These tidal cycles are especially important for marine and coastal organisms, and people have rather implausibly wondered whether some kind of species memory of our marine ancestry survives in our monthly reproductive cycles. That may be far-fetched, but it is a matter for intriguing speculation how different life would be if we had no orbiting moon. It has even been suggested, again implausibly in my opinion, that life without the moon would be impossible.

  What if our planet didn’t spin on its axis? If it kept one face permanently towards the sun, as the moon does towards us, the half with permanent day would be a roasting hell, while the half with permanent night would be insufferably cold. Could life survive in the twilight hinterland between, or perhaps buried deep in the ground? I doubt if it would have originated in such unfriendly conditions, but if Earth gradually spun down to a halt there would be plenty of time to accommodate, and it is not implausible that at least some bacteria would succeed.

  What if Earth spun, but on an axis that was not tilted? I doubt if that would rule life out. There would be no summer/winter cycle. Summer and winter conditions would be a function of latitude and altitude but not time. Winter would be the permanent season experienced by creatures living close to either of the two poles, or up high mountains. I don’t see why that should rule life out, but life without seasons would be less interesting. There would be no incentive to migrate, or to breed at any particular time of the year rather than any other, or to shed leaves or to moult or hibernate.

&n
bsp; If the planet were not in orbit around a star at all, life would be completely impossible. The only alternative to orbiting a star is hurtling through the void – dark, close to absolute zero temperature, alone and far from the source of energy that enables life to trickle upstream, temporarily and locally, against the thermodynamic torrent. Darwin’s phrase ‘cycling on according to the fixed law of gravity’ is more than just a poetic device to express the relentless and unimaginably extended passage of time.

  Being in orbit around a star is the only way a body can remain a relatively fixed distance away from a source of energy. In the vicinity of any star – and our sun is typical – there is a finite zone bathed in heat and light, where the evolution of life is possible. As you move away from a star into space, this habitable zone dwindles rapidly, following the famous inverse square law. That is, light and heat diminish not in direct proportion to the distance from the star, but in proportion to the square of the distance. It is easy to see why this must be so. Imagine concentric spheres of increasing radius centred on a star. The energy radiating outwards from the star falls on the inside of a sphere and is ‘shared’ evenly by every square inch of the internal area of the sphere. The surface area of a sphere is proportional to the square of the radius (ESK).* So if sphere A is twice as far from the star as sphere B, the same number of photons has to be ‘shared’ over an area four times as great. This is why Mercury and Venus, the innermost planets of our solar system, are scorching hot, while the outer ones, such as Neptune and Uranus, are cold and dark, although still not as cold and dark as deep space.