The Greatest Show on Earth (40 page)

‘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.
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.
The Second Law of Thermodynamics states that, although energy can be neither created nor destroyed, it can – must, in a closed system – become more impotent to do useful work: that is what it means to say that ‘entropy’ increases. ‘Work’ includes things like pumping water uphill or – the chemical equivalent – extracting carbon from atmospheric carbon dioxide and using it in plant tissues. As already spelled out in Chapter 12, both those feats can be achieved only if energy is fed into the system, for example electrical energy to drive the water pump, or solar energy to drive the synthesis of sugar and starch in a green plant. Once the water has been pumped to the top of the hill, it will then tend to flow downhill, and some of the energy of its downward flow can be used to drive a water wheel, which can generate electricity, which can drive an electric motor to pump some of the water uphill again: but only some! Some of the energy is always lost – though never destroyed. Perpetual motion machines (you can’t say it too dogmatically) are impossible.
In life’s chemistry, the carbon extracted from the air by sun-driven ‘uphill’ chemical reactions in plants can be burned to release some of the energy. We can literally burn it in the form of coal, which you can think of as stored solar energy, for it was put there by the solar panels of long-dead plants in the Carboniferous age and other past times. Or the energy may be released in a more controlled way than actual combustion. Inside living cells, either of plants or of animals that eat plants, or of animals that eat animals that eat plants (etc.), sun-made carbon compounds are ‘slow-burned’. Instead of literally bursting into flames, they give up their energy in a serviceable trickle, where it works in a controlled manner to drive ‘uphill’ chemical reactions. Inevitably, some of this energy is wasted as heat – if it were not, we’d have a perpetual motion machine, which is (you can’t say it too often) impossible.
Almost all the energy in the universe is steadily being degraded from forms that are capable of doing work to forms that are incapable of doing work. There is a levelling off, a mixing up, until eventually the entire universe will settle into a uniform, (literally) uneventful ‘heat death’. But while the universe as a whole is hurtling downhill towards its inevitable heat death, there is scope for small quantities of energy to drive little local systems in the opposite direction. Water from the sea is lifted into the air as clouds, which later deposit their water on mountaintops, from which it runs downhill in streams and rivers, which can drive water wheels or electric power stations. The energy to lift the water (and hence to drive the turbines in the power stations) comes from the sun. This is not a violation of the Second Law, for energy is constantly being fed in from the sun. The sun’s energy is doing something similar in green leaves, driving chemical reactions locally ‘uphill’ to make sugar and starch and cellulose and plant tissues. Eventually the plants die, or they may be eaten by animals first. The trapped solar energy has the opportunity to trickle down through numerous cascades, and through a long and complex food chain culminating in bacterial or fungal decay of the plants, or of the animals that prolong the food chain. Or some of it may be sequestered underground, first as peat and then as coal. But the universal trend towards ultimate heat death is never reversed. In every link of the food chain, and through every trickle-down cascade within every cell, some of the energy is degraded to uselessness. Perpetual motion machines are . . . all right, that’s enough repetition, but I won’t apologize for quoting, as I have done in at least one previous book, the marvellous saying of Sir Arthur Eddington on the subject:If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations – then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation – well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.
When creationists say, as they frequently do, that the theory of evolution contradicts the Second Law of Thermodynamics, they are telling us no more than that they don’t understand the Second Law (we already knew that they don’t understand evolution). There is no contradiction, because of the sun!
The whole system, whether we are talking about life, or about water rising into the clouds and falling again, is finally dependent on the steady flow of energy from the sun. While never actually disobeying the laws of physics and chemistry – and certainly never disobeying the Second Law – energy from the sun powers life, to coax and stretch the laws of physics and chemistry to evolve prodigious feats of complexity, diversity, beauty, and an uncanny illusion of statistical improbability and deliberate design. So compelling is that illusion that it fooled our greatest minds for centuries, until Charles Darwin burst on to the scene. Natural selection is an improbability pump: a process that generates the statistically improbable. It systematically seizes the minority of random changes that have what it takes to survive, and accumulates them, step by tiny step over unimaginable timescales, until evolution eventually climbs mountains of improbability and diversity, peaks whose height and range seem to know no limit, the metaphorical mountain that I have called ‘Mount Improbable’. The improbability pump of natural selection, driving living complexity up ‘Mount Improbable’, is a kind of statistical equivalent of the sun’s energy raising water to the top of a conventional mountain.* Life evolves greater complexity only because natural selection drives it locally away from the statistically probable towards the improbable. And this is possible only because of the ceaseless supply of energy from the sun.