Remembering Richard Thorpe and Norman Harding – friends whose humour and outlook I miss.


‘It is an eternal cycle in which matter moves, a cycle that certainly only completes its orbit in periods of time for which our terrestrial year is no adequate measure, a cycle in which the time of highest development, the time of organic life and still more that of the life of beings con­scious of nature and of themselves, is just as narrowly restricted as the space in which life and self-consciousness come into operation; a cycle in which every finite mode of existence of matter, whether it be sun or nebular vapour, single animal or genus of animals, chemical combination or dissociation, is equally transient, and wherein nothing is eternal but eternally changing, eternally moving matter and the laws according to which it moves and changes. But however often, and however relentlessly, this cycle is completed in time and space; however many millions of suns and earths may arise and pass away; however long it may last before, in one solar system and only on one planet, the conditions for organic life develop; however innumerable the organic beings, too, that have to arise and to pass away before animals with a brain capable of thought are developed from their midst, and for a short span of time find conditions suitable for life, only to be exterminated later without mercy – we have the certainty that matter remains eternally the same in all its trans­formations, that none of its attributes can ever be lost, and therefore, also, that with the same iron necessity that it will exterminate on the earth its highest creation, the thinking mind, it must somewhere else and at another time again produce it.’

Frederick Engels Dialectics of Nature, ca. 1876

Writing Stepping Stones: The Making of Our Home World in the late 1990s was fun; a self-indulgent opportunity freely to explore a history that is also our own unimaginably long and tortuous inheritance. The project grew from my having fallen into an odd career. Since 1972 I had worked at the British Open University as a member of course teams aiming to teach lone, distant students about the breadth of Earth science. Working in teams of up to two dozen gave me three points of departure: a grasp of how to engage the solitary learner; a lively engagement with colleagues who carried different intellectual baggage from my own; and a growing urge to express as an individual what I had learned (but often suppressed) while teaching ‘in committee’ for more than two decades. Oddly, the eventual trigger for me to begin writing independently and freely was reading a book that W.S. Deller had said should be read by all scientists – Steven Pinker’s The Language Instinct – one of a growing genre of popular but revolutionizing syntheses that draw together, and so begin to free sectors of modern science, previously corralled behind specialist academic subdivisions, to anyone who has a curiosity about the natural world.

The 1990s seemed to demand that Earth science too should be brought to a wider audience. Bar a few books with a broad appeal that took ancient life and its fossils as their topic, my own subject had done little to shed its popular image of earnest, anorak-clad figures glimpsed in road cuttings or as faint dots on far-off, moist crags. There will always be some truth in that, but geology is full of surprises. Bits and pieces, usually of the scary-monster and disaster-movie kind, did get a public airing now and then. Continental drift, the meteorologist Alfred Wegener’s brave idea at the start of the 20th century, was the lone ‘coat hanger’ for a wider view of the Earth, outside geology’s stalwart ranks. Those ranks were also thinning, from a lack of funding for its traditional work – charting the Earth in space and time – towards more mundane matters and skills for a shrinking market place. Few geologists launching an oddball career dream of gravel or where to site refuse dumps. Billions of years that shaped a planet and its occupants attract curious people. Because most of that history has left a cryptic record, a vivid imagination rather than a stout pair of blinkers is obligatory kit.

Around 1972 I read Frederick Engels’ Dialectics of Nature. I am not keen on epigraphs, and use only one, at the head of this Preface. Engels’ two monumental, final sentences in his Introduction expressed for me both curiosity and an outlook of continual change. They show his audacity and breathtaking optimism that are hard to match anywhere in literature today. It sustained me through the premature obituaries for science, politics,  society, history and of any opportunity for moving forward as humans that 18 seedy years from 1989 imposed on Britain, and to some extent the rest of the world. Stepping Stones sets out to break from the ethos of ‘There is no alternative!’ (M. Thatcher). I try to view what we now know of the world with the delight that Engels took in the interconnection of its parts and the surprises that its unfolding presents to those who choose to study it.

In the last three decades, old certainties in geology have begun to tumble before every kind of question. Thanks to a flood of technical advances, either borrowed from other spheres or developed by Earth scientists themselves, there has been an explosion of new information. It is that more than anything that throws up questions. Attempting to answer them unmasks an all-sided, multi-scale, multi-rate turmoil rather than a set of eternally fixed laws. This emerging picture of continual motion and change was masked since the founding of our branch of natural science by the very ‘laws’ that some of its founders imposed on it. It is perhaps no accident that its birth while industrial capital began to flourish was followed by more than two centuries of faltering development in Earth science, as that economic system met with crisis upon crisis. It is ironic that today’s globalized capital, in a riot of disorder felt daily by billions, coincides with a period of new insight in every branch of science. There is a drawing-together of knowledge not felt since the Renaissance, when the dark ages of feudalism were approaching their nemesis. Stepping Stones tries to reflect that unification by weaving into the Earth’s complex story fundamental threads from chemistry, biology and physics.

Science is not only done by humans; primarily we do it for ourselves. Exploration and curiosity are not for the sake of their object, the surrounding universe, but in order better to harness it to human ends. The last Part of Stepping Stones sees those matters of the spirit and of being in relation to early humans’ uniquely transforming parts of their world in order to survive over times of wild fluctuations. Through making rudimentary tools the first humans changed their relationship to the rest of nature. That laid the basis for changing themselves into beings with increasingly conscious power over their surroundings. Gradually, it expanded their horizons to the entire planet and beyond. Through the book’s account of the Earth’s history won from rocks runs a central strand of the chemical preconditions for life, its origin, evolution and survival of barely imaginable upheavals. This is our link to the rest of the universe and processes which operate irrespective of life and consciousness.

Life is a fragile and possibly rare phenomenon on the cosmic scale. Chemical processes that lie at its root can only function within strict limits of temperature, pressure and the concentration of matter. On the only known inhabited planet life is peripheral, a thing of the outermost skin. It extends a mere 2 to 3 kilometres beneath the rocky surface, a little more in the oceans and it interacts with air and incoming solar radiation. Life exists within a narrow, not-too-hot, not-too-cold envelope around a common-or-garden star. That environment presents a window of inorganic chemical opportunities among about 15 chemical elements. They assembled as complicated molecules to became self-replicating and therefore truly living. Life’s molecules are made up mainly of just four of the most abundant, light elements in the cosmos: hydrogen, carbon, oxygen and nitrogen. That this foursome is so overwhelmingly common stems from the way in which stars fuel themselves. Earth’s own star, however, is incapable of producing any but a tiny proportion of carbon from the simpler, less energy-demanding processes that fuse hydrogen to make helium. Life’s heavier chemical elements and those comprising its home world must therefore have arrived from other parts of the cosmos. They can only have formed in supermassive stars and in the cataclysmic death throes of such giants.

Springing from an inorganic setting with tight limits, life is indissolubly bound to it. It interacts with all parts of its bubble-like milieu, depending on it yet modifying it by its own selective chemical processes. Changing conditions demand that life itself performs a chemical balancing act. On Earth, and quite likely anywhere else that it exists, life goes on because of a chemical equilibrium between two simple molecules, water and carbon dioxide. They form carbohydrates, which living processes use as both building material and fuel. Carbon dioxide oddly acts as a way-station for energy received from our star by delaying part that would otherwise immediately be radiated back to space. Without this so-called ‘greenhouse’ effect, Earth in its early stages would have been icy, and so reflective that the feeble, early Sun might never have warmed it to support liquid water at its surface – carbon-based life could never have emerged. The process of living draws down this gas, particularly when sediment buries dead tissue. That is just as well. Without life, and while its star slowly heated as it evolved, the CO2-veiled Earth would probably have become the kind of sterile inferno that its twin Venus is today. Quite predictably, this simple gas is a central player in Stepping Stones.

Looked at just as a relationship between life and its basic building blocks, conditions in the inhabited bubble must be regulated once life arises to mediate between solar heat and the gas that helps retain it, or so simple mathematics demand. So long as the Sun refrains from ‘going out’, or more likely expanding eventually to engulf its planetary system, it might seem that life not only regulates itself but its supporting bubble too. In short, life as a chemical phenomena sustains its own conditions, the most general part of which is climate. That balancing is a second thread in Stepping Stones. But it is not the profoundly comforting fable first brought to wide attention by James Lovelock’s Gaia metaphor for the living world. There are other strands of which ‘Gaians’ scarcely speak, if at all. One surprise among many is another all-pervading, but hidden chemical agenda, in which the metal iron figures repeatedly. That is central to all living things, once acted as a regulator for the composition of air and water, and is a major metal inside the Earth. It links life’s bubble with what lies beneath it.

Because it assembled from matter flung out by long-dead stellar catastrophes, the inner Earth contains a few elements whose nuclei have been unstable from their birth. By disintegrating at a regular pace they too release energy, whose passage to escape from the surface sets the depths of the Earth in slow yet continual turmoil. This inner motion, which geologists now resolve, in part, as the opening and closing of oceans with the drift and collision of continents, forms a more familiar third theme. That underlying process involves two things: first, a supply to the surface of new material, dominated by molten rock, that is balanced by the ultimate resorption of older crust to the deep interior; second, perpetual changes in the Earth’s surface shape – its elevation, and how land and oceans are disposed. The second bears fundamentally on how solar heat held briefly in air and water moves around the planet. The first brings with it gases that escape from pressurized solution in molten rock. Among them is carbon dioxide – a source of climate change that is largely independent of surface events. That is crucial, for Earth’s release of its inner energy is not so steady as once believed. Intermittently it sheds stored heat in great belches of gas-carrying magma. Such events link to evidence for episodes of climatic heating, and also to periodic crises for life, some that almost extinguished it. Such seemingly odd coincidences are not restricted to the Earth’s own behaviour alone.

Earth is a far livelier planet than its rocky neighbours. Their senescent faces record awesome encounters with much smaller worlds. Asteroids and comets perturbed by complex gravitational forces pay visits from far off, occasionally to pockmark the bigger planets and moons. Earth cannot have escaped such bombardment, but its continual repaving hides most of the evidence. Biological evolution links with both our planet’s inner workings and those of the Sun-centred envelope that birthed and sustains life. The 150 million-year reign of dinosaurs met Nemesis in the shape of a major asteroid strike 65 million years ago, but that now seems to be one astronomical event of many. Strangely, the timing of this impactor matches with one of the biggest volcanic upheavals. Other mass extinctions that form the main boundaries in geologists’  timescale seem more likely to reflect internal and superficial upheavals with directly terrestrial driving forces. Each near lifeless world formed an empty ‘pool’ into which surviving organisms launched renewed adaptive radiations from their genetic heritage. Life’s course owes much to chance and none to design, except that made by itself in the wider context of how the world works irrespective of its passengers.

Our own origins came in strange times, while our planet’s surface cooled. That epoch, which continues now, was mainly a consequence of shifting continents and multifarious drawings down of carbon dioxide from the air. At the depth of this cooling it took on a strange and regular dynamism of frigid and warm periods, which connect to the emergence of the first conscious humans in Africa and their successors’ repeated outward migrations. Climatic pulses match closely, but not exactly the wobbles of the Earth’s motion through space, brought on by other worlds’ changing gravitational influence. The deviation of climate’s oscillations from an astronomical metronome show that Earth’s own web of systems has a beat as well. Actions within the present economic command of human society seem destined to intervene in this complicated ‘forcing’ of climate, perhaps to bring chaos in the manner of a fibrillating heart. Whatever our reflections on our surroundings and their history, and whatever we need to do in future are inextricably bound with universal processes that continue independently of the thinking brain. And that brings us full circle to the outlook of Frederick Engels.

Do not expect what you will meet in Stepping Stones to conform to literary niceties; it is not a ‘story’ in the sense of a neatly ordered beginning, middle and end. I chose a sequence that seemed to provide a nice way through the science. Nowhere will you find any ideas chiselled in stone, and perhaps the only abiding ‘rule’ is that everything moves and changes, scientific ideas included. Because its linkages are truly universal, so events on our home world are inherently contradictory and hard to predict. Keeping the brief new spark of consciousness glowing may well depend on a fuller grasp of how our world works.

About the revised edition (2016)

Stepping Stones did quite well in its original guise. As with all books about science it became outdated and declared out-of-print once demand fell. I didn’t re-read it for 15 years, but I thought it might be worth updating when I retired, even if only to stave off the horrors of daytime TV. Plucking up the nerve to do that, four years after I retired from the Open University, to my relief I still liked the style and structure that I had adopted, the main surprise being that it didn’t need a complete rewrite. I had kept in direct touch with all the themes in Stepping Stones through my Earth-Pages blog which Blackwell Science commissioned me to write in 2000. My brief was to keep readers informed about what I thought were the most important developments in the geosciences by summarising and making extemporary critiques of research publications on a weekly basis. In truth, I was allowed to write what took my fancy. Fifteen years of weekly production covered a lot of material, most of which was in the context of Stepping Stones. As with writing the first edition, revising the book has also been fun, despite finding some appalling howlers and thickets of impenetrable prose. I hope that there are no longer so many such ‘sticking points’. Oxford University Press having reverted all rights to me, I decided that the revised edition should be free to all.

Taking things further

Compared with a textbook or one intended for a coffee table, Stepping Stones is not copiously illustrated. But compared with 1999 the web is bulging with superb, freely available pictures and explanatory diagrams on any topic for which you might search. For convenience I have highlighted keywords where they first appear. Use them, or any other topic that I have covered, in the web browser of your choice. Web searches may also help some readers who may find some passages more difficult than their background has prepared them for, while others may want to learn more. I have learned a great deal, very quickly over the last 10 years doing exactly that. It is partly the reason for Wikipedia, and there are a great many other on-line sites that prove to be superb sources

Accessing peer-reviewed publications, such as those in the Reading lists that follow each Part of Stepping Stones, is not so easy. In many cases the ‘paywall’ is a barrier if you don’t have paid-for access to electronic journals through an academic library. However, you can always get to journal web sites to view abstracts and find authors’ addresses; there’s no harm in asking an author for a PDF of a paper that you particularly want. Open Access is slowly becoming more common, and it is also worth tracking down authors to their web presence at host institutions, where you may find free access to pre-print (un-reviewed), post-print (reviewed and corrected) or even published versions of papers. In the Reading lists, which are as up-to-date as possible, I have tried to find free links for recent publications. Additionally, since 2000 Earth-Pages has commented on hundreds of what I considered to be the most useful articles bearing on many of the themes in Stepping Stones. Most of them are included in the Reading Lists with a link to the relevant Earth-Pages post.

Finally, Stepping Stones is my view of the way the world works and has evolved. Like any other academic, I am not entirely objective and in some cases may be ‘flying a kite’ or simply ‘up a tree’! So, feel free to point out errors and omissions and to comment. One great advantage of electronic publishing is the ease of correction and amplification.


As well as being eclectic, a book like this is inevitably derivative to a large degree. Most information in Stepping Stones came from reading the work of many other individuals, most of whom I have never met. I have borrowed many ideas, giving some my own ‘spin’, though a few are mine. Owing much to many presents a difficult choice; I cannot possibly acknowledge all, so I mention none. The lists of further reading will reveal who were most influential. Several people have kindly read a few parts and commented. Others have listened, criticised and encouraged, even helped me to write what I mean as well as meaning what I write. They are Jef Leinders of the Dutch Open Universiteit, Margaret Andrews Deller and Cyril Smith whose outlooks and wisdom I value. Thanks also to Ian Francis, Earth Science Commissioning Editor at Blackwell Science, who invited me to author Wiley-Blackwell’s Earth-Pages blog. Maintaining a steady flow of commentaries there has been the main avenue to updating and improving the book.

Steve Drury, Cumbria, June 2016


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