Introduction

While Salvador Allende’s social democratic regime was being overthrown by the Chilean military in 1973, Swedish-Argentinean cameraman Leonardo Henrichsen was filming the drama on the street outside the presidential palace in Santiago. In Henrichsen’s footage we see an officer turn to glower at the camera, raise his pistol towards us and then to fire. The film jerks but continues and the officer turns away dismissively. An ordinary soldier fires a second shot, misses. A third soldier joins in, fires two shots. The footage jerks. His rifle jams, yet the camera continues to roll. After his third shot the scene falls to the sidewalk. The unfortunate Henrichsen felt himself isolated from events because he was peering through a lens. Many scientists consider themselves to be part of such a Fourth Estate, dispassionate observers as the world unfolds.

Being ‘objective’ supposes that any part of the material world, indeed the whole of it, can be ring-fenced and observed without bias from beyond its perimeter. It demands not only excluding the observer from the observations, but in many cases ignoring everything except a particular phenomenon. Any social scientist, an anthropologist, psychologist or even a student of other animals’ behaviour knows that this approach is, for them, an unattainable delusion. Is it possible to be ‘objective’ in the study of things that do not move perceptibly, such as rocks, or which are immensely remote, such as planets, stars and galaxies, or even the time spanned by their history?  Earth’s history goes back for almost 5 million millennia, nearly 5 billion years, so surely Earth scientists who attempt to grapple with it can be detached from its course?

The science of geology amounts to this: people with special skills, experience and knowledge examine rocks and time’s record of events locked in them, but they are people above all else. Their objective is not rocks and the information that they present, nor even the history of the world and the way it works, but a general enrichment of human knowledge through each individual’s self-enrichment. Rocks, like anything else, are pored over for entirely human motives not for their own sakes; so that canals might safely transport coal; to find gold and, more important, water; to warn of impending disaster; to understand why soils are fertile or not; to know why landscapes are the way they are; even for curiosity or a cussed feeling that everyone else is wrong!  Studying the Earth can never be ‘objective’. Scientists can end up as nervous wrecks striving to attain the god-like standard of ‘objectivity’. Others can sit smugly on the sidelines flinging the ultimate epithet of ‘subjectivity’, of allowing the human element in. But that is what people do, in every walk of life, simply because they are human; not ‘only human’, but conscious, self-creating beings whose only true wealth is shared knowledge of their world and the culture that it supports.

It is trite and mechanical to look on our planet either as a machine or as some sort of system described and modellable using mathematical equations or the ‘thought-experiment’. But a lot of scientists have a go. Earth is inseparable from the rest of the cosmos, and is not subject to special laws of its own. Processes that act upon and shape our home world span every conceivable scale, from that of galaxies down to sub-atomic interactions. Equally, their pace differs by many orders of magnitude, from hundreds of millions of years to those which occupy times less than a microsecond. Space and time are not the only dimensions involved, for the myriad processes consume or produce energy and, depending on their scale and rate, have different powers. Work is and always has been done in the natural course of events. Those thermodynamic measures have underpinned a continual transformation of matter in all its forms. Despite the seemingly enduring nature of rocks, they record above all else the progress of change, both gradual and revolutionary.

We can now point with considerable precision to a time when the Earth came into being. Since then it has evolved, without repeating any stage in its history. Its fundamental components, the chemical elements, were largely in place at the outset. How they have interacted chemically with each other has had much to do with their relative proportions, but even more so with the physical conditions that prevailed upon and within the planet. Those conditions provided several unique windows of opportunity that set our planet on its evolutionary course. One of them permitted the assembly of information-rich molecules capable of exactly reproducing themselves, and opened the way to life. Life’s survival and evolution has not simply been determined by the course of inorganic events, but contributed itself to shaping overall conditions, even extending its indirect influence to the Earth’s interior. But all did not proceed cosily. Unimaginably high deliveries of energy and matter, which humans have never yet witnessed, fortunately, punctuated the co-evolution of Earth and life at surprisingly regular intervals. Such powerful events came in two ways, literally out of the sky and through cataclysms driven by forces thousands of kilometres beneath the surface. A gentler touch, outwith living and geological processes, also conditioned the surface environment, and continues to do so. Gravity in the planetary system around the Sun slowly fluctuates in a complex, but nonetheless predictable way, because massive planets orbit slightly out of kilter beyond the realm of Mars. This affects the Earth’s orbital geometry, and always has. The amount of heat from the Sun and the seasons continually change through an astronomical control over weather in the very long term.

Learning how the home planet works means following a web of interconnected threads that lead everywhere. No single person can hope to follow all these sidelines, but they constitute what nature is, and people are irretrievably bound up in its web. Studies of all aspects of Earth and the life upon it have, over the last decade or so, reached a stage where the fabric’s weave has suddenly snapped into focus, transforming our view of ourselves into the bargain. As far as I have been able, I have tried in Stepping Stones to link some of the more fruitful, intriguing and revolutionizing alleys in the science of the Earth. This has required picking up threads from studies that outwardly might seem divorced from the popular image of the lonely geologist with hammer and sample bag. As well as being a necessary device to show how natural processes unfold from their roots, it is a reflection of what unites all knowledge – the development of humanity in its entirety.

Humanity is surprisingly easy to define. We are the only living things known to us that are potentially free to define themselves!  That is neither a clever play on words, nor a mere attribute of being naturally conscious and curious. Humans set out to change the world through their actions as social beings – nothing could be clearer from today’s world economy, nor from the simplest of human societies. In doing so we transform ourselves, both in our consciousness and physically. Our bodies, our culture, the way we view our surroundings are the evolutionary products of about two hundred thousand generations of human activity. Our own evolution built on those of all earlier biological processes on the Earth and the physical and chemical forces that shaped the inorganic environment. Evolution by natural selection favours those individuals (and the genes that they carry) that are more likely to survive the conditions of life to reproduce successfully ‘down the line’ of later generations. That is the essence of Darwin’s much abused phrase ‘the survival of the fittest’. Fitness is conferred by genes that control the potential of an organism to pass on those genes to its progeny. Whether or not it succeeds lies beyond genetics and within its physical and biological surroundings, and in what it does. The most fit individual imaginable can succumb to lightning, a rock fall or a hidden poison. The least fit might strike it lucky!. Evolution is neither determined by genes, nor is it the outcome of pure chance, but an interplay between both, within a complex and continually changing world. To a biologist, the outcome of perhaps four billion years (Ga) of evolution might seem dominated by genetic factors, as Richard Dawkins has argued in his book The Selfish Gene, but reality transcends his narrow, deterministic view. The emergence of organisms that consciously change the world to improve their chances of survival adds a completely new dimension. They become increasingly in control of their own destiny. Part of nature, they are in a conscious battle with it.

Humanity first showed itself 3.3 million years (Ma) ago. To most people conditioned by the notion of our ancestral species as club-wielding Flintstones huddled deep within a cave that will come as a surprise and perhaps a provocation. That there were conscious beings so long ago springs out from a single discovery. Gravels and silts of that antiquity, in parts of Kenya, contain sharp-edged stone objects. They are not particularly impressive, indeed a sceptic might claim that similar broken bits could be found in the back garden. Closer examination reveals that the sharp edges are not the result of a single clean break, as might happen if one pebble banged into another. Instead, they are jagged from repeated smaller breaks. They all fit snugly in the hand, without annoying edges that cut the fingers (surprisingly most fit best in the right hand). They are, without doubt, tools that someone deliberately carried to the site after careful selection – none of the objects are made from rocks that will not take and retain an edge. The variety of shapes suggests a tool kit, including scrapers and borers (with possible connotations for the use of skins for clothing). The problem is that there is no fossil sign of whoever made and used them. But humans were definitely around. There is an obvious reason for that conclusion. No other species makes tools, however rudimentary. In fact, without being shown how to craft one of these choppers or cutters, you or I would take days of trial and error (with some nasty wounds) to become proficient.

Humanity’s hallmarks are making and using tools in a social milieu. Sure, many other animals display seemingly social graces, such as mutual grooming and communal hunting, and some even pick up natural objects and use them for one function or another. But not one transforms natural objects, and none use them within a social structure. It is quite common to see scraps of what we do among this or that species, be it bird or mammal, but outside of our own all these attributes are never assembled together. Tools are parts of the natural world, taken from it and transformed, to be used as means of intervening in natural processes. How such materials are transformed to become tools is passed on as a central element in culture, as part of a universal sharing of labour and goods, whatever economic form this takes, and within whatever social group we belong to. Through tools we humans consciously create new conditions that improve our chances of survival; a sort of ‘buffer against nature’ taken from the rest of the natural world, yet used within it. Our fitness is conferred by what we create, added to our physical attributes that stem from genetic inheritance. That is our biological uniqueness. Billions of individuals in the human line survived to reproduce, not because they were necessarily fit in a purely biological sense, but because they were cushioned from ‘nature, red in tooth and claw’ by their own conscious efforts and those of other people, increasingly through tools.

So, finding an ancient tool is more than a cause for celebration. It shouts ‘Human!’, whether or not its maker brachiated through the trees, had a brain the size of a walnut, or the table manners and face of a baboon. Whatever, he or she was conscious. No discovery raises the hairs in the back of the neck so dramatically.

‘My uncle probably dropped it!’ joked Tesfamichael, on revealing a fine hand axe that he had just found by a spring in Tigray, northern Ethiopia. It was made of flint, certainly not a rock that a geologist would expect in that part of north-east Africa, and assuredly one that its maker had carried from far off. Thereby hangs a tale that says a great deal about the ‘objectivity’ of science. Fourteen Land Cruisers packed with geologists from three continents and two dozen cultures were making their dusty way around Tigray and the neighbouring country of Eritrea. Not one of us, including Tesfamichael, had come looking for hand axes. In March 1996, we had an earnest objective and a unique opportunity. Until five years before, a savage and generally unreported war had raged across both countries. No geologists, except those who had volunteered to liberate their countries from an iron-heeled dictatorship, had roamed these hills for more than thirty years.

What drew us there was the chance to learn how part of a supercontinent called Gondwanaland (after the Gondi people of western India that once formed a part) had assembled from beaded volcanic islands strung across what was once a great ocean. This had happened between 900 to 650 Ma ago, when living things were neither big enough nor had parts hard enough to survive as fossils. The Earth was very different then. Rugged Eritrea and Tigray have more unvisited exposures of the evidence for these events than you can poke a stick at. Some of us hoped to check if the area was like those of similar age elsewhere in Africa and Arabia, and even in parts of northern Scotland; perhaps new research opportunities might open up. Others eyed the chances of rediscovering the Queen of Sheba’s lost gold mines, for the Tigrinya-speaking people as well as Haile Selassie’s offspring claim descent from her dalliance with Solomon himself. Gold there is, and it concentrated in pockets as the volcanoes rose and then were crushed together. One or two would in another life have collected matchboxes or gazed at locomotives. Their clear intent was to grab the oldest-looking samples and scurry off to date them to help complete their African ‘set’. Margaret, a recent convert to Earth science, was as fascinated as any of us by the ‘Pan African’, but had another quest. She hoped to begin research on somewhat younger rocks in the area, which were reputed to contain evidence for a glacial epoch about 250 Ma old. Not that her thoughts were on icy processes, for a group of NASA scientists had speculated that bouldery clays – traditionally equated with glaciation – might also form by the catastrophic blast of comet impacts. At 250 Ma ago more living things were extinguished than ever before or since; Tigray might just reveal a link. The majority were young Eritrean and Ethiopian geologists whose careers had been blighted by war and isolation from the mainstream of their subject. Tesfamichael had helped organize this ‘field conference’ to put his younger colleagues in touch with the most experienced international geologists who cared to come. So those of us from far afield had a wider responsibility than pondering on what we saw. Wrinkles of knowledge, tricks of the trade, discussion of ideas and, if money could be found, opportunities for overseas study were our side of the implicit bargain.

Tesfamichael found the hand axe shortly after observing that we were driving on a plain underlain by the 250 Ma old boulder clay. We crossed a stream that 100 metres from the road plunged into a chasm leading to the Tekesse River. A short walk down should reveal at least a glimpse of Margaret’s objective. But the gorge was horrific, so back we clambered, Tesfamichael in the rear thoughtfully studying the ground beneath his boots. For an ex-fighter, this was an old habit, for who knows what useful strand of wire might appear, or even a land mine  We did eventually reach a magnificent outcrop of boulder clay a few days later, but it proved a let-down; it seemed to have been deposited by raging torrents. All its cobbles were rounded and untouched by ice or comet. The experience with an ancestor did leave its indelible mark though. As time and tyres wore on, the heat grew as we descended to the lowlands of western Eritrea. Patience, tempers and ‘objectivity’ frayed as the expedition became more political than geological. There was another agenda. On the pretext of visiting the ‘oldest’ rocks – which woke up the ‘train-spotter’ – we were formally introduced to the new Eritrea.

It is a chastening experience to sleep in a barracks, across whose floors scuttle large scorpions, waving their claws menacingly like demented out-fielders. Sawa had been a liberation-fighters’ stronghold, but is now a remote camp for thousands of urban Eritrean conscripts on ‘National Service’, about 20 kilometres from the Sudanese border. We were on the front line across which Islamic fundamentalist terrorists infiltrate from the Sudan. Relations between the two neighbours are not good. Mohammed, now a professor in the USA, is Sudanese. There were Ethiopians that had shed Eritrean blood on these hot, dusty plains. Those of us from Europe knew full well what horrors nationalism can lead to. There were misgivings as soon as it became plain that we were about to take leave of scientific protocol. Brian, a geophysicist, has been to some very odd places indeed. He could easily have become anxious, bored and fractious crossing the Barka gravel beds. Instead, looking at his boots for other reasons than Tesfamichael’s, he began seeing stone tools!  Not the clear sign of that leap in consciousness from artifice to art signified by a symmetrical hand axe – visualizing a useful shape in a stone that waits to be liberated by design – but something more primitive. What he found were tools of the kind that date back three and a half million years. Even today, local nomads knap bits of hard, brittle rock to make disposable butchering tools (they value their fine steel knives enormously), so Brian had not necessarily found a site on which archaeologists might swoop, but he lightened the near mutinous mood.

Eritrea stands across the route that took humans out of Africa to colonize the planet in wave after wave, starting around 1.8 Ma ago. Its geological grain conveniently runs north-south, and so do its mountains and plains. Everywhere there are more recent cairns and small monuments to the passing of nomadic sheiks and the martyrs to its liberation struggle. Danakil, now the hottest place on Earth and 150 metres below sea level, is one of these routes pointing to more hospitable regions. Two million-year old outcrops in Danakil contain extinct elephants, hippos, antelopes and pigs, and there too are remains of humans, those who did make and use tools like those that Brian found. As well as humanity’s recent signature, the near-billion-year old dockyard where volcanic islands eventually welded together a supercontinent, and the misconceived evidence for glaciers or a comet’s hot breath, two other sets of features command attention in this scarred land. One draws the eye instantly.

Everywhere on our route appropriately blood-red cliffs atop a leathery landscape, mottled in oranges, yellows and creams, seemed to lie around every bend. The cliff-forming rocks look like massive brick walls. And indeed that is their only use, both as stone building blocks and as feedstock for brickworks that post-war reconstruction so badly needs. They are red because they are held together literally by rust, an oxide of iron, and clay. Though now riven with gorges it is plain to see that they form a sheet, no more than ten metres thick, capping the more ancient Pan African basement. At their highest they rise three kilometres above sea level, but descend gently west to a mere four hundred metres, and lie at the foot of the escarpment that plummets eastward more than 3000 metres in 50 kilometres to the Red Sea. These sporadic tablelands define a bulging and a tearing apart of the crust beneath, that came after the blazing red rocks were laid down flat as a pancake across not only Eritrea and Tigray, but most of Africa. The cliffs are formed of a soil named laterite after the Latin word for bricks. Such soils form today beneath tropical rain forest. Those in NE Africa contain no fossil trees because the conditions of their formation were intensely corrosive, and that is why they sit on this leathery landscape. The once crystalline basement beneath is rotted through and through, even to depths of a hundred metres or more. Minerals formed at igneous temperatures and modified by continental squeezing at depths once as far down as 10 kilometres succumbed to noxious juices leaking from rotting plant life. The laterites show that much of Africa when they formed, about 40 to 50 Ma ago, was an immense jungle swamp; hot and amply supplied with rainfall. Disappointed that all on the expedition knew little about laterites and simply dismissed them as superficial soils, Margaret determined to map and analyse them.

Plastered across the top of the old red soils are layer upon layer of basalt, once molten lava that poured out to spread far and wide. The lavas that rise to more than 4 kilometres form plateaus, once connected across the Red Sea to the Yemen Highlands. Averaging 2 kilometres thick, these plateau basalts used to extend across 1.5 million square kilometres, so the total effusion of molten magma that built them up was around 3 million cubic kilometres. If that is not impressive enough, the bulk of the eruptions 30 Ma ago lasted only about a million years – on average three cubic kilometre per year. A single flow can extend over hundreds of thousands of square kilometres, signifying that the lava was very fluid and spewed out in volumes that dwarf any observed today. Flood basalt is a particularly apt term. The Ethiopian basalt flood is one of the youngest of several, but by no means the largest. We can be relieved that events of this kind are not happening today, although there is a rough pulsation recorded in those of which we know. Its periodicity is … about 30 Ma!   Whatever their origin, drenching the surface with lava at more than 1200oC was not their only outcome. Magma contains dissolved gas that froths out in the manner of soda pop when the pressure drops. The most abundant, water vapour and carbon dioxide, trap heat in the atmosphere, while the next, sulphur dioxide either falls as sulphuric acid rain or resides as reflective droplets in the upper atmosphere. Basalt floods present the choice of frying, steaming, chilling or being corroded. For life they are stressful. Such events have played a big role in our planet’s evolution and they are implicated in life’s long-term ups and downs. Their driving force seems to be a periodic inability of the deep Earth to digest huge lumps of its outer rind that continually sinks into it.

Like the laterites always immediately beneath them, Ethiopia’s flood basalts bulged and rifted in more recent times. These particular deformations had a signal effect on our own origins. They divided a once monotonous continent into two climatic domains along a great system of rift valleys and flanking mountains that runs from Israel to Mozambique. Africa to the west stayed humid and forested. The east dried and forests were displaced by grassland and savannah, which forest apes now trapped east of the rift had to cope with. The major rifting and swelling dates only to 5 to 10 Ma ago, when it seems that our own line of descent in East Africa parted company with that of our nearest ape relatives, the chimpanzees of the west. Arabia split away to focus any diffusion from the human birthplace along the Red Sea coast, smack bang through Eritrea.

The Horn of Africa is one of a few microcosms that yield evidence for several great events in the Earth’s past, but it is a unique one that gives us a surprisingly detailed glimpse of what underlay our own origins. Even the most blinkered geologists, no matter where they work, cannot really hide from the full wealth that records times gone by. Nor can they escape the rest of human activity, much as some might try. Which of those scientists who visited Tigray and Eritrea carried most away, those who stuck rigidly to their ‘objectivity’, or those who were sidetracked by what they stumbled upon?  Stepping Stones follows the upheavals in knowledge set in motion by those scientists who dared to stray from the straight and narrow. Margaret, by the way, completed her research on laterites (PhD, M.E. Andrews Deller, British Library); they are no longer merely reddish fossil soils to the scientific community.

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