Body of Evidence

The most direct legacy that we can leave to future geology is that of our own mortal remains. Today, in reconstructing the long-vanished Jurassic landscapes, we put the mighty, charismatic dinosaurs square in the foreground. This focus we have—well-nigh a fixation—seems to us almost self-evident. Were they not the rulers of their empire, just as we are of ours, literally bestriding their domain as colossi of scale and blood and bone? Their skeletons, avidly sought, intensely studied, painstakingly reconstructed in museum displays, are the symbols of those times, iconic, mesmerizing. Might we not hope for similar awe and reverence from our future excavators? There is no guarantee, of course, that these as yet unborn explorers of a future Earth will share this perspective. Perhaps their focus will be on what, among all the diverse living inhabitants of this planet, is most important in preserving this living tapestry. They may well regard the myriad tiny invertebrates, or the bacteria, of the world as much more important to that (in planetary terms) rare phenomenon, a stable, functional, complex ecosystem. If these future explorers took this view, at the risk of off ending what little there might then remain of our amour propre, they would have a point. Take away the top predator dinosaurs, and the Jurassic ecosystems would have been a little different, to be sure, but no less functional. Take away humans, and the present world will also function quite happily, as it did two hundred thousand years ago, before our species appeared. Take away worms and insects, and things would start seriously to fall apart. Take away bacteria and their yet more ancient cousins, the archaea, and the viruses too, and the world would die. But, let us imagine our excavators as being, in true science fiction style, just as obsessed with their relative position in the food chain as we are. Let us assume that, in their excavation of the Earth’s history, they will be looking for the power brokers of the ancient past, that they will be digging for bones and bodies.

High Water, Low Water

In almost everybody’s natural lifetime, the sea is one of the great unchanging certainties of life. There is land; there is sea; and in between is that magical place, the seaside, which is sometimes knocked about a bit by the waves, but always manages to recover for that next idyllic summer. There are, one remembers, those faintly disquieting legends, about a remarkably well-organized and ecologically aware person called Noah, and about a Deluge. But these, of course, should not be taken seriously. They were a jumpy and superstitious lot, our ancestors, always prone to making up scary stories. It was a good way to keep the children in order. With a longer perspective, things seem a little different. Take any one location on the globe, for instance. Track it over millions of years. At that one location, there may be a change from deep ocean, to shallow sea, to a shoreline, and thence to terrestrial swamps and flood plains. And then, perhaps, to the absence of evidence, a horizon of absolutely no thickness at all within a succession of rock strata, in which a million years or a hundred million years—or more—may be missing, entirely unrecorded. It is that phenomenon called an unconformity, all that is left of the history of a terrestrial landscape pushed up into the erosional realm. On that eroding landscape, there may have been episodes of battle, murder, and sudden death among armoured saurians, of fire, flood, and storm, and of the humdrum day-to-day life of the vast vegetarian dinosaurs, chewing through their daily hundredweights of plants. Of this, no trace can persist. Only when that landscape is plunged again towards sea level, and begins to be silted up, can a tangible geological record resume. The Earth’s crust, as we have seen, is malleable, can be pushed downwards or thrust upwards by the forces that drive the continents across the face of the globe. Many of the sea level changes that can be read in the strata of the archives are of this sort, and mark purely local ups and downs of individual sections of crust, with no evidence that global sea level was anything other than constant.

Dynasties

Developing a methodology is everything in a science. Once you have it, you can go on to extract information, facts—a narrative—from the natural world. To human scientists and non-scientists alike, the use of fossils as evidence of past events on Earth is now taken for granted, is indeed ingrained into popular culture. Dinosaurs, for instance, stalk through our TV screens and cinemas and shopping malls, as virtual animations and plastic models and soft fluffy toys and comic book covers. An Age of Dinosaurs is widely accepted as a long-vanished era, a world lost within deep time. Our extraterrestrial investigators will, at some stage in their studies, be ready to try to recreate for themselves the eras of long-vanished animal and plant dynasties on this planet, to construct a coherent history out of the scattered relics preserved in the Earth’s abundant strata. By coming to understand the Earth’s marvellously regulated heat-release engine, that drives the tectonic plates, they will appreciate the continuous creation and preservation of strata. By getting to grips with the more subtle puzzle of how sea level has risen and fallen, they will have some idea of the finer controls on the preservation of the stratal record. And, as they grapple with these problems, they would undoubtedly try to put the strata themselves into some sort of order, just as did our Victorian and pre-Victorian predecessors. These pioneering geologists, after all, could recognize a prehistory when they saw one, even as they were still far from divining the workings of the Earth machine that lay at the heart of the story they were pursuing. What kind of strata will be available for study, one hundred million years from now? Many, if not all, of the classic fossil localities that we treasure today will have gone forever, eroded into scattered grains of sedimentary detritus that will ultimately accumulate on sea floors of the future. The Solnhofen Limestone of Germany, that yielded the archaeopteryx, will likely be gone. The Burgess Shale of British Columbia, with its wonderful array of early soft-bodied organisms from the Cambrian Period, half a billion years back, is almost certain to disappear, perched as it is high up a fast-eroding mountainside.

The Strata Machine

History is bunk—or so Henry Ford is reputed to have said. Folk memory, though, simplifies recorded statements. What Henry Ford actually told the Chicago Tribune was ‘History is more or less bunk. It’s tradition. We don’t want tradition. We want to live in the present, and the only tradition that is worth a tinker’s damn is the history that we make today.’ So folk memory, in this case, did pretty well reflect the kernel of his views. Henry Ford also said that ‘Exercise is bunk. If you are healthy, you don’t need it; if you are sick, you shouldn’t take it.’ Henry Ford was a very powerful, very rich man of strongly expressed views. And he was quite wrong on both counts. Not having known Henry Ford, interplanetary explorers may have their own view of history. As, perhaps, an indispensable means of understanding the present and of predicting the future. As a way of deducing how the various phenomena—physical, chemical, and biological—on any planet operate. And as a means of avoiding the kind of mistake—such as resource exhaustion or intra-species war—that could terminate the ambitions of any promising and newly emerged intelligent life-form. On Earth, and everywhere else, things are as they are because they have developed that way. The history of that development must be worked out from tangible evidence: chiefly the objects and traces of past events and processes preserved on this planet itself. The surface of the Earth is no place to preserve deep history. This is in spite of—and in large part because of—the many events that have taken place on it. The surface of the future Earth, one hundred million years from now, will not have preserved evidence of contemporary human activity. One can be quite categorical about this. Whatever arrangement of oceans and continents, or whatever state of cool or warmth will exist then, the Earth’s surface will have been wiped clean of human traces. For the Earth is active. It is not just an inert mass of rock, an enormous sphere of silicates and metals to be mined by its freight of organisms, much as caterpillars chew through leaves.

Echoes

It would seem to be like searching for a needle in a haystack. One hundred million years on, strata containing recognizable fossils of multicellular creatures will extend through a 640-million-year span in time, and will have piled up, in total, to many kilometres in thickness. Somewhere in those endless stretches of rock might be the layer in which traces of humanity may be preserved, awaiting discovery by our curious visitors. Would they happen upon this by chance? Or would they be led to it by following a trail of clues, much as a detective is led to the perpetrator of a crime by the ripple effects of the act itself: the wealth gone astray, the scattered victims, the damaged property, the spree in Monte Carlo. In the case of humanity, there have been victims, and damage, and stolen wealth. It has been a quite singular felony. It will leave echoes, collateral changes, that may act, in the far future, as signposts. Some of these may give clear directions to a heist that was quite out of the ordinary. On a planetary scale, in fact. But first, how thick a section of strata should we take, as the geological target to be searched for, analysed, interpreted for signs of a vanished civilization? One might start by taking ten thousand years’ worth. To us, that represents a gigantic stretch of time (just imagine trying to peer ten thousand years into the future). Geologically, it barely counts. There are a hundred such intervals in a million-year span, and a million-year span represents the small change of geological history. Nonetheless, there are good reasons for choosing this duration, for it represents the span during which human activities can be said to have left a detectable imprint upon the geological record—an imprint beyond the odd vanishingly rare bone of an obscure bipedal hominid. Ten thousand years ago, half of the large mammals of the Earth abruptly disappeared, and it seems increasingly likely that this disappearance was mainly the result of hunting by humans.

100 Million Years AD

A storyteller arrives, one hundred million years from now, to tell the tale of the human species. It is an interval that will add a couple of per cent to the age of the Earth and a little under one per cent to the age of the Universe. Geologically, it is the near future. Cosmologically, we are almost there. There will be an Earth, that which we now call our own. On it there will be, very probably but not quite certainly, oceans of liquid water, an oxygen-rich atmosphere, and an abundance of complex, multicellular life. The Earth is abnormal, and that will draw any interstellar travellers in. The spaceship’s sensors—a simple spectroscope will suffice here—will immediately register the highly reactive surface chemistry that is out of any sort of normal equilibrium. An oxygen-rich atmosphere is not normal. Even from a distance of many millions of miles, this will be a planet that is obviously alive. Closer up, the living skin on the planet, regulator of that planetary surface chemistry, can begin to be glimpsed, as the green wavelengths that mingle with the blue of the oceans of liquid water and the brown of the rock surfaces. Our future visitors would not yet be aware of chlorophyll, but that unexpected signal shining through in the light spectrum would certainly arouse their curiosity. Rock, oceans . . . and green stuff. The geography of the Earth, to our own human and contemporary eyes, would look oddly familiar, but distorted: as though remodelled by Salvador Dali. Familiar landmasses will be displaced. But where to? Unfortunately, we cannot predict where the Earth’s continents will be in one hundred million years’ time. Will the Atlantic Ocean continue to widen, and the Pacific Ocean shrink? Will the East African Rift expand into an ocean? Will the continents aggregate into super-continents, as has happened in the past? Long-term tectonic forecasts, like long-range weather forecasts, are subject to such uncertainties that detailed prediction becomes useless; there are simply too many possible alternative futures. Our planet’s physiography will simply be different, one hundred million years from now, though with elements we would find partly familiar, rearranged as though by the hand of some gigantic and playful child.

Perspective

The purest of science fiction. The Earth, in a post-human future, many millions of years hence, being re-explored. By . . . whom? Perhaps extraterrestrial explorers or colonists, just as we now peer at images of rock strata sent back by the Mars landers. Or perhaps a new, home-grown intelligence: say, a newly evolved species of hyper-intelligent rodent. No matter. What would such explorers, of whatever ancestry, find of our own, long-vanished, human empire? A frivolous question, perhaps. But perhaps not. It is hard, as humans, to get a proper perspective on the human race. We know that the Earth has a history that is long beyond human imagination, and that our own history is tiny by comparison. We know that we are animals, and yet we have transcended our natural environment to live in surroundings that, mostly, we have manufactured for ourselves. We know that this created environment is evolving at a speed that is vastly more rapid than the normal evolution of biological organisms or communities. We do not understand, quite, how our created environment and our activities interact with the natural environment, and we do not know what the long-term consequences will be. Let us take one view. We are simply one species out of perhaps 30 million currently inhabiting the planet (reputable estimates range from some 5 million to over 100 million). We are briefly in the golden age of our power, our dominance. But we are destined to extinction also, just as the dinosaurs became extinct. The world will then go on as before. Once a geological age or two has passed, there will be nothing but the odd bone or gold ring to show that we were ever here. In this scenario, comparison with the dinosaurs is apt. They were the top predators of their day, as our single species is now. But consider, also, the differences between us and the dinosaurs. The dinosaurs existed on this Earth for about a hundred million years, and included many species adapted to different environments. Homo sapiens is but one species, and has been around for less than a quarter of a million years, less than a tenth of an average species’ longevity.

Meeting the People

If one has found the ruins of an ancient civilization on a distant planet, and the skeletal remains of some of the civilization-builders amid those ruins, there would follow a flood of questions—if the discoverers possess anything like human levels of curiosity, that is—about just what these beings were like. War-like and aggressive, or peaceful and harmonious? Socially cohesive and communicative, or individualistic? Sedentary or highly mobile? Rational or superstitious? Good or bad . . . What could an impartial observer, coming fresh to such a scene, make of humanity’s actions and habits and motives? It is hard—well, yes, impossible— for us to think through the mind of an extraterrestrial alien. It is hard enough to think through the mind of another human being. In the study of past humanity one might take the example of Stonehenge, a magnificent, geometrically highly ordered structure, designed and constructed by members of our own species with great ingenuity and eff ort. It served, quite obviously, a highly important purpose. But just what was that purpose? All we have is the structure itself, and the archaeological evidence that surrounds it (for there is a stratigraphy too in such studies, of placing physical evidence within a time framework). We have lost all contact with the constructors, for the chain of word-of-mouth history-telling has long been broken, and if any kind of written records were ever produced, none have been unearthed. But this is an artefact of our own species, remember, and there are many potential parallels, in modern and ancient culture, that we can draw on. Yet basic questions about its function continue to be debated. Was it produced for ritualistic purposes, from deeply felt religious motives? Or for purely practical purposes, to do with the organization and management of goods and labour. Or as an awe-inspiring symbol of naked political power? Did the people of those times go there as people go to church today, or as they go down to the pub, or to a wedding, or to the labour exchange or to Buckingham Palace or perhaps to a village council meeting? Or perhaps they went for different reasons at different times: the structure was built and embellished over something like a thousand years, so its function and context may well have evolved.

Traces

What fossils did you make today? If you imagine a fossil to be a dinosaur skeleton in the grand entrance of a museum, you might think that you would not have the opportunity to become a fossil until the day of your funeral. However, your fossil-making capacity is far greater. You may, for instance, be making a contribution to the potential fossil record each Wednesday evening, as you leave the wheelie-bin out at the end of your drive. Humans have the capacity to make fossils all the time: each time, for example, that one defecates, or walks through the park. Evidence of past life comes in two main varieties: the actual bodily remains of a once-living organism and any traces left of the activities performed by that organism. Humans, clearly, produce both types, termed ‘body’ and ‘trace’ fossils respectively. Future palaeontologists, though, trying to characterize life in the Human Period will undoubtedly produce an incomplete and perhaps hugely misleading reconstruction. The fossil record of human beings, like the fossil record that we study today, has inherent biases. How will future palaeontologists categorize human trace fossils? It is first necessary to emphasize that there will not be one type of human trace fossil, because humans have not one but many types of behaviour. This is a first principle of trace fossils: a single type of organism can make many different traces. Let us choose an example and follow him through the course of a day. We may call him Robinson. Robinson is on a cruise when—catastrophe!— his boat capsizes in a storm and he washes up on a deserted island. He crawls up on the beach, and lies there for a while, catching his breath, leaving an imprint of his body where he lay on the sand. He walks around a little on the beach, leaving a trail of footprints (he lost his shoes in the swim for the island), looking for food. Robinson remembers that beaches are a good place to find shellfish. He decides on this as a strategy for lunch.

Tectonic Escalator

How does it work, this engine that produces the world’s strata, those storehouses of an almost infinite history? Our future explorers might be sorely puzzled, for the Earth’s motor is quite specific in its mechanism. There is nothing else like it in the solar system, and even reasonably close duplicates of it may be rare among planetary systems generally. A problem is immediately encountered in any attempt to construct a history of the Earth’s life and environments from the stratal archives. For the question will extend beyond simply explaining why the strata that formed on ancient sea floors happen to be present high up on land. Any explorer, in trying to construct a coherent history of the Earth, will find anything but coherence in those rock layers, once they try to put them back into their original order. For in many regions of the earth the strata are tilted, or are upside down, or are crumpled into huge folds, or have been sliced into segments in which the primary stratal layers are markedly off set from one another. Some layers show signs of having been recrystallized by heat and pressure, showing that they must have somehow been carried down to great depths below the Earth’s surface, and then carried back up again to lie exposed at the surface. The strata of neighbouring Mars, by contrast, have nothing like the richness of the Earth’s—but neither do they possess such formidable structural complications. These crazy Earthly stratal geometries, just as much as earthquakes or volcanoes, are indisputable signs of an active planet, in which the seemingly solid and stable crustal surface is, in reality, highly mobile. Our future explorers should take it for granted that strata are essentially made of sediment that was eroded from topographic highs (say, mountains) and was carried down to topographic lows (say, the floor of a lake or of a deep sea). This is straightforward. It has happened, say, on Mars. But on Mars it essentially happened as one cycle, a long time ago, where the highlands represent the eroded areas and the flatlands are an accumulation of the sediment derived from them.