There is no way back into the past. 
The choice is the Universe – or nothing. 
		- HG Wells.

Look around. You are at the centre of a once-great city. But there is no artificial light, no traffic noise - no people.

It is the year 2010. The last human disappeared a year ago.

The first stages of the destruction of the cities are familiar to every urbanite battling to keep the weeds from her patio or drive. After a year, weeds like dandelions are growing in the gutters and the frost-cracked concrete. Some imported shrubs like buddleia are more aggressive, with roots powerful enough to penetrate bricks and mortar. The debris from these first plants forms a soil over the concrete, allowing the spread of other plants. Grasses, shrubs and trees take root in the deepening soil; tree roots smash through what is left of the pavement.

We can see this process in action today. The modern city of Pripyat was built to house nuclear workers from Chernobyl. After a decade of abandonment its open spaces were green, its paving stones so smashed and lifted by tree roots they looked as if they had been through an earthquake.

After five to ten years, storm-sparked fire is likely in the leaf-strewn cities. The burned material provides nitrogen to speed the growth of the plants.

Any gutted buildings surviving the fires may be pounded by another great natural force: floods. The embankments and barriers protecting London from the Thames, for example, would soon yield to erosion and subsidence, and land along the river bank would revert to marsh.

As the plants recolonise the cities, butterflies, bees and other insects follow - then, as the food chain builds up, the birds, and at last mammals.

Not all animals fare well. Animals like pigeons and rats which depend on humans for food disappear, in favour of field mice and other countryside species. Descendants of larger dogs survive in the wild; and cats descended from pets once fondly spoiled by old ladies have little trouble recontacting their wilder instincts.

But the balance of nature is not as it was before man. Many non-native species of animals and plants, imported for gardens, parks and zoos - maples and conifers, parakeets, feral pigs, deer - survive here, living clues to any alien archaeologists that intelligence was once at work here, disturbing the primeval order.

Amidst the spreading greenery, the surviving buildings are battered by weather and erosion. Without repainting and maintenance, bridge spans do not last more than fifty years. Our modern steel and concrete buildings, like city-centre office blocks, fare better: ivy dangles from the skyscrapers that still protrude, for a time, from the green layers of forest. In fact, the air and rain is clean of the acidic pollutants that damage our buildings in the present day. But they have lost their window glass, and are scorched by fire.

Wooden structures are the first to vanish completely, their fabric destroyed by the patient attention of insects.

Stone buildings last longer - at least, those far from the unstable land close to the rivers - but as more trees take root among the rubble, more walls are brought down. The very mortar of Hadrian’s Wall (built circa 122 AD, almost 2000 years old) has long crumbled to dust, and frost and lichen are destroying the stones themselves. After five centuries, the stone buildings are reduced to hummocks under the turf.

The skyscrapers survive a little longer, as long as their foundations hold against the rise of the water tables. But corroding concrete at last exposes the steel bars that reinforce the buildings. After that, the collapse is swift.

It seems unlikely that any modern building would last as long as the great stone constructions of the Middle Ages. The oldest castle in the UK, at Chepstow, Monmouthshire, dates from 1067 AD. Many buildings and structures have survived longer still: the oldest Egyptian pyramid is the Djoser at Saqqara, dating back to c 2630 BC, and the earliest part of Stonehenge dates from c 2950 BC. Modern buildings in a city like London – built on a flood plain - rest on much less stable foundations, subject to inundation and natural earth movements.

The destruction of the buildings, subject to weathering and the action of life – and without anybody to maintain them – is steady and inexorable. Surely not even the most robust of our structures will survive more than a few tens of millennia.

Fossil human skeletons will surely turn up here and there. We are very numerous and, with our habit of burying our dead, we are giving our remains a head start in preservation.

It is unlikely that any soft parts would survive more than a few tens of millennia, however. Today we occasionally find exceptionally preserved humans – bog bodies, ice men, mummies - their remains protected, by accident or design, by freezing, pickling or drying. In 1900 the head of a Siberian mammoth was thawed after being frozen for 40,000 years; the meat was well enough preserved that wolves were prepared to eat it. But such preservation can only work as long as environmental conditions, such as temperature and humidity, stay constant. Over a million years or more, this is unlikely.

Still, soft body parts can leave traces. Skin impressions from dinosaur ‘mummies’, a hundred million years old, have occasionally been found, in places where the skin lasted long enough to leave a mark in the enclosing, hardening rock. Bodies trapped in rocks formed in volcanic eruptions, like the hollows left by the burial of the inhabitants of Pompeii, can leave similar ‘negative’ impressions.

It is strange to think that the best clue to the shape of our bodies in the remote future might be the impression left by a 1920s gangster in his ‘concrete overcoat’ ...

Our abandoned artefacts - buried with our bones in the gathering layers of sediment – may fare better.

The oldest weapon discovered dates from c 200,000 BC: a wooden spear found near Clacton in Essex in 1911. The oldest tools may be 2.5 million years old, a collection of chipped pebbles and scratched bones found with the remains of an early hominid called Australopithecus. Compared to that, even deep-buried nuclear waste is short-lived; plutonium has a half-life of twenty-four thousand years, so that only a few per cent remains after just a hundred thousand years. It may be that the simple tools of chipped stone left by our ancestors may last longer than any remnants of our more sophisticated age.

But not just bones, tools and body parts leave traces for the future. We have observed dinosaur nests, burrows and even footprints. As a species we have worked much harder at disturbing the Earth than the dinosaurs ever did. Our ‘footprints’ will include roads, foundations, hard-wearing artificial rocks like concrete and brick, iron, cooper, steel and plastic: a thick layer left by centuries of building, shot through with pipes, tunnels, cables and pilings reaching deep underground. Even after the buildings are gone, such a stratum can survive indefinitely – if it is buried sufficiently quickly, and escape erosion.

On the surface of Earth, erosion is savage and unrelenting and – on geological timescales – surprisingly rapid. Wind carries away particles from rocks and soils; rivers carry sediments to the sea; waves grind up pebbles and eat away at cliffs. Earth’s dynamic geology keeps its surface young - compared to static, smaller worlds like the Moon and Mars. But it is a process which leads to the inexorable destruction of traces of human presence.

As the continents work through their stately dance, some parts of the Earth’s surface are rising, and some are falling. Surprisingly, perhaps, it is the parts that are falling that will be best at preserving traces of mankind, for they allow remains to be buried before being eroded. In regions of uplift, even the foundations of the greatest cities will crumble and wash away to the welcoming sea. This will probably be the fate of the cities of West Coast America, Japan, and stable Australia. But European cities like Amsterdam and Barcelona, and cities in India, central Asia and the African rift valley will probably leave long-lasting traces.

Sea level change is also crucial, however. A rapid rise in sea level – caused for instance by the disintegration of the West Antarctic Ice Sheet – while catastrophic for mankind, would serve to drown and therefore preserve many coastal cities.

Our surviving remains will, of course, be crushed by an increasing weight of sediment above them. But this need not destroy their identity; fossil shells, long buried, are still recognisable as shells.

Concrete may be destroyed – or at least altered – by chemical changes. But the humble brick will probably survive. A brick is just a lump of clay shock-heated so its minerals change and fuse: mudstone, mud baked by lava, is a natural analogue, and can survive for billions of years, if buried. Glass is unstable over millions of years, becoming opaque as tiny crystals form – but glass fragments retain their shape. Milky-white bottles that once contained beer or soda might persist for millions of years in the strata of the future.

It may be, however, that the most identifiable signature of our presence will not be an artefact at all, but a chemical remnant of our pollution. We have spread concentrations of a host of metals like lead, mercury, cadmium and copper across land and sea, an exotic stratum that will survive deep into time.

Artefacts away from Earth – such as probes in deep space – will survive longer.

But all such objects in the inner Solar System are subject to a constant sandblasting from micrometeorites, and will eventually erode to dust. On the patient Moon, the last traces of Apollo - footprints, flags, plaques and all – will surely be gone after ten million years.

On longer timescales, of course, Earth itself is doomed. In five billion years the sun will start burning helium instead of hydrogen, and expand to a red giant. Earth itself may be engulfed - rocks, tectonic plates, artefacts and all - and any last trace of ourselves will finally be destroyed.

Our most enduring monuments of all will surely be the four interstellar spacecraft we have launched: the Voyagers and Pioneers, launched to the outer planets in the 1970s by NASA, and now escaping from the Solar System altogether.

Right now Voyager One is flying high above the plane of the ecliptic, that invisible sheet in space which contains the orbits of the major planets. Voyager weighs about a tonne, and is about as big as a small house. Racing across space at more than a million kilometres per day, Voyager One is heading for the stars.

It takes twenty thousand years for Voyager to cross the Oort Cloud, the sun’s immense swarm of comets. Then, its power and radio transmitter long dead, Voyager embarks on an endless circling of the heart of the Milky Way Galaxy.

In the silent calm of interstellar space there is almost nothing to damage the derelict craft. The stars are so sparsely scattered that Voyager never encounters another stellar system. But at last the slow sublimation of metal causes Voyager’s aluminium structure to collapse. The fragments of the spacecraft - instrument booms and power generator, pitted and tarnished, metal walls reduced to a paper thickness - drift away from each other, so that the ruin of the spacecraft is surrounded by a cloud of glittering metal dust.

Thus Voyager dies: billions of years after the first clumsily chopped stone axe, twenty thousand light years from the sun, the last human artefact in existence.

A possible future. But not the only future.

The choice is ours.