Deep future: What will our descendants know about us? What clues to the way we live today will archaeologists unearth in the millennia to come? What will endure, and what will fade away?

WHEN humans in the far future are piecing together a picture of the primitive civilisation of 2012, archaeology will surely be the best way to go about it. After all, the best libraries, archives and museums can be undone by a single fire, amply illustrated by the fate of the library of Alexandria.

So what will archaeologists working 100,000 years from now discover about us? Only the luckiest of artefacts will avoid being crushed, scattered, recycled or decomposed. You, personally, will almost certainly leave nothing behind that survives that long. To get a sense of why, just point time's arrow the same distance in the opposite direction. Around 100,000 years ago, anatomically modern humans were just emerging from Africa to populate the world. Most of what we know about them is guesswork, because the only clues that remain are sharp stone tools and a handful of fossils.

You are especially unlikely to leave your bones behind. Fossilisation is an exceedingly rare event, especially for terrestrial animals like us - though with 7 billion people on the planet, at least a few of us will no doubt achieve lasting fame.

Luckiest - and rarest - will be the "instant fossils". These form when people or animals die in calcium-rich seasonal ponds and wetlands, or in caves. In both situations, bones can mineralise quickly enough for fossilisation to win the race against decomposition, says Kay Behrensmeyer, a palaeobiologist with the US National Museum of Natural History in Washington DC. One wildebeest toe-bone in southern Kenya soaked up calcium carbonate so quickly that it began to turn to stone within two years of death.

Future fossil hunters won't be looking for us in graveyards since bodies buried there crumble into dust within a few centuries. Instead, the richest human bonebeds will likely be found in the debris of catastrophic events, such as volcanic ash or the fine sediments left by the recent tsunamis in Asia, Behrensmeyer says. A few bodies might be mummified in peat bogs or high deserts, but they will decay if conditions change, as is likely over a span of 100,000 years.

Those same changes will also lay waste to other important clues to our civilisation: our homes. Climate change and rising sea levels are likely to drown coastal cities such as New Orleans and Amsterdam (see "Where will we live?"). In these cases, waves will probably destroy the parts of buildings above ground, and basements and pilings will soon be buried by sediments. While concrete may dissolve over the millennia, archaeologists will recognise the precise rectangular patterns of sand and gravel that remain as a sign of purposeful design. "There is nothing at all in nature like the patterns we make," says Jan Zalasiewicz, a geologist at the University of Leicester, UK.
Building our own geology

Nowhere will these designs be more unmistakable than in our biggest structures. A few human artefacts, such as open-pit mines, are essentially geological features already, and will last for hundreds of thousands of years as testimony to our earth-moving powers. Our largest dams, such as the Hoover dam in the US and China's Three Gorges dam, contain such an immense volume of concrete that some pieces will certainly survive that long, too, says Alexander Rose, executive director of The Long Now Foundation, based in San Francisco, California. A few structures - most notably the Onkalo nuclear-waste repository in Olkiluoto, Finland - are even being engineered to survive intact for 100,000 years.

We have also been busy building another massive legacy that will be the real bumper crop for future archaeologists: our garbage. The landfill sites where most of our goods eventually end up are almost ideal places for long-term preservation. When full, modern landfills are typically sealed with an impermeable layer of clay, so that the contents quickly become devoid of oxygen, the biggest enemy of preservation. "I think it's fair to say that these sites will remain anaerobic over geological time," says Morton Barlaz at North Carolina State University in Raleigh. Under such conditions, even some organic materials such as natural fabrics and wood are likely to avoid decomposition - though over the millennia they will gradually transform into something resembling peat or soft coal, says landfill expert Jean Bogner of the University of Illinois at Chicago.

A few materials will be preserved just as they are. We don't make much from stone any more, but a few statues might survive, buried safely away from erosion. Ceramic plates and coffee mugs should last indefinitely, too, just as the potsherds of early human civilisations have. Some metals, such as iron, will corrode quickly, but titanium, stainless steel, gold and others will last much longer. King Tut's gold, after all, looks almost unchanged after 5000 years. "There's no reason to think that wouldn't be the same after 100,000 years," says Rose. Indeed, titanium laptop cases, their insides long since corroded, may end up as one of our civilisation's most lasting artefacts. Who knows - scholars of the future may construct elaborate theories about our religious practices based on these hollow tablets and the apple-shaped figure etched into their surface.

The fact is that no matter how much we may try to preserve a legacy for future generations, we can never know which aspects of our civilisation will interest our descendants. Today, for example, our study of early humans is informed by Darwin's theories, a perspective that was inconceivable only a century ago. Even if the objects in our museums survive, they will only tell future generations what we thought of ourselves. What they will think of us is something no one reading these words today can fathom.

Lowly grail

They say diamonds last forever, but you might not expect that alongside those sparkly gems, a future museum may well be showcasing our polystyrene coffee cups.

That's because the petroleum-derived pellets of polystyrene cannot be biodegraded by any known microorganism. This stuff could last a million years.

Having said that, in the wild the cups will likely crumble into unrecognisable lumps. And if recent efforts to engineer fungi that can decompose polystyrene succeed, not even these will survive. But protected in a landfill and undisturbed for millennia, the cups could retain their shape sufficiently to allow future archaeologists to deduce what we used them for.