For whom the bell tolls: £20m 'Memo' project takes shape on Dorset's Jurassic coast

Sebastian Brooke was out of work, a jobbing stonemason without a job, when in the autumn of 2006 his brother said to him: "Make me something." So began Brooke's journey through the history of the Earth; down into the Underworld, like a modern Orpheus, and coming back up again to bear witness to all the lost souls, saving them from oblivion with a plan to carve an epitaph to every species that once inhabited our planet but is no longer around. It's reincarnation in rock.

The story of this "Memo" project begins with the Great Fire of London. In 1666, most of London – made largely of timber and thatch – was destroyed after a fire broke out in a bakery in Pudding Lane. London as we now know it rose from the ashes, largely thanks to Christopher Wren, who built St Paul's and much of Whitehall. But the Great Fire was also the making of Robert Hooke, then a journeyman scientist specialising in microscopy, who became surveyor to the City of London.

Standing on the site of the Royal Exchange in 1668 and inspecting the great stone blocks that had been hauled up from Portland to be used in its construction, Hooke observed that they were pitted and veined with strange geometries. And he realised that the shapes were the traces of creatures that had once inhabited the Earth. These giant ammonites ("of a prodigious bignesse") were so unlike any living species that they must represent the remains of a species "totally destroyed and annihilated".

It came as a shock to religious belief: the notion that the Apocalypse began shortly after Genesis subverted the notion of a fixed Chain of Being put in place by God for all time. The scientific theory of extinction was not fully established until the end of the 18th century. But the vision of extinction was born right here. "The Earth itself," Hooke wrote, "so neer us, under our feet, shews quite a new thing to us, and in every little particle of its matter, we now behold almost as great a variety of creatures as we were able before to reckon up on the whole Universe it self."

By an odd quirk of synchronicity, several thousand miles away on the island of Mauritius, in the Indian Ocean, around the same time that Hooke was surveying London, the last surviving dodo was trying to sprint away from its pursuers. Until the end of the 16th century, this flightless bird lacked predators. Suddenly, it was surrounded by them. It was reputedly fearless of humans, which was its big mistake. Cue extinction, less than a century after its discovery. Over 3ft tall, and weighing up to 50lb – and now it was nothing but a memory, almost a myth.

In a similar vein, I once went on a moa hunt in New Zealand. Rather like the dodo, the moa had been large, feathered, but wingless. And had duly disappeared. But this didn't stop one hopeful New Zealander (with me in tow), trying to track one down a few years back. Need I add, we were out of luck. The moa, if it is hiding, is tucked away on that same plateau somewhere in South America that Arthur Conan Doyle conjured up in The Lost World – a Shangri-La of non-extinction. Memo – the Mass Extinction Monitoring Observatory – is probably the closest we will get to discovering that Lost World.

According to the Red List, an inventory kept by the International Union for the Conservation of Nature, more than 800 species have died out since 1500. One estimate has it that more than 99.9 per cent of all species have already disappeared – and what we are seeing now (including us) is the merest leftover. Of these, nearly 20,000 are now on the brink. What we are witnessing is "the sixth Mass Extinction Event" – an event being caused by us.

Sir Crispin Tickell, chairman emeritus of the Climate Institute in Washington and a patron of the project, believes that the current rate of extinction makes our geological epoch, the Holocene, comparable to the Cretaceous and its great dinosaur holocaust. "There are a few species k that have survived over long periods," he says, "but they are the exception. For species to keep going, environmental circumstances have to remain constant. And, of course, they don't. So you adapt or die." Dragonflies have been around for 300 million years; birds are the descendants of dinosaurs – but the Earth's crust is a giant graveyard. As Tickell says, "We are walking around on the remains of living organisms."

And, in recent times, our species has been responsible – via deforestation, habitat destruction, global warming, and chemical pollution of the biosphere – for stomping them down there in the first place. Tickell argues that future geologists will refer to this era not as the Holocene (which began with the end of the last Ice Age around 12,000 years ago) but the "Anthropocene" – dominated by anthropos, humankind – "because we have changed the character of the earth, the sea, and the sky".

Back in 2006, Brooke's big idea was this: to carve the likenesses of a few non-existent species or rare and precarious ones. He had no idea he was starting a global snowball. Soon the Royal Society took an interest, and then so did the Eden Project creator Tim Smit. Edward O Wilson, the Harvard biologist acknowledged as the father of biodiversity, signed up as a patron. When Brooke joined forces with David Adjaye, the London-based architect, the vision began to take concrete form: or rather, Portland stone form. And in Portland. For while Memo is universal, it is also site-specific.

Smit, who is sceptical about the "taxonomic arrogance" of museums in general, waxes lyrical about Memo. "It's a cathedral to an idea: that we are part of something, not apart from it. It gives you a spiritual awareness of the oneness and fragility of a planet flying through the middle of nothing." It could be, he reckons, a cultural icon, but also "a place to go and reflect on Gaia, the interdependence of all life, and to feel a greater sense of humility".

Portland bears some resemblance to a lost world itself. Geomorphologically speaking, this 6km by 2.4km island, the southernmost point of Dorset, is unique. Take the train to Weymouth, 120 miles south-west of London, then weave along the narrow strip of Chesil Beach that connects the "isle" tenuously to the mainland, past countless windsurfers and kite-surfers, then keep on climbing 500ft right up to the highest plateau. There, on the day I visit, stands Brooke, sounding a bell outside the walls of a castle built by Henry VIII. Or rather he is encouraging an old lady to whack his bell with a rubber hammer. "Why has it got all these bits hanging off the outside?" she chirps.

This is a smaller brother of the 10ft-diameter bell that will be tolled in the Memo belltower to signal the demise of yet another species. At the rate we are going, it's going to be as regular as Big Ben. The bell was originally cast in stone by Marcus Vergette – something not done since the advent of the Bronze Age – using the form of Portland "roach" stone first analysed by Hooke, shot through with the imprints of Jurassic sea entities, like the name Brighton running through a stick of rock. "I thought it had been dredged up from somewhere," says the old lady.

Finished in bronze, the bell now outwardly displays all those species on its surface, making it crusty and blobby, like a miniature Gaudi cathedral. "They've been holes for 150 million years," Brooke says, referring to the ex-lifeforms entombed in stone. "Now we're giving them a voice." It's like a 3-D snapshot of extinction. And its tone is at once both joyous and melancholy.

Something similar could be said of the Adjaye building, a helical enclosure of Portland limestone open to the sky in which the belltower will sit. Adjaye was so inspired by the project that he donated the design. He originally thought of an hour-glass shape to mark the sense of time passing. But the narrow, waspy waist would have required disproportionate ingenuity (and expense). So he chopped it in two – half-an-hour-glass – and worked instead on the concept of the spiral or helix. The final design is an elegy to the ammonite. "The inspiration came from the rock," Adjaye says. "The beginning of life on our planet. The spiral was intrinsically architectural. But think of the solar system and the structure of DNA – we're all swinging through these concentric spirals." The tower will be a continuous spiral lined with stone carvings all the way up – but with a great void in the middle, which is symbolic (of extinction) but also useful for staging mass events. Educational displays will probably be located in a section buried below ground.

At the age of 43, Brooke has something of the rainforest about him, something organic, rugged, weather-beaten: crooked nose, uneven teeth, windblown hair, untended beard. We stand on the spot, on the west of Portland, where Memo will rise up out of the Jurassic Coast (our only World Heritage Site). The sea below is a perfect azure; the quarried blocks beneath our feet a wonderful creamy, chalky, whiter shade of pale. With creatures – thousands of them. Millions: limestone is nothing but the remains of living things. It's ex-plankton. When Dr Johnson wanted to disprove Bishop Berkeley's argument that nothing exists, he kicked a large stone and said: "I refute it thus." It exists; we exist. Brooke's stone is like that: it shouts out existence – the lives and endlessly reiterated deaths of so many creatures. And it is a narrative that will be added to again and again, so long as species are dying. Like an extinction blog – written in part by schoolchildren, whose work will be exhibited inside the building alongside the creations of professional stone-carvers.

"It speaks to me of absence," Brooke says moodily, looking out over the quarries that have been the life of the island. "For centuries people have come here, dug up great chunks of the place and hauled it away with ropes and barges to build other places." Indeed, since Roman times, 6 million tons of what used to be Portland have been carted away and turned into London or New York. There are holes here the shape of St Paul's, the Old Bailey and the Cenotaph. "Now I want to put something back again – to restore it to itself." Some Portlanders wanted Memo buried in the earth, so it wouldn't spoil the skyline. But it had to be visible – and audible.

The project has now been greenlighted to go ahead on land owned by the Albion Stone and Crown Estate. The cost: £20m. The timetable: 18 months' fundraising (starting now), another 18 months to complete. Yet it already has a soul. And I can't help wondering: what manner of creature will remain to toll the bell when the species that has already destroyed so much is finally itself destroyed? When the final homo sapiens go the way of the dodo? For this, surely, is what that old lady and I realised when we looked into one another's eyes and listened to the tones of that beautiful bell: send not to know for whom the bell tolls; it tolls for thee.

The Memo bell will toll in Portland on Wednesday to mark the Earth Summit in Brazil (see memoproject.org). Andy Martin is the author of 'The Boxer and the Goalkeeper: Sartre vs Camus'.

Film credit: Norman Lomax; www.memoproject.org

Dearly departed: Five species that met their end at the hands of mankind:

Dodo

Living undisturbed by predators and surrounded by low-lying fruit for so long, through time, this Mauritian bird lost the ability to fly. Which meant that when the Dutch decided they were an easily accessible source of fresh meat on the island, it was less than a century before they disappeared, in 1681.

Great Auk

Agile in water but easy prey on land, these penguin-esque birds lived on the coast of the North Atlantic in Canada, Greenland, Iceland, the British Isles and Scandinavia. They were heavily hunted by man as a resource of meat and its feathers as fashion items. The final two confirmed specimens were killed in 1844.

Tasmanian tiger

Despite its name, this species, extinct since 1936, bore more similarities to the kangaroo for its pouch. Australian farmers began a campaign to kill these carnivores in the late 19th century, after their livestock was attacked – and they were later backed by a government-led cull.

Yangtze Dolphin

Historically accorded demi-god status by fishermen, who saw them as reincarnations of drowned princesses, these river dolphins dwindled in number from thousands in the 1950s to extinction in 2007 as a result of illegal overfishing and the clogging of river arteries, driven by China's industrialisation.

Javan Rhino

This species was thought to have been killed off during the Vietnam War – until a single subspecies was discovered in 1988. Yet it lasted only another 23 years, with blame for the loss of the final 10-15 rhinos blamed on large-scale habitat loss, agricultural encroachment – and poachers.

Inês Klinesmith