by Steve Jones
At Down House, the longest-running biological experiment in the world is still under way but, ancient as it might appear, the worm-stone has been in place for no more than an instant of geological history. Darwin realised that in the abyss of time his own life and the span of his own experiments were fleeting indeed. He saw that the remnants of ancient structures scattered over England gave him a better chance to test his subjects’ powers. In late middle age, he began a tour of the stately ruins of England and - ever a busy correspondent - wrote to dozens of people who might give him information.
The head of excavations at Wroxeter, near Shrewsbury, came up with a strong hint of what worms could do, given time. The city had been founded by the Romans to act as the capital of a British tribe, the Cornovii. Viroconium, as it was called, at its peak held six thousand people. In time, it fell into decay and became, in legend, the site of King Arthur’s court. Camelot, the archaeologist responded, was in some places buried under more than a metre of vegetable mould. Much of that was due, Darwin had no doubt, to the efforts of earthworms.
In 1877, men at work on the restoration of Abinger Hall in Surrey, the grand house of his friend Thomas Henry Farrer, who had earlier helped with the experiments on hops and other climbers, discovered the remains of a Roman villa. The Sage of Downe came to visit. He saw how the creatures crawled through the rotten concrete floor of the ancient structure, and brought up material from below. At the time, and for long afterwards, antiquarians assumed that the layers of earth found above decorated pavements and the like were the remnants of later and less civilised inhabitants, who had settled down in the houses of their erstwhile masters and left their household rubbish behind. The supposed squatters were, in truth, worms.
Darwin was impressed to discover burrows almost two metres beneath the modern surface. The animals could even mine into the ancient structure’s thick walls. Farrer observed their activities for several weeks, and saw them hard at work as they heaved the soil. A quick sum showed that their labours were more than sufficient to bury a Roman house within a few centuries. At a villa with a mosaic floor on the Isle of Wight, Darwin’s son William was told that so many castings were thrown up between the tiles that the ground had to be swept every day to keep the pattern in view. William also visited Beaulieu Abbey in Hampshire and found that as a result of their labours the bottom of a hole dug down to the ancient floor twenty years before was already covered.
The trip to Stonehenge was also part of the worm project. It showed that, active as the animals might be in rich soils, in some places they achieved rather less. Emma herself noted that they ‘seem to be very idle’ and in that thin soil the animals had done no more than enough to sink some of the ‘Druidical’ stones by twenty centimetres or so since they had toppled (they rested on the chalk layer beneath, into which the creatures could not penetrate). John Lubbock, who lived close to Down House, had dated the stones to the Bronze Age, which began in Britain around 2100 BC. The latest estimates push their masons further into antiquity at close to 2300 BC - a period when the Britons began to cut down their forests and replace them with fields. Some of the monoliths fell long ago, in part through the efforts of the worms themselves, whose work, and that of the rain, weathered away the soil that once supported them. Others fell - or were pulled down - within the past few centuries (one major collapse happened in 1797), which suggests that perhaps the burrowers were not as idle as Emma imagined. Indeed, they buried the stone chips left by the first modern excavators of the site in the 1920s to a depth of about five centimetres or so in thirty years, which was almost the same rate as that measured at Down House.
In Charles Darwin’s sesquicentennial year of 1959 a plan was hatched for an improved version of his experimental millstone, built on a grander scale, to test the destructive effects of such creatures on the ancient monuments of England. The British Association - that stamping ground of Victorian evolutionists - set up a Committee to Investigate by Experiment the Denudation and Burial of Archaeological Structures. A long pile of chalk, with a ditch alongside, of about the size and shape of a typical section of an English barrow or burial mound of three thousand years ago, was built at Overton Down, not far from Stonehenge itself. Plant spores and bits of broken flowerpot were scattered on the surface. Just thirty years later, natural weathering and the efforts of Darwin’s favourite excavators had caused large parts of the wall to collapse into the ditch, and both were covered with a layer of grass and soil. The pieces of broken pottery moved by around three centimetres a decade, and the spores were carried several centimetres into the depths. The worms were at work; and a similar structure built at about the same time on an acid heath in Dorset, with far fewer of those animals, was far less disturbed. At Overton, the experimental barrow now looks much like others a hundred times older. Once again, most of the change in the soil took place in the first few years after it had been disturbed. The next survey is planned for 2024, when, no doubt, the British Association Barrow will be almost impossible to tell from those built by the associated British long before.
Life’s underground frenzy soon blurs the record of the past. At Abinger Hall, several Roman coins were found; but among them was a halfpenny dated 1715. An incautious student would gain an odd view of British history if he took that observation literally. In a five-thousand-year-old Indian mound in Kentucky, the constant activity of soil animals has been enough to turn over and mix up the whole of the site fifty times over since the original inhabitants left. Sites with moist, rich soils are at more risk of disturbance than are deserts or cold uplands - but as men and worms have similar tastes in places to live, the news for those who hope to reconstruct ancient history is bad.
On Leith Hill, the highest in south-east England, Darwin tried to test the extent to which the material dug up would slide downwards to fill valleys and plains. He found that the castings soon rolled downhill and reckoned that for a steep slope a hundred metres long ten kilograms of earth would be washed to the bottom each year. His estimate is close to those made today, and is a tribute to the worms’ importance as architects of the fertile fields of southern England - and of the hungry pastures on the hills above. The wind, too, can transport their excreta, to add another weapon to the animals’ armoury as soil engineers. Wind-blown soils make up large parts of China, the Great Plains and the Rhine Valley. A strong gale moves stones and gravel, but such large elements soon fall to earth and the finest, and most nutrient-rich, particles - those of the worm-casts included - are blown furthest of all. That valuable powder can even cross the Atlantic. On the last leg of the Beagle voyage, the young naturalist noted a fall of white dust on to the deck of the ship as it sailed off South America. Some of that came from North Africa, thousands of kilometres away. Silt around Lake Chad - in part the product of worms and their fellows - is picked up by gales, and sifted finer and finer as it travels, until it becomes filled with valuable salts of nitrogen and phosphorus. More than ten million tons of the stuff fall on the Amazon rain forest each year and bring fertility to those thin and hungry lands. The good work of the worms can, it appears, cross great oceans.
Nowhere is their power better seen than when they themselves traverse the seas. Some species - the ‘peregrines’ - are keen migrants. In New Zealand at the end of the nineteenth century, farmers found to their surprise that what had been thin pasture had been transformed into lush loam. The immigrants were at work as they broke down soil into compost. They can move into empty pastures at ten metres per year. In today’s New Zealand, as they continue to spread, they can bury metal rings - a modern version of the worm-stone - at twice the speed measured in the Down House garden. Now, the Europeans are on every continent apart from Antarctica and in many places far outnumber the natives.
Their ability to improve the ground is so impressive that the animals are sometimes introduced to heal the damaged earth. After mining is finished, or all the peat has been stripped from a bogland, the intruders do a lot to help a landscape to recover. In the
Kyzylkum Desert of Kazakhstan and Uzbekistan vast numbers were moved in Soviet times to isolated oases, with salutary effects. Waterlogged Dutch polders, too, had their drainage improved by a hundred times after the animals were called in to help the engineers who had recovered the fields from the sea.
Long before they began to move, the worms helped make the landscapes of the agricultural regions of the world (southern England included), and as an incidental maintained innumerable farms and gardens in a fertile and healthy state. Now, those who till the ground - like their predecessors when farming began, but at a far greater rate - are undoing the animals’ work. Like skin stretched too tight on an ageing face, the Earth’s epidermis - the soil - has grown thinner with the years. Like age itself, the process is slow but impossible to resist, and like the signs of decay in a human body, the process speeds up with the years.
Man has flayed his home planet for ten millennia. Soil is hard to make, but easy to destroy. A modern plough shifts hundreds of tons a day, which is beyond the capacity of the most vigorous invertebrate. It digs down to no more than a metre or so, to make a solid and impermeable layer at just the depth of the blades. Another problem arises when fifteen-ton tractors roll across the surface. Their wheels compact the loose soil into a material almost like concrete, in which nothing will grow. In addition, continued ploughing breaks up the topmost layer and allows vast quantities to wash away. Every farm’s raw material is on the move, from hill to plain, from plain to river and from land to sea. The evidence is everywhere. My parents’ house overlooked the Dee Estuary (the Welsh rather than Scottish version). What was, a few centuries ago, a broad waterway has become a green field with a ditch in it and the local council is exercised about what to do about the sand that blows on to its roads. The reason lies in the fertile fields of Cheshire and North Wales. They have been ploughed again and again and their goodness has disappeared downstream.
The process is speeding up. The amount of organic carbon in Britain’s lakes and streams has rocketed in the past twenty years and in some places has almost doubled. Waters that once ran clear now flow with the colour of whisky, which itself gains its hue from the carbon-rich streams that run through peat bogs to feed the stills. On the global scale, matters are even worse. Twenty-four billion tons of the planet’s skin are washed away each year - four tons for every man and woman - and although some is replaced and some has always been lost to the rain and to gravity, the figure is far higher than once it was.
Man has long been careless of the deposits in his soil bank. Again and again, as a civilisation grows it empties its underground accounts, goes into decline and collapses. Usually it takes around a thousand years. Marx himself noticed, for he wrote: ‘Capitalistic agriculture is a progress in the art, not only of robbing the worker, but of robbing the soil.’
Darwin compared the work of the worms with that of the plough. Since his day, farm machines have become far more powerful. His experiments showed how earth could slip and churn, but he had no more than primitive tools to measure how much movement there now is. A sinister spin-off of modern technology has come to the aid of science. From the first atom bomb in Nevada in 1945 to the last air-burst of a hydrogen bomb in 1968, vast quantities of radioactive fallout spilled across the world. Radioactive caesium, which has a half-life of around thirty years, binds to soil particles. In an undisturbed site, the element is most abundant near the surface, but after a plough has passed the radioactivity is dispersed to the depth of its blade. As the disturbed ground is washed away, the irradiated soil is lost, to accumulate in places where the mud settles. In the Quantock Hills of Somerset, soil is, in this era of industrial agriculture, lost at around a millimetre every year. As the land sinks, the ploughs dig deeper, and the relics of the Romans that lie beneath will be smashed within a century - although, thanks to the worms, they have been preserved for two thousand years. Already, most of the treasures picked up by metal detectors come from ploughed fields, proof of how fast man’s machines are stripping the fragile surface of the globe.
The Romans themselves paid the price for their abuse of the soil, for at the time of their decline and fall so much damage had been done to Italy’s farmlands that a large part of the Empire’s food had to be imported. The fertile fields around their capital lost their goodness and vast quantities of grain were shipped in from Libya, which in turn became a wasteland as its surface was stripped by ploughs. In Rome’s last Imperial years, it took ten times more Italian ground to feed a single citizen than it had during its heyday.
The damage had begun long before. The first large towns appeared in the Middle East around eight thousand years ago. Quite soon their growing populations began to demand more food. The farmers exploited their precious mould with no thought of replacing its goodness or allowing their fields to rest. Instead they attacked it with ceaseless vigour. The plough was invented soon after oxen were domesticated. In a few centuries the topsoil was gone and many villages were abandoned. Within a couple of millennia, all the fertile land of Mesopotamia was under cultivation. Many irrigation canals were built. The soil was soon washed away, and the canals became blocked with mud. Enslaved peoples such as the Israelites were forced to clear it (and the abandoned city of Babylon is still surrounded by dykes of earth ten metres high, the remnants of their labours). Abraham’s birthplace, Ur of the Chaldees, once a port, is now nearly two hundred and fifty kilometres from the sea, and the plain upon which it sits is the remnants of what were fine fields, lost to leave a desert. Salt poisoned the last of the land, and what had been the Fertile Crescent - and the civilisation it fed - collapsed.
The same happened in China, where the Great River was renamed the Yellow River two thousand years ago as swirling earth changed the colour of the water. The ancient Greeks, too, faced the problem when the country around Athens was stripped bare. Plato blamed the farmers.
The real disaster for the skin of the Earth came in the Americas. The Maya bled their landscape dry, as did the inhabitants of Chaco Canyon and Mesa Verde - the abandoned Native American settlements that now form part of the deserts of the American south-west. The Europeans were even worse. Virginia’s ‘lusty soyle’ was ideal for tobacco, but the plant sucks goodness from the ground as much as it does from the bodies of those who consume it. In three or four years of that hungry crop the soil was drained of goodness, but farmers saw no need for fertiliser (‘They take but little Care to recruit the old Fields with Dung’) and simply moved on to the next piece of land. George Washington himself complained that exhaustion of the soil would drive the Americans west, as it soon did. Charles Lyell, Darwin’s geological mentor, used the huge gullies that scarred the devastated surface of Alabama and Georgia to examine the rocks below and commented that soon American agriculture would collapse.
The nemesis for vegetable mould - and a major threat to our own future - began in 1838 when John Deere invented the polished steel plough - ‘The Plow that Broke the Plains’, as the memorial at his childhood home calls it. Soon thousands of his devices were tearing up the prairies.
Since farmers began to work the Great Plains, the soil has lost half its organic matter. The Mississippi is what Mark Twain called America’s ‘Great Sewer’, and the amount of silt that pours down it has doubled since John Deere’s day. As the crew of the Beagle had noticed, yet more is taken by the wind. A thin layer of sod had kept the prairie soil in place and soon it began to blow away. The Dust Bowl followed. A huge gale in May 1934 blew a third of a billion tons of dust eastwards from Montana, Wyoming and the Dakotas. The dense cloud reached New York two days later, and petered out far out in the Atlantic. The finest material was blown furthest, which meant that the New Englanders gained the nutriment-filled dust that had passed through the guts of Montana worms, while the unfortunate westerners were left with just a rough and hungry silt. The gales returned again and again until by the mid-1930s more than a million hectares of prairie had been replaced by desert.
The situation elsewhere is worse. W
orldwide, an area larger than the United States and Canada combined has already been despoiled. In Haiti, almost all the forest has gone and thousands of hectares of ground are now bare rock. Less than a quarter of the island’s rice, its staple food, is now home-grown and over the past decade food production per head has gone down by a third. In China, the Great Leap Forward exhorted the peasants to ‘Destroy Forests, Open Wastelands!’ They did, and the soil paid the price. A parallel problem in Africa explains some of the continent’s chronic instability. Across that landmass, three-quarters of the usable land has been bled of its nutriment by farmers, who cannot afford fertiliser and whose fields are, as a result, no more than a third as productive as those elsewhere. Its earth still leaches its goodness into the water, or into dust. The Sahel, the area of thin soil to the south of the Sahara, is becoming a dust bowl and loses two centimetres of surface each year. Hundreds of millions of people go hungry as a result. A planet ploughed by man is far less sustainable than it was when tilled by Nature.
In 1937, after the Dust Bowl disaster, President Franklin D. Roosevelt said in a letter to state governors that ‘The nation that destroys its soil destroys itself.’ His Universal Soil Conservation Law became the first step to putting right the damage done to the precious fabric of his nation. It promoted careful ploughing, the use of windbreaks and a ban on the reckless destruction of forests. Within a few years, most of the American Dustbowl returned to a semblance of health. To plough with the contours, rather than against them, makes a real difference (and the Phoenicians had the same idea). In China, too, the Three Norths project plans a five-thousand-kilometre strip of trees in an attempt to stop the light earth from being taken by the wind. Even the Sahel has gained hope from low technology, with lines of stones set across the slopes to stop the thin earth from washing away. In Niger alone, fifty thousand square kilometres of land have been put back into cultivation.
