Darwin's Island

by Steve Jones

  Worms are among the simplest creatures to have a central nervous system, with a distinct brain connected to a set of nerve cords (although even after the brain has been removed the animals can mate, feed and find their way through a maze). A section of the worm book is headed ‘Mental Qualities’ - but its first sentence reads ‘There is little to be said on this head.’ Even so its author set out to see just what their lowly wits were capable of. He noted that they often pulled in leaves to seal the mouth of the burrow, perhaps, he thought, to protect themselves from the cold. Whatever crosses the animal’s mind as it drags fronds into its home, it acts with a degree of foresight. Worms, he found, prefer to grasp a leaf by its tip. More than nine-tenths of broad leaves were pulled in from that end, but for narrower kinds, which are easier to slip into a small opening, just two in three. Those of the rhododendron curl up when on the ground, so that some were narrower near the base, and others near the tip. The creatures, more often than not, pulled them in by the narrow end. They were just as smart when it came to pine needles, which could be dragged in only base-first.

  Darwin admired such rationality, for it forged a link between the lowest creatures and the most noble: ‘one alternative alone is left, namely, that worms, although standing low in the scale of organization, possess some degree of intelligence’. In the first real experiment on invertebrate psychology Charles and his son Horace presented the animals with paper triangles cut into various shapes - and once again they acted in the most efficient way, for on most occasions they seized the pointed end. Other studies of their intellectual universe involved the choice of foods - meat, onions, starch or lettuce (with beads and paper used in an attempt to trick them). In a series of midnight expeditions to the lawns of Down House, the father-and-son team shone lamps upon the animals, warmed and cooled them, and subjected the unfortunate creatures to tobacco smoke. The subjects were ‘indifferent to shouts’ and just as unconcerned by the shrill notes of a metal whistle or the deep tones of a bassoon. They did respond to vibration, and became agitated when placed on top of a piano. They were ‘more easily excited at certain times than others’, and a series of taps upon the ground made them emerge. Hungry birds could often be seen doing just that to persuade their prey to venture forth. There was, no doubt, a wide gap between their mental world and that of the naturalist - but profound as it was, it had been bridged by the same system of slow change that moulded the physical universe of each.

  Those patient experiments on the inner life of the burrowers were an introduction to their wider role in the world of the soil and their ability to modify their own habitat and that of those who stride the ground above. Most of Darwin’s book is devoted to the animals’ impressive ability to disturb and fertilise the ground.

  That talent had been noticed long before. Aristotle described the worms as the ‘Earth’s entrails’. Cleopatra herself decreed them to be sacred animals, and established a cadre of priests devoted to their welfare (although they were less important than scarabs, those other recyclers of dung, whose image was universal in Pharaonic times). Cleopatra’s interest arose because the creatures were so important to the fertility of the mud laid down by the Nile (and they were also useful in weather forecasting). Herodotus knew as much when he wrote that ‘Egypt is the gift of the Nile’, and most of the immense deposit of the great river that comes down in the annual flood does indeed begin as eroded worm-casts from the Ethiopian highlands, far upstream. The same is true closer to home. In 1777, the English naturalist Gilbert White wrote, in a letter unknown to Darwin, of their ‘throwing up infinite numbers of lumps of earth called worm-casts which, being their excrement, is a manure for grain and grass … the earth without worms would soon become cold, hard-bound and void of fermentation, and consequently sterile’. As he put it in The Natural History of Selborne, ‘Earthworms, though in appearance a small and despicable link in the Chain of Nature, yet, if lost, would make a lamentable chasm.’

  Without their help, we would ourselves fall into a real abyss. Those simple creatures play a role in both economics and history. They improve drainage and break organic matter into fine particles: ‘all the vegetable mould over the whole country has passed many times through, and will again pass many times through, the intestinal canal of worms’. That unromantic product determines the fertility of the soil, which does a lot to dictate the nature of the society that lives upon it.

  In the mid-nineteenth century gardeners saw the animals as mere pests, relatives of tapeworms and such unpleasant beasts (the word vermin, indeed, has the same Latin root as does ‘worm’). Books advised how to get rid of the unwelcome visitors by driving them from their burrows with mallets, poison or steel rods inserted into the ground and played with a bow (a pastime known as ‘worm-grunting’ to the American fishermen who still use it to collect bait). Soil was considered to be a product not of biology but of chemistry and physics, for it came from the mechanical dissolution of rock and the chemical decay of vegetation. The Comte de Buffon, mentioned in The Origin as a pioneer of the notion of natural selection, was, like his British successor, interested in what made the Earth’s outer cloak. He noted that many soils contained grains of minerals such as iron, and that cover tends to be thinner on mountain slopes than in valley floors. All this was, he claimed, proof of the breakdown of rocks and the importance of rain, rivers and gravity in disturbing the surface. The Russians of the period - obsessed as they were with the vastness of the steppe and its effect on the Slavic psyche - were pioneers in the study of the deep and dark chernozem, the ‘black earth’ that fed the masses and nourished the nation’s soul. They, too, emphasised the role of chemical decay. Why should such mundane creatures as worms play any part in the sacred soil of Mother Russia?

  Physics and chemistry do, no doubt, help build the ground beneath our feet. Chalk and limestone dissolve in the rain and even sandstone and granite can be eroded away to make earth. Tiny cracks fill with water, which shatters rock when it freezes. The surface tension that holds water to the walls of minute channels also exerts huge pressure as the liquid warms and cools. Clay itself - little more than tiny particles of ground rock - is a product of such insidious action.

  The earliest fossil soil is three thousand million years old, almost as ancient as land itself. It was made with no help from biology. Three hundred and fifty million years before the present the first land plants moved on to a sterile landscape. Since then, life has fed on soil and soil on life, in a great cycle that enriches both.

  The labour of the worms did a lot to improve the Down House garden. Its topmost layer is filled with channels, most of them thinner than a human hair, and around half filled with water. Below it lies a sheet of material with little air and no worms. A large part of the animals’ contribution to fertility comes from their ability to open the ground to air and water. A hectare of rich and cultivated ground is riddled by ten million burrows - which, together, add up to the equivalent of a thirty-centimetre drainpipe. Half the air beneath the surface enters through burrows, and rain flows through a disturbed soil at ten times the rate of unperforated ground.

  The surface of the Earth, when watched for long enough, is as unruly as the sea. Everywhere, soil is on the move. Gravity, water, frost and heat all play a part, but life disturbs its calm in many other ways. Living creatures - from bacteria to beetle larvae to badgers and to worms themselves - form and fertilise the ground. What lies beneath our feet forms the largest reservoir of diversity on the planet, with a thousand times more kinds of single-celled organism in a square metre than anywhere else. The soil contains more species than the Amazon rain forest. Its vast variety of inhabitants, large and small, burrow through the topmost layer, draw in air, digest its goodness, excrete into it and turn over so much material that the skin of our planet is in constant eruption. The ‘biomantle’, the organic layer near the surface, can be metres deep or be no more than a thin sheet. Its base is marked by a layer of pebbles that sinks to a depth at which the stones can no longer be
disturbed by the animals that agitate the ground above. As the mantle churns, the relics of man’s labour - from ancient tools in Africa to the pots of the first European settlers in Australia - sink through the topsoil, and accumulate, with the stones, just where the tillers abandon their efforts.

  Darwin’s subterranean subjects have many assistants. A shovelful of good earth contains more individuals than there are people on the planet. Most soils have hundreds of thousands of tiny mites and springtails in every square metre. Roots exude sugars and other substances that feed the millions of single-celled creatures that teem around them. They add their remains to the helpful productions of the worms’ rear ends. Bacteria and fungi possess powerful enzymes that can break down material that even earthworms cannot digest. They feed roots, break down vegetation - and produce antibiotics. Until the 1930s, a diagnosis of tuberculosis was a death sentence. The disease had killed a billion people since Darwin’s birth. Then it was found that a soil suspension attacked the bacteria responsible - and soon streptomycin was discovered and the disease was, at least temporarily, defeated. The microbial world beneath our feet is still almost unexplored and may have far more to offer. Molecular probes that pick up known genes in unknown species suggest it contains innumerable members of a very distinct group of creatures called Archaea. They look rather like ordinary bacteria, but in fact occupy a separate kingdom of life. Once seen as eccentric denizens of hot springs, we now know that there may be a hundred million of them in each gram of soil, many times more than bacteria. Each burns up ammonia and other waste products and helps maintain the Earth’s fertility.

  Worms stir their habitat without cease, and as roots grow they push barriers out of the way, and die to leave channels into which soil may collapse. As the roots suck in water, the soil settles, and as trees lash back and forth in storms they disturb the ground. A large tree can shatter solid rock as it falls, and the hole it leaves may take centuries to fill. Small animals do even more. Insects, mites, spiders and subterranean snails, together with the worms, may make up fifteen tons of flesh in a hectare of soil - an elephant and a half’s worth (and a single pachyderm needs several times that area to feed itself).

  The elephant under the grass is a voracious beast. Earthworms are earth-movers, but in the tropics ants and termites may do more, for they carry up material from several metres down. Alfred Russel Wallace was astonished by the richness of the ground in some parts of Brazil: ‘a layer of clay or loam, varying in thickness from a few feet to one hundred … over vast tracts of country, including the steep slopes and summits … of a red colour, and is evidently formed of the materials of the adjacent and underlying rocks, but ground up and thoroughly mixed’. It had been mixed by ants.

  Larger creatures also help to stir up the soil, and elephants themselves often paw away at the surface. Below ground, moles, prairie dogs, marmots, wombats, meerkats, badgers and other excavators join in, each in its own part of the world. They are helped by aardvarks, armadillos and anteaters as they scratch away in search of food. A colony of naked mole rats can build burrows a kilometre long. In the south-western United States, tens of thousands of symmetrical piles of earth up to two metres high and fifty metres across mystified historians for years. They were, they imagined, sacred sites of a lost tribe of Indians. The truth about the Mima Mounds is more prosaic. They are built by gophers, which over the millennia push tons of earth uphill to provide a dry refuge in a marshy place. Even the bottom of the sea is not safe, for manatees and narwhals dig up food, skates do the same and shrimps work away at the top few centimetres of mud. Enthusiasts for the process trace it back to the Cambrian explosion, around five hundred and forty million years ago, when the first animals with hard shells emerged. They were able to dig into the thick layered mat of microbes that had until then covered the seabed. As they did, a whole new way of existence sprang into being. The revolution of the burrowers marked the origin of modern life, and their descendants are still essential to keep it healthy.

  Today’s worms are merchants as well as miners, for they are major players in the vast traffic in chemicals that passes from the world of life to that of death and back again. Darwin knew as much when he wrote that ‘All the fertile areas of this planet have at least once passed through the bodies of earthworms.’ In an English apple orchard they eat almost every leaf that falls - two tons in every hectare each year, and in the same area of pasture certain kinds munch their way through an annual thirty tons of cow dung. A few tropical species pile their casts into mounds twenty centimetres high and his book refers to the gigantic castings on the Nilgiri Hills of southern India as an indication of the vast amount the animals must chew. Most of their endless meal is ground down in the muscular gizzard. Rather little is absorbed. Even so, it undergoes chemical changes. The experimenter fed some of his subjects with soil laced with red iron oxide powder and noted that it lost its colour when excreted; proof that acid and enzymes had done the job. Their potent guts change the soil, for the chemistry of clay is much modified when passed through their bodies. It is ground even finer than before, which helps it retain water and nutriments - and the tiny particles left after the worms have done their work mean that clay has ten thousand times the surface area of an equivalent volume of sand.

  Some species of worm have the unexpected ability to draw - like certain plants - carbon from the air and convert it into soluble substances that can be recycled. Vegetable Mould suggests that the small grains of chalk found in the digestive glands are waste products. The truth is more remarkable. Radioactive labels show that the glands extract carbon from free carbon dioxide - abundant beneath the soil - at a considerable rate (an unusual talent for an animal) and combine it with salts of calcium. The particles of chalk so produced are excreted and also return to the earth when the creature dies. The worms hence do a lot to increase soil carbon and to improve fertility.

  The constant flood of slime pumped out as they burrow recycles other minerals such as nitrogen. Plants and animals die, and farmers pour fertilisers, manure and treated sewage on to their lands and the worms do their bit to pull them into the earth. Their casts contain five times as much nitrogen and ten times as much potassium as does the soil itself. A large part of that emerges from their busy inner life; to the bacteria that live in the oxygen-free world of the gut. Each worm intestine is a tiny fermentation chamber in which bacteria chew up manure. They make useful fertiliser - but with the side-effect that they also pump out nitrous oxide, a greenhouse gas (and, as ‘laughing gas’, a primitive anaesthetic), which gives their hosts an unexpected role in global warming.

  A simple experiment shows the power of the worm to disturb the underground world. A mouse carcass was placed in a glass jar with some fine rubble and leaves, plus an added earthworm. In just three months, the bones had been scattered sideways across about ten centimetres and some had been dragged the same depth into the soil. In wormless jars, the corpse stayed undisturbed. Darwin, too, set out to test his subjects’ powers of burial. On morning after morning, in the garden at Down House, he counted the number and size of casts - each the undigested remains of a worm’s meal - and found dozens in a typical square yard. His cousin Francis Galton joined in and, ever keen to use statistics, counted the number of dead worms he saw on paths in Hyde Park. He found, on the average, a corpse every two and a half paces. The worms, he calculated, brought seven to twenty tons of earth to the surface in every acre of his local fields each year. At that rate, worms would lay down half a centimetre of topsoil in a twelvemonth. In fact, their labours are even more impressive, for most of what they excrete remained beneath the surface, invisible to the eye.

  The number of worms is so huge, and their labours so sustained, that in time they can do great things. In a follow-up of his youthful observation at Maer, and soon after moving to his own grand house, Charles Darwin scattered quantities of broken chalk and brick over a field near Downe to test how fast it sank. Twenty-nine years later he dug a trench across the chalk site,
and found most of the chalk buried some fifteen centimetres down. The bricks, on thinner soil, took longer but even they disappeared in the end. By 2005, the fragments of brick had sunk to the level of a solid band of flinty clay into which the worms could not penetrate, while the chalk had been dissolved away.

  Darwin’s garden had ten or more burrows in every square metre. Given the ability of each animal to chew through earth, if they acted with equal enthusiasm in every cubic centimetre the whole mass would be disturbed to a depth of a metre or so in about five thousand years. That was not at all the case, for stone tools of that age are often found at shallower levels. In addition, many species of worms reuse their burrows and that economical habit also reduces the extent to which they agitate the ground. As a result, an object that falls on the surface may sink quite fast in its first few decades, but then slow down.

  In his final decade, Darwin started an experiment to test their sepulchral power. He placed a lump of rock - a hefty millstone forty centimetres across - in a corner of his lawn. A long brass rod was pushed deep into the soil through a hole in the centre. The movement of the rock in relation to the rod measured the efforts of the burrowers as they worked away below. In its first days, it sank by around twenty millimetres a year. Charles died before the experiment was complete, but his son Horace continued the study and found that the worm-stone sank by twenty centimetres in ten years. Today’s stone, admired by the curious as it might be, is a copy of the original and has been moved since it was first put in place. Nowadays it sinks more slowly than it did. Sir Arthur Keith (who became wrapped up in the Piltdown Man scandal before writing an early biography of Charles Darwin) retired to live close to Down House in the 1930s, and re-examined the sites used in the chalk and brick experiments. Eighty years on, the marked stones had sunk little more than they had in the lifetime of those who set them there, as further proof that the worms are most active near the surface.