Life doesn't just inhabit the Earth—it shapes it, physically, on every scale. What can we learn from its methods?
The following is an excerpt from “Our Earth, Shaped by Life” by Olivia Judson, originally published on Aeon.
A few months after wishing for annihilation should he be proved wrong [about his theory of coral atoll formation], Darwin published his study of earthworms. At the time, the book was wildly popular, selling 3,500 copies within the first month. Today, though, it is little read, and often dismissed as the eccentric afterthought of a great man whose life was drawing to a close.
But it wasn’t an afterthought. True to form, Darwin had studied earthworms for decades. In November 1837, just over a year after returning from his voyage around the world, he gave a brief speech about the animals at the Geological Society of London and, a couple of years later, he published a short paper about them. In this paper, Darwin described several fields that he had visited with his uncle. Each of the fields had, some years previously (though no one could remember exactly when), been covered with a superficial layer of materials such as cinders. But, in each case, the layer had mysteriously disappeared. When Darwin dug holes in the fields, he discovered that the overlaid materials were now at some depth below the surface. His uncle speculated that this was due to the activities of earthworms. As they burrowed through the soil, the worms were, his uncle suggested, acting as slow-motion ploughs. Altogether, the observations were so interesting that William Buckland, an eminent geologist of the day, suggested that Darwin had identified “a new Geological Power”.
"It may be doubted whether there are many other animals which have played so important a part in the history of the world as these lowly organized creatures." ––Charles Darwin on earthworms
As their family began to expand, Darwin and his wife decided they wanted to move out of London. Accordingly, in August 1842, a few months after his book on reefs was published, the Darwins bought a house in the country; it came with some land. Soon after moving in, he set aside some of that land to test his uncle’s hypothesis. From the outset, this was a long-term project: Darwin intended that the field should lie undisturbed for many years. In his science, as in his thoughts about the workings of the world, he embraced a long, slow view.
Earthworms burrow through soil by eating it; they also nibble on organic matter such as dead leaves. To defecate, they generally come to the surface, where they eject, as Darwin put it, “little intestine-shaped heaps” known as castings. On the basis of his conversations with his uncle, Darwin suspected the worms of tilling the soil, bringing fine particles from deep in the ground up to the surface.
As a result of these repeated actions, he thought, the soil would be slowly turned over and mixed. New soil would be raised up, while objects at the surface would become dusted with earth and, as the years went by, would gradually become buried. To measure how long such a burial might take, in 1842, just before Christmas, Darwin had lumps of broken chalk spread on top of the field that he had set aside. Twenty-nine years later, he had a trench dug across the field. The chalk now formed a line through the soil, roughly 18 centimeters (7 inches) below the surface. From this, Darwin calculated that in each of the intervening years the worms had covered the ground with a layer of topsoil that was, on average, 0.56 centimeters (0.22 inches) thick. Something similar happened to a row of flagstones that, in 1843, he’d had placed to make a path across his garden lawn. “During several years the path was weeded and swept; but ultimately the weeds and worms prevailed, and the gardener ceased to sweep.” The stones had gradually disappeared beneath the grass.
After helping to shape the land, the molecules that were a worm of course make a new progress, working into the muscles, bones, and feathers that carry robins and their chicks from the land to the sky.
Never one to settle for a single piece of evidence, Darwin did not stop there. Once again, he mustered an enormous mass of material, from as many sources as possible, to support his claims and overwhelm potential objections. He embarked on experiments to see how worms perceive the world. Were they creatures of tastes, with active preferences? Yes, they like to nibble on cabbage leaves, but disdain thyme and sage. Moreover, leaves did not only serve as food. Darwin observed that earthworms often line their burrows with leaves, perhaps “to prevent their bodies from coming into close contact with the cold damp earth”. He also compiled huge numbers of observations from others, enlisting the help of four of his sons, as well as correspondents in places as diverse as India, Australia, Brazil and Venezuela.
Much of this work was unglamorous, consisting as it did of collecting and weighing those little intestine-shaped heaps of excreted soil. One of the most valiant efforts was made by his niece, Lucy Caroline Wedgwood. For a year, on a near-daily basis, she collected and weighed earthworm castings from two designated plots, each just under a square meter in size. On the less productive plot, the worms brought up just short of 2 kilograms of soil per square meter per year, which doesn’t sound like much. But scale up these numbers across space and time, and the results become impressive. Darwin estimated that worms move between 18.98 and 45.49 tonnes per hectare per year (7.56 to 18.12 tons per acre per year), depending on where they live. He reckoned that, considering the worm-friendly areas of England and Scotland together, earthworms would move more than 325 million million tonnes––in today’s parlance, that’s 325 trillion tonnes––of earth over the course of a million years.
This Japanese woodblock map from 1840 provides a colorful illustration of the scale of Darwin's global correspondence and coordinated scientific observations.
If worms bury bits of chalk, Darwin reckoned that they would also bury other objects dropped on the ground––coins, gold jewelry, ancient tools. Nor was that the end of their powers. By analogy with the sinking flagstones in his garden, he suspected that worms could cause the burial of ancient ruins. By burrowing away below, worms would cause buildings to subside and sink into the soil; and by bringing soil to the surface, he thought that they would gradually cause the ruins to become covered up. And so off he went to find out.
Picture a hot day in August 1877. Darwin, by now, is an old man, the leaping pole long since discarded. He has a long white beard that makes him look vaguely like an Old Testament prophet or, as one of his eulogists would write, “an ancient philosopher”. He has travelled some distance from home to attend the excavation of the ruins of a Roman villa discovered beneath a field in Surrey.
On each of several mornings after the villa’s atrium had been excavated and swept of soil, Darwin knelt down to inspect the tiled floor––and found several little heaps of soil left by worms. The worms, it turned out, had come up through small gaps between the tiles. By snatching away the fresh castings, Darwin even managed to surprise several worms in the act of retreating into their burrows. From what he observed at the site, Darwin concluded that worms had been the chief agents of burial. “Archaeologists are probably not aware how much they owe to worms for the preservation of many ancient objects,” he declared.
A site similar to the one Darwin visited, Great Witcombe Roman Villa now features replica buildings and the remaining foundations of ancient structures, reclaimed from the burying action of earthworms.
Taking his results together, Darwin showed that earthworms have several important effects. The animals do not just mix the soil by bringing deeper material up to the surface. By munching on fallen leaves, and by pulling those leaves down into their burrows, they also create new soil and enrich it with a nutritious compost. In addition, as earthworms eat their way through the soil, they grind it into smaller particles, breaking up small stones and milling the soil to a finer consistency. Earthworm burrows––which, in some places, can be well over a meter deep––also serve as channels that irrigate and aerate the soil, and make it easier for plants to send down their roots.
Over time, these activities transform the landscape. Worm castings don’t always stay put: rain, wind and gravity conspire to shift some of the soil that the worms upheave, tending to send it downhill. By measuring worm castings before and after wind and rain on slopes of different steepness, Darwin showed that, each year, some fraction of the soil excreted by worms flows downhill. While, from one year to the next, this would be an imperceptible creep, over many centuries, it adds up. Or, as Darwin put it:
When we behold a wide, turf-covered expanse, we should remember that its smoothness, on which so much of its beauty depends, is mainly due to all the inequalities having been slowly levelled by worms. It is a marvellous reflection that the whole of the superficial mould over any such expanse has passed, and will again pass, every few years through the bodies of worms. The plough is one of the most ancient and most valuable of man’s inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earthworms.
Worms might appear insignificant, but because there are so many of them, little by little, they sculpt the contours of the world.
Visit Aeon to read the complete “Our Earth, Shaped by Life” by Olivia Judson.
About the Author
Olivia Judson is an evolutionary biologist and writer. She is a former journalist for The Economist, a former online columnist for The New York Times and has published in a number of other publications, including The Atlantic, Smithsonian, and National Geographic. She is the author of Dr Tatiana’s Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex (2002), and a recipient of a 2020 fellowship from the John Simon Guggenheim Memorial Foundation. She is writing a history of life and Earth, and lives in Berlin, Germany.
The Collection
Dig into the world of worms and other soil-dwelling creatures, and explore the ways they interact with and shape the face of the Earth.
