Darwin's 40-Year Experiment
In 1842, Charles Darwin moved to Down House in the Kent countryside. It was here, in his own garden, that he began a series of observations that would last for four decades. His final book, published in 1881, was not a treatise on evolution but a deep investigation of a creature: the earthworm. The book was titled The Formation of Vegetable Mould, through the Action of Worms, with Observations on their Habits.
Darwin’s primary experiment involved what is now known as the "wormstone." He laid a millstone on his lawn and, with the help of his son Horace, designed an instrument to measure its gradual sinking into the soil. Horace's measurements showed the stone sank by about two millimeters per year. Darwin hypothesized this was due to the ceaseless activity of earthworms bringing soil from below and depositing it on the surface as castings. He had started a related experiment nearly 30 years earlier, spreading chalk and cinders on a patch of ground and later digging a trench to find they had been buried seven inches deep by worm activity. From these long-term studies, Darwin calculated that worms could move more than ten tons of dry earth per acre, per year. He concluded that the entire layer of topsoil in the country had passed, and would pass again, through the intestines of worms.
The Cambrian Substrate Revolution
Darwin was documenting a process called bioturbation—the mixing of sediment by living organisms. He did not know that this same process, occurring on a global scale 540 million years ago, fundamentally re-engineered the planet. Before the Cambrian Period, the seafloor was a different world. It consisted of firm sediment covered by cohesive microbial mats. The water and sediment were sharply divided, and the sediment itself was largely anoxic and toxic with hydrogen sulfide produced by bacteria.
The beginning of the Cambrian saw a dramatic change. For the first time, animals evolved the ability to burrow vertically into the seafloor. This diversification of burrowing, known as the Cambrian Substrate Revolution, had significant consequences. These early ecosystem engineers broke up the microbial mats, mixing and irrigating the sediment with oxygenated water from above. This action detoxified the substrate, released buried nutrients, and created a three-dimensional habitat where none had existed before. The very chemistry of the oceans and atmosphere may have been altered. This agronomic revolution destabilized the old seafloor ecosystem but opened up new ecological possibilities, potentially fueling the diversification of animal life known as the Cambrian Explosion. Darwin's backyard observations were a miniature modern version of this ancient, world-changing event.