Mud. Muck. Dirt. Although we have plenty of words for it, we rarely give soil a second thought. But without soil, we would certainly be dead.
Soil is crucial to almost every aspect of life on land, from water storage and filtration to climate regulation, flood prevention, nutrient cycling and decomposition. The dirt beneath our feet is also an exceptionally high source of biodiversity: some estimates suggest that at least one quarter of all species live in or on the soil. And we are still discovering its treasures: in January 2015, scientists announced that the first new antibiotic in 30 years had been found in soil bacteria.
The UN has named 2015 the Year of Soils and 5 December also happens to be World Soil Day. If there was ever a time to celebrate this underappreciated substance, it is now. But where did soil come from originally, and why is it so fundamental to life on land?
At the birth of the solar system, before our planet formed, the building blocks of soil were lurking in the inky blackness of space. Evidence for this comes from meteorites known as carbonaceous chondrites that date from the dawn of the solar system and that are rich in the clay minerals that made up the earliest terrestrial soils.
Following the formation of Earth, about 4.6 billion years ago, these clay-rich primeval soils developed across our young planet. But conditions were harsh: frequent and massive meteor impacts would have melted and pulverised large volumes of these early as quickly as they formed.
Almost from the moment of its origin, life began to influence – and be influenced by – soil
“There is debate about whether the whole surface of the Earth was melted,” explains Gregory Retallack, an expert in ancient soils from the University of Oregon in Eugene, US. He supports the theory that no more than half of Earth was molten at any one time.
Around 3.8 billion years ago, conditions on Earth began to stabilise. The constant meteorite bombardment that had made the planet an inferno until that point began to subside, and liquid water could condense, forming lakes and seas. This marked an important point in the soil story. The liquid water weathered and eroded Earth’s rocky crust, generating mineral matter and forming more permanent soils.
The first life on Earth probably appeared a little later, about 3.5 billion years ago; some of the earliest evidence comes from fossilised structures that formed on rocky shores and resemble microbial mats called stromatolites, which are still found on Earth today.
Almost from the moment of its origin, life began to influence – and be influenced by – soil. For instance, those first microbial mats were built up from photosynthetic organisms, which could produce huge volumes of organic material using energy from the sun. This organic matter gradually built up on the shoreline, where it mixed with the minerals freed up by eroding rock to create what was arguably the first true soil.