The planet's first poison
Deep beneath the red sands of the Kalahari Desert lies a geological formation that records one of the most violent transformations in Earth's history. The Kalahari Manganese Field is the largest land-based deposit of manganese on the planet. The story of its formation begins around 2.4 billion years ago when the oceans were rich in dissolved iron and manganese, and the atmosphere was almost entirely without oxygen. In these ancient seas, the dominant life forms were anaerobic microbes, for whom oxygen was a deadly poison.
Then, a new type of bacterium evolved: cyanobacteria. Through photosynthesis, these microorganisms began releasing vast quantities of oxygen into the oceans and atmosphere. This "Great Oxidation Event" was a biological catastrophe. The newly introduced oxygen was toxic to most existing life, causing a mass extinction that allowed oxygen-breathing organisms to evolve. The event also permanently altered global chemistry. Dissolved iron and manganese in the seawater reacted with the new oxygen, causing them to precipitate and sink to the ocean floor. This process, happening on a global scale, created massive deposits called banded iron formations and manganese formations. The Kalahari field is the most significant of these manganese graveyards.