Micro-organisms may be too small to see but we must never underestimate their importance. They are the foundation of the biosphere, both from an evolutionary and an ecological perspective. They were the first organisms on earth and have been present for 3.7 billion years, 3 billion years ahead of plants and animals.
The diversity of the microbial world is both mysterious and spectacular. Some live at boiling temperatures, or higher, in hot springs and deep-sea thermal vents, others live at temperatures below freezing sea ice.
Billions of micro-organisms are present in the human (and all animal) digestive tracts, making digestion and the absorption of vitamins and amino acids possible. Without them, we could not exist. Plants too, rely on micro-organisms to provide nutrients for growth.