Microorganisms are tiny single-celled organisms such as bacteria, algae, and fungi. They are the simplest and yet the most diverse creatures on earth, and have been present for longer than any of the more complex plants and animals that have evolved from them. Their presence has not only led to the evolution of other plants and animals, however, but it has changed the way the planet itself has evolved and changed, because microorganisms have affected the evolution of rocks, mountains, and even the earth’s atmosphere.
Microorganisms are amongst the earth’s most effective weathering agents, and all kinds of rocks are susceptible to microbial weathering. In this kind of weathering, colonies of bacteria dissolve the rocks in order to gain access to the minerals contained within the rocks. The dissolved and crumbled rocks are then washed away by rain, and thus the minerals that had been trapped in the rocks become available to other life forms.
Microorganisms have also had an influence on the atmosphere, and almost certainly played a huge part in its evolution, because the earth’s earliest atmosphere contained no oxygen. The first life forms were believed to be marine blue-green algae (cyanobacteria) that can live either without oxygen (anaerobically) or with it (aerobically), and it is these microorganisms that are believed to have created much of the oxygen in the atmosphere. Only when oxygen was present were oxygen-using organisms able to develop, and these are the majority of the life forms currently on the planet.
Early microorganisms may have also created the methane in the early atmosphere of earth, before oxygen became widespread. Methane is a greenhouse gas that is far more effective than carbon dioxide, and its presence allowed the temperatures of the earth to rise, which could in turn lead to faster evolution of primitive plants and then animals.
Microorganisms are also known to live in clouds, and have even been found in the stratosphere, many miles above the surface of the earth. Little is known about the microbes found in clouds, but they are known to nucleate ice crystals within the clouds, and they may be at least partly responsible for the formation of clouds, and perhaps even for the production of rain and snow. They affect the composition of the atmosphere in addition to their effects on the climate.
Microorganisms were the first forms of life to appear on earth, appearing over three billion years ago. They were able to occupy virtually every niche, and they are still capable of occupying niches no other organism can live in, being found in environments as inhospitable as hot springs, boiling water from underwater vents, ice shelves, and rock glaciers. They have even been found living inside nuclear reactors. It is this amazing ability to survive almost anywhere that enabled them to thrive, and more complex plants and animals to evolve from them.
Microorganisms have had other effects on the evolution of the plants and animals that followed them, since microorganisms include bacteria that can produce disease in plants and animals. Disease can act as an evolutionary mechanism, since it can kill the weaker individuals, allowing the fittest to survive.
Microorganisms also help in the evolution of plants and animals since they break down the waste products and the bodies of dead plants and animals. Without their presence, other life forms would soon be swamped and killed by their own waste products and the bodies of the dead.
Microorganisms have therefore been extremely important in the evolution of the Earth, its atmosphere, and the plants and animals that now thrive on the planet.