Microorganisms are so numerous and varied that they have served not one but many roles in earth’s evolution. In the modern world, microorganisms are found everywhere and fulfill numerous roles. In the oceans, algae, protozoa and bacteria are the most numerous and important members of the plankton communities that are the basis of all marine food chains. On land, bacteria dominate in soils and are the most important decomposers, being responsible for the recycling of almost all organic materials. Microorganisms are the chief causes of transmittable diseases and affect the evolution of their hosts as weaker organisms succumb to disease while stronger individuals survive to pass on their genes. These roles are all important but it is four billion years ago that microorganisms changed the earth forever.
Life evolved from large organic molecules about four billion years ago. At that time the earth was still hot, with a poisonous atmosphere. At some point, the boundary was crossed from non-life to life when RNA and DNA molecules began to self replicate and then used proteins and lipids to encase and protect themselves in the first cells. These simple cells had no nuclei or other organelles. They were primitive but alive, being capable of growth, movement and reproduction and they shaped the earth’s atmosphere and future evolution over a period of one to three billion years.
These first microorganisms were heterotrophs, consuming loose organic molecules and nutrients, plus each other. There must have come a time though, when there was a shortage of available nutrients and in this crisis the first cyanobacteria used the pigment chlorophyll to capture sun energy and make sugars from water and carbon dioxide. These first autotrophs (learn difference between autotrophs and heterotrophs), the ancestors of modern blue green algae, were to radically alter the earth’s atmosphere by producing oxygen, as a byproduct of photosynthesis.
During the next three billion years, microorganisms continued to evolve. They developed the ability to manufacture a huge number of chemicals, becoming sophisticated biochemical factories. They also developed organelles to make themselves more efficient. These cells evolved nuclei where the chromosomes are stored and where the control functions of the cells occur. Ribosomes developed and other cell organelles, important for microorganisms but also important later when cells joined together to form multicellular organisms. Without the biochemical abilities of individual cells which evolved by microorganisms, multicellular life would not be possible. Our structures and abilities all come from the information stored at the cellular level and the chemicals that they can make, from the digestive juices that allow us to process our food to the pigments that colour our eyes, hair and skin, to the oxygen carrying capacity of our blood: all was made possible by our microorganism ancestors.
The other major evolutionary advance made by microorganisms that changed evolution forever was sexual reproduction. This amazing change brought about the explosion of life forms in the Cambrian Period. Up to that point, evolution could only proceed slowly at the rate of mutations. With the sexual revolution and subsequent genetic recombination, new forms evolved quickly and life really took off.
By the Cambrian Period there was enough oxygen in the ocean and the atmosphere for numerous new aerobic life forms to evolve. All the major phyla of animals appear in this period, as if life was experimenting on a grand scale with all the potential shapes, forms and types. Some succeeded while others became extinct, but in all that period and up to the present, microorganisms continued to survive, compete, thrive and adapt. Some took on parasitic lifestyles, taking advantage of larger multicellular hosts to protect themselves. Parasites and disease-causing microorganisms are important in driving natural selection, taking out weak individuals and thus promoting the evolution of fitter species.
Many higher organisms also took advantage of microorganisms and formed symbiotic relationships with them. Without microorganisms, termites could not digest wood and cows could not digest grass. We too are dependent on our gut flora to help digest our food. Virtually every plant and animal has symbiotic bacteria that they are dependent upon for their survival.
In short, microorganisms have played pivotal roles in the evolution of life on earth and continue to do so in many ways. Love them or hate them, they have been more important than our own species for billions of years and will probably be here long after we are gone.