Evolution of Microorganisms
Microorganisms came into being on earth over a period of about 1.2 to 1.5 billion years. Fossil microbes have been found in rocks 3.3 to 3.5 billion years old. Since then, microorganisms have had the principal task of recycling organic matter in the environment. As such they are absolutely essential to the health of the earth. Without them, the earth would be a gigantic, permanent waste dump. Microorganisms are responsible for recycling the huge masses of organic matter synthesized by plants as life on earth evolved. Furthermore, microorganisms—the cyanobacteria or their DNA in the chloroplasts in plant cells—were the source of most of the free oxygen in the early atmosphere. They also oxidize ammonia (the universal end product of protein metabolism) to nitrate, which is the only nitrogen source used by plants and is therefore essential for production of our plant foods. Microorganisms also are responsible for cellulose hydrolysis in the rumens (first stomach compartments) of cattle, facilitating the production of animal protein for human consumption. And, in recent times, microorganisms have been the sources of antibiotics that have enabled the cure of numerous diseases. Blue-green algae (cyanobacteria) are prokaryotes (that is, their cells have no distinct nucleus). They are very independent nutritionally since they can perform photosynthesis using chlorophyll a. Thus they can synthesize sugars for energy from carbon dioxide using the sun's radiation. They also release oxygen. They can respire aerobically and can fix nitrogen, generating amino acids and protein. They require only water, nitrogen gas, oxygen, carbon dioxide, some minerals, and sunlight. The evidence is that they were on earth 3.2 billion years ago. The cyanobacteria are among the earliest microorganisms and very important even today. Green algae are eukaryotes (that is, their cells have a distinct nucleus). They evolved about one billion years ago. They contain chlorophylls a and b, which enable them to convert carbon dioxide, through sunlight radiation, to sugars, and to polymerize sugars to starches, hemicelluloses, and celluloses—some of our most important sources of food energy. Green algae are still major sources of food in the oceans. Green algae were likely the life forms that evolved into plants, which first lived primarily in the oceans but moved to the land about 450 million years ago, about the same time as the amphibians and first land animals evolved. It is believed that the first mammals evolved about 150 million years later, along with insects and reptiles, which were dominant. Another 150 million years later, dinosaurs and the first birds evolved, along with the first flowering plants. During the entire period from 3.6 billion years ago, microorganisms were consuming and recycling the organic matter from themselves and other forms of life as they lived and died. For several billion years, bacteria, algae, and other microorganisms served as food for other microbes and for higher animals as they evolved. When plants evolved in the oceans and then subsequently moved to land, they became the major sources of food for other forms of life, including microorganisms, animals, and eventually humans.