The simplest form of life and yet the most important is bacteria. It consists of a single cell. The most important component of bacteria is its DNA. It is wrapped up like a ball of string and floats in a watery fluid contained in an outer wrapper called the cell membrane. The fluid inside the membrane called the cytoplasm is mostly water and a mix of organic chemicals such as enzymes that the cell has manufactured along with amino acids, glucose etc. At the center of the cell is the ball of DNA that carries the genetic code of the bacteria.
Some bacteria have limb like structures attached to their membrane called the flagellum that help propel bacteria. Different bacteria have different number of flagellum. Some have none. The shape of different bacteria too differs. Some are shaped like an ellipse, others may be squeezed in the middle or sides etc. Human cells have the same basic features as bacterial cells along with additional advanced features.
The cytoplasm of bacteria contains numerous chemicals such as enzymes. A simple bacterium contains numerous different enzymes manufactured by instructions provided by the bacterial DNA. Each enzyme helps to manufacture proteins required by bacteria to grow and multiply. Some bacteria are photosynthetic. They can make their own food from sunlight, just like plants and produce oxygen. These are some of the first life forms that must appear on any planet where life has to emerge since they have not much else in terms of food to consume. Some bacteria can live off unusual "foods" such as iron or sulfur whereas other help to convert nitrogen compounds of the atmosphere into a food suitable for plants to come later in the evolution of life on a planet. The variety of foods, bacteria can consume is truly amazing. They are not at all choosy like higher life forms about what they consume. Bacteria are all around us. Animal and human bodies carry billions of these little organisms along with viruses. These microbes live on your skin, under your fingernails, in your hair and inside your bodies and it has nothing to do with cleanliness. They are a part of us. Some of us view bacteria as disease causing agents. A few do cause disease but the vast majorities do not. Many are even essential for digestion of food.
Since bacteria are the first life forms, they have been around much longer than other animals and have even become a part of their bodies. A single human body carries more microbes than all the humans that live on earth. Humans depend on bacteria for digestion. Besides breaking down tough cellulose, the bacteria that live in the intestine also produce essential vitamins that humans could not make themselves. Without them a person would have a whole host of problems. Without vitamin K, which is necessary in the clotting process of blood, a person would be prone to uncontrolled bleeding. Without vitamin B12, a person would suffer from anemia. Bacteria are the reason people develop digestive problems when taking an antibiotic. The drug kills the useful bacteria along with undesirable ones.
Animals like cattle, sheep, goats, giraffes, and camels are incapable of digesting cellulose, a tough, protective structural substance that forms plant cell walls. Yet leafy greens and grass are their prime food source. The only creatures known to digest cellulose are microbes, so the only way grazing animals can get any nutrition from plants is with the help of bacteria. The bacteria in a cow’s stomach break it down into glucose that the cow can then use.
Bacteria are found everywhere on earth, high up in the atmosphere, in the arctic region and in hibernation inside ancient glaciers. They live deep inside the earth too. They have been found at depths of more than a thousand feet. One teaspoon of soil may contain 10 million bacteria. The oceans are full of bacteria. It is estimated that the total mass of microbes on your planet may be more than twenty five times the total mass of all animals and humans put together.
Like all living things it grows, reproduces, produces waste, moves around, and communicates with other microbes, and all this with just its one single cell. As already mentioned, bacteria have a wide range of choice for their food. Some depend on photosynthesis for their food just like plants and they give out oxygen as a waste product. Bacteria are the ancestors not only of animal and human life but also that of plants. Bacteria can feed off other bacteria, wood, dead animals, glue, and a wide range of minerals that cannot be digested by higher life forms.
Bacteria can reproduce by doubling their cells rapidly. One cell divides into two and so on. Some can do it as frequently as once in twenty minutes. A single bacterium can grow into millions in a matter of hours provided their food supply is maintained. Yet unlike higher life forms they have the capacity to hibernate as well. They do this when no food is available or if frozen. Indeed they can survive forever if not destroyed by heat or some other agent that kills them. Bacteria do not have a limited life span like animals and humans. Besides being the carriers of genetic codes, bacteria carry on a number of other processes necessary for life. Bacteria keep the planet's life cycles turning. Animals breathe in oxygen and exhale carbon dioxide (CO2). Plants breathe in CO2 and release oxygen. Microbes beneath the sea pump out about 150 billion kilograms of oxygen every year, producing the oxygen we breathe. If that system were to stop, all the oxygen in the air would be exhausted within decades. Cyanobacteria are aquatic and photosynthetic, that is, they live in the water, and can manufacture their own food. They are one of the largest and most important groups of bacteria on any habitable planet. The other contribution of the cyanobacteria is the origin of plants. The chloroplast with which plants make food for themselves is actually a cyanobacterium living within the plant's cells.
The nitrogen cycle is a similar story. Nitrogen exists in every living cell and is necessary for building proteins. Although nitrogen is the most common gas in atmosphere, animals cannot use it in that form. Animals get their nitrogen from eating plants or eating plant eaters. Most plants are unable to take nitrogen from air; they get their nitrogen from the soil. They cannot get the nitrogen they need without the help of nitrogen-fixing bacteria. These bacteria modify nitrogen by combining nitrogen in atmosphere with other elements to form organic compounds. The nitrogen, now in a modified form is available to the plants through their root systems with the help of other bacteria in the soil. In return, the plant supplies the microbes with nutrients for their growth. Some bacteria simply live in the soil surrounding the roots, but other kinds of bacteria actually live inside the roots of plants.
Viruses unlike bacteria are not living organisms and are generally smaller. They may be visualized as a part of a bacteria, a part consisting of a set of genetic material that is transferable to other cells. Viruses are not fully life forms because they cannot consume food, grow and reproduce on their own. In order to reproduce viruses need the help of other living cells. They can get inside the membrane shell of bacteria and then multiply with the help of the bacteria. A cell so hijacked by a virus may start to make hundreds of copies of the virus instead of reproducing itself. Even though some viruses cause disease they are as essential for life as bacteria are. Viruses like bacteria contain DNA (and sometimes RNA instead) in a protective shell.
The essence of the evolutionary model here is that life arrives on a planet from the cosmos in a bacterial form. It develops further by evolution, primarily assisted by viruses that carry genetic information back and forth between species and bacteria. Bacteria return the new evolutionary information generated in the planet to the cosmos from time to time.
Viruses are the workers that carry genes not only to higher organisms but also other bacteria. In the early 1950s it was observed in Japan that multiple antibiotic resistance was developing in a single step in patients with enteric infections. In 1960 T. Akiba, T. Koyama, Y. Isshiki, S. Kimura, and T. Fukushima (Jap. med Wochschr. 1866:45) described these phenomena. In 1961 T. Watanabe and T. Fukasawa reported that this multiple drug resistance was being transferred by a plasmid. Bacteria because of their simple structure can modify, mutate and evolve much more rapidly than higher life forms.