Bacteria are the fundamental units of life. They are prokaryotes, meaning they do not have a nucleus ("pro" meaning "first"). Bacteria are not animals, they are bacteria. The first animals are called "protozoans," literally meaning "first animals." Protozoans have a nucleus. Bacteria are far older, and far simpler. By "fundamental unit of life" I mean that bacteria are the most basic organisms to meet the definition of life, and were the first form of life to exist. Bacteria have been evolving longer than anything else on Earth, and there is more evolutionary difference between a bacteria and an amoeba than there is between you and a mushroom. They are cells with the capacity to have sex horizontally, which means they can swap DNA within their genome instantly without actually reproducing. Bacteria are constantly exchanging DNA, sharing a knowledge base that allows them to exist under almost any conditions. During the past several billion years of evolution, bacteria have exchanged more information than the human race could ever approach, even if we were to live through the death of the sun. Bacteria are solely responsible for the 20% of the atmosphere that is oxygen, which is a highly reactive gas that took millions of years of evolution to harness correctly. If there was a way to kill all bacteria, the amount of oxygen on Earth would drop to zero, as it literally floats right out of the atmosphere and must be created constantly. While plants definitely play a role, they themselves could not exist without bacteria.
Bacteria outnumber humans by a proportion that approaches infinity. In your body, there are ten bacteria for every one cell that contains your own DNA. The majority of your genetic code has been manipulated by bacteria, who due to their small size "only" make up ten percent of your dry body weight. Every single cell in your body contains mitochondria, which are self-replicating "organelles" that at one point were parasitic bacteria. Mitochondria are solely responsible for the process of respiration that allows you to process oxygen; they contain their own DNA that is completely different from the DNA in your cells' nuclei. They still maintain the ability to have horizontal sex, and there is some suspicion that renegade mitochrondria might have a signficant role in the development of certain cancers.
In every plant and form of algae there are organelles called "chloroplasts" which basically perform the opposite function of the mitochondria. Chloroplasts also reproduce on their own and contain their own DNA. Chloroplasts are nearly genetically identical to cyanobacteria, formerly known as "blue green algae." Without chloroplasts, plants would be unable to produce oxygen from carbon dioxide. Cyanobacteria were the first form of life to transpire and create oxygen in atmosphere.
The fact is that without bacteria, the entire Earth's ecosystem would collapse. Not only do "formerly bacteria, now organelles" occupy every nucleated cell on Earth, but true self-sufficient bacteria are required by all plant life to take nitrogen from the atmosphere and put it into the plant's vascular system. So basically, without bacteria there would be essentially zero oxygen in the atmosphere, and no way to create protein, which requires captured nitrogen to exist. The entire food chain rests on bacteria; not only are they at the very bottom, but they are also at the very top, being natural predators of the human race (only certain forms, of course). Long-established bacteria are less likely to hurt us, as they "genetically realize" that symbiosis works. The new strains we create every day aren't quite aware of this sometimes.
Most people think bacteria are dangerous invisible "germs" that exist to make life miserable. In fact, without bacteria, none of us would exist. The only reason certain strains of bacteria are becoming more and more deadly is because of overuse of antibiotics. Bacteria are capable of controlling the ecosystem, and are often responsible for the creation of viruses that wipe out over-zealous organisms like humans who fail to see their impact on the biosphere. While it would be impossible to scientifically verify it as of now, it is highly likely that much your DNA actually codes for the creation of bacteria and viruses that are essential for your very survival.
If you could kill every bacteria in your body, you would probably die within a week, definitely within a month. A single E. coli bacteria reproducing without any competition every 20 minutes would create a population in four days that outnumbers the (of course an estimated figure) number of quarks in the Universe. Without a balance of diverse bacteria in your body, one strain can wreak havoc, but within each and every one of you there is an intricate balance that can't easily be upset. Due to the sterility of hospitals, most newborns must be injected with Vitamin K at birth because they lack the bacteria in the digestive tract that otherwise would have created it for them.
To sum it up, bacteria are pretty much the only form of life that isn't parasitic (there are of course certain strains that are). They could live without us, but not vice versa. In fact, experiments done with E. coli have demonstrated that they are fully capable of genetically reprogramming themselves to consume what we could consider waste products. In what truly is a self-sufficient ecosystem, some E. coli will "eat" what is readily available (even noxious chemicals like salicin), and others will "eat" the toxic by-products of the bacteria that consume the primary nutrient, a feat we as human beings should admire and try to mimic as much as possible.
The best book on this subject (in my opinion), by the way, is Lynn Margulis' masterpiece, Microcosmos.
I recently received a /MSG disputing my claim that "bacteria are the fundamental units of life." As I explained above, they are fundamental in the sense they were the first cells to exist on Earth, and all cells are descended from them. All prokaryotes are bacteria; all eukaryotes are essentially long-descended symbiotic groupings of bacteria. It has already been clearly demonstrated the mitochondria and chloroplasts contain their own genetic code and replicate on their own, even as they form an essential component of the nucleated cell. Even the mechanical workings of the nucleus itself during cell division are the result of what once could be considered spirochete bacteria. This is how complex life began; prokaryotes eventually coalesced together into more complicated eukaryotes. Whether it's valid to call a nucleated cell a grouping of cooperative bacteria is more of a semantics issue than anything else (as would be the argument that a human being is a collection of eukaryotic cells, or a collection of sub-atomic particles for that matter), but the biological fact that all cellular life (which is how all life is defined on Earth) derived from bacteria is not a topic of much debate (except perhaps among creation mythologists who throw all scientific theory out the window whilst continuing to use computers, CD players, and the like in pursuit of a model that does not clash with their perception of reality), and for these reasons I will maintain my stance that bacteria are the fundamental units of life.
Gritchka says...What hasn't been mentioned in this node is that Bacteria is now the taxon name for the domain of "eubacteria" (bacteria other than Archaea) -- see The Three Domains...