Origin and Evolution of Bacteria

The first bacteria may have come from meteorites in space, been directed here by alien life forms, or, most likely, evolved from complex chemical reactions on the surface of the Earth. Many sorts of material systems are known to become more complex when exposed to flows of energy; energy flows are also known to cause cycles, both chemical and physical, to arise. A flow of energy represents complexity pre-existing in the environment that reaches thermodynamic equilibrium, sometimes by forming complex structures. Beyond the actively researched details of the origins of life from nonlife, one can assume that the first living beings were chemical cycles becoming more complex. Energy from the environment, such as geothermal energy from the earth, solar energy, and energy stored in chemical compounds, funneled through cell membranes. Cell membranes are amphiphilic, meaning that they tend to keep the oily compounds of life inside them and the water of the environment outside. Any form that began actively to seek energy sources, such as the first faithfully reproducing bacteria with DNA, would have quickly spread to oust other energy-driven complex systems. Researchers today, by looking at the details of modern cyclical cell metabolism, may be able to discover the ancient steps leading from thermodynamic systems to the first bacteria.

Viruses are smaller than bacteria, and they are composed of relatively few genes and proteins. They are not true organisms or cells, however, because they can reproduce only by using the genetic and protein-making apparatus of living cells. And truly functioning cells, today, are all bacteria. The smallest bacterium known is Mycoplasma, one form of which causes venereal diseases in human beings. Like viruses, however, Mycoplasma must be considered a derivative or degenerate form: it appears to be the evolutionary result of earlier, more self-sufficient cells that reproduced on their own. When cells team up, as occurs often in evolution (the phenomenon is known as symbiosis), organisms may be taken care of, even sometimes inhabiting the insides of other cells, and thus lose parts of themselves; viruses and Mycoplasma, although small, do not represent the oldest life forms.

The oldest known life forms are microfossils of bacteria from Australia and South Africa. Radioactively dated at some 3.5 billion years, these microfossils show evidence of bacteria being already widespread on the planetary surface. Indeed, because the early Hadean geological eon during earth's formation was so hot, older microfossils of bacteria would not have survived in the rock record. Thus, as soon as there could be evidence of fossil bacteria, there is.

Genetically, the earliest bacteria are thought to have been archaebacteria, a classification that includes methane-producing bacteria, halophiles that survive conditions too hypersaline (salt-rich) for most other organisms, and sulfur bacteria able to tolerate the extreme heat radiating from earth's interior. What ties these diverse sorts of bacteria together is their

RNA, which exhibits a long stretch of similar base pairs, one that is substantially different from the rest of bacteria, sometimes known as eubacteria. Such bacteria, able to inhabit extreme conditions inhospitable to other living beings, are known as extremophiles. Geological evidence from the earth's crust provides strong evidence that free atmospheric oxygen did not exist early on in the earth's history; breathable oxygen in the air, it turns out, was probably put there by life—bacterial life. The change from an early oxygen-poor (anoxic) planetary environment to a modern planetary surface rich in oxygen (oxic) was one of the most dramatic events in the history of the biosphere. The presence of extremely hardy extremophiles able to tolerate conditions no longer prevalent on the earth's surface points to a past in which bacteria not only dominated the biosphere but were also its sole inhabitants.

Obviously, we cannot know exactly what happened during the course of evolutionary history. However, it is clear that bacteria played, and continue to play, a key role both in early evolution and in present global ecology. It is impossible to appreciate the importance of bacterial diversity fully without an understanding of the major impact that they have made on evolutionary history.

The first bacterial cells may have been fermenters, gaining energy from hydrocarbon compounds produced naturally before life by the rays of the sun. Like modern fermenting cells, they would not have required atmospheric oxygen, which had not yet accumulated in the atmosphere. Alternatively, the first life may have been photosynthetic anaerobes, such as the purple bacteria that today use the energy of sunlight and the hydrogen of hydrogen sulfide (H2S) rather than water (H2O) to make the hydrogen-rich compounds of their bodies. Biologist Jack Corliss, one of the first to go down into the ocean abyss in the submersible Alvin, was part of the team that discovered thriving ecosystems beneath the Galapagos Islands. He proposes that similar thermal upwellings were the site of first life. Here, at the bottom of the ocean, pogonophoran tubeworms are internally fed by symbiotic bacteria who themselves feed not on sunlight or other organisms. Instead, they take their energy directly from the oxidation of hydrogen sulfide gas seeping up from deep within the earth's crust. Such sulfide bacteria, which are genetically classified with the most ancestral types, could have been among the first life forms. Corliss argues that the first life forms fed off similar sulfide redox gradients, which would have been more prevalent on the earth's surface during the earliest eons (the Hadean and Archean) of planetary history.

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