Evolutionary origin of bacteria and viruses

Earth formed between 4.5 and 6 billion years ago. Conditions initially remained inhospitable for the potential development of life. By about 3.0 billion years ago, however, an atmosphere that contained the appropriate blend of nitrogen, oxygen, carbon, and hydrogen allowed life to commence. The formation of proteins and nucleic acids led to the generation of the genetic code , contained in deoxyribonucleic and ribonucleic acids, and the protein machinery to translate the information into a tangible product.

Fossil evidence indicates that one of the first life forms to arise were bacteria . The planetary conditions that were the norm four to six billion years ago were much different from now. Oxygen was scarce, and extremes of factors such as temperature and atmospheric radiation were more common than now. Although the exact origin of bacteria will likely never be known, the present-day bacteria that variously tolerate extremes of temperature, salt concentration, radiation, pH and other such environmental factors may be examples of the original bacteria.

Such "extremophiles " are part of the division of life known as the Archae, specifically the archaebacteria.

Whether bacteria originated in the sea or on land remains a mystery. The available evidence, however, supports the origin of bacteria in the sea. With the advent of molecular means of comparing the relatedness of bacteria, it has been shown that most of the bacteria known to exist on land bear some resemblance to one another. But, only some 10% of the bacteria from the ocean are in any way related to their terrestrial counterparts. In support of the origin of bacteria in the ancient seas is the discovery of the vast quantities and variety of viruses in seawater.

The discovery in the 1970s of bacteria thriving at hydrothermal vents deep beneath the surface of the ocean suggests that bacterial life in the ancient oceans was at least certainly possible. Such bacteria would derive their energy from chemical compounds present in their environment. It is also likely that bacterial life was also developing concurrently in response to another energy source, the sun. Indeed, the evolutionarily ancient cyanobacteria are photosynthetic microorganisms , which derive their energy from sunlight.

One type of bacteria that is definitely known to have been among the first to appear on Earth is the cyanobacteria. Fossils of cyanobacteria have been uncovered that date back almost 4 billion years. These bacteria are suited to the low oxygen levels that were present in the planet's atmosphere at that time. The cyanobacteria produced oxygen as a waste gas of their metabolic processes and so helped to create an atmosphere containing a greater amount of oxygen. Other, oxygen-requiring bacteria could then develop, along with other life forms.

In contrast to bacteria, scientists debate if viruses are alive. They are not capable of their own reproduction. Instead, they require the presence of a host in which they can introduce their genetic material. Through the formation of products encoded by the viral genetic material and by the use of aspects of the host's replication machinery, viruses are able to direct the manufacture and assembly of components to produce new virus.

The nature of viral replication requires the prior presence of a host. It remains unclear whether the first virus arose from a prokaryotic host, such as a bacterium, or a eukaryotic host. However, the appearance of prokaryotic life prior to eukaryotic life argues for the origin of viruses as an evolutionary offshoot of prokaryotes.

Scientists are in general agreement that the first virus was a fragment of DNA or ribonucleic acid (RNA ) from a eventual prokaryotic or eukaryotic host. The genetic fragment somehow was incorporated into a eukaryotic and became replicated along with the host's genetic material. Over evolutionary time, different viruses developed, having differing specificities for the various bacteria and eukaryotic cells that were arising.

The evolutionary origin of viruses will likely remain conjectural. No fossilized virus has been detected. Indeed, the minute size of viruses makes any distinction of their structure against the background of the rock virtually impossible. Likewise, bacterial fossilization results in the destruction of internal detail. If a virus were to be present in a fossilizing bacterium, any evidence would be obliterated over time.

Some details as to the evolutionary divergence of viruses from a common ancestor are being realized by the comparison of the sequence of evolutionarily maintained sequences of genetic material. This area of investigation is known as virus molecular systematics.

The comparison of a number of gene sequences of viral significance, for example the enzyme reverse transcriptase that is possessed by retroviruses and pararetroviruses, is consistent with the evolutionary emergence of not one specific type of virus, but rather of several different types of viruses . The present day plethora of viruses subsequently evolved from these initial few viral types (or "supergroups" as they have been dubbed). So, in contrast to an evolutionary "tree." viral evolutionary origin resembles more of a bush. Each of the several branches of the bush developed independently of one another. Furthermore, the consensus among virologists (scientists who study viruses) is that this independent evolution did not occur at the same time or progress at the same rate. In scientific terms, the viral evolution is described as being "polyphyletic."

The evolution of viruses with life forms, including bacteria, likely occurred together. On other words, as bacteria increased in diversity and in the complexity of their surfaces, new viruses evolved to be able to utilize the bacteria as a replication factory. Similarly, as more complex eukaryotic life forms appeared, such as plants, insects, birds, and mammals, viruses evolved that were capable of utilizing these as hosts.

See also Bacterial kingdoms; Evolution and evolutionary mechanisms; Mitochondrial DNA