Microbiology and Immunology

Evolutionary origin of bacteria and viruses WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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changes in the numbers of different forms of a gene (allelic

frequency) that result from sexual reproduction. Genetic drift

can occur as a result of random mating (random genetic drift)

or be profoundly affected by geographical barriers, cata-

strophic events (e.g., natural disasters or wars that signifi-

cantly affect the reproductive availability of selected members

of a population), and other political-social factors.

Natural selection is based upon the differences in the

viability and reproductive success of different genotypes with

a population (differential reproductive success). Natural selec-

tion can only act on those differences in genotypethat appear

as phenotypic differences that affect the ability to attract a

mate and produce viable offspring that are, in turn, able to live,

mate and continue the species. Evolutionary fitness is the suc-

cess of an entity in reproducing (i.e., contributing alleles to the

next generation).

There are three basic types of natural selection. With

directional selection, an extreme phenotypeis favored (e.g.,

for height or length of neck in giraffe). Stabilizing selection

occurs when intermediate phenotypeis fittest (e.g., neither too

high or low a body weight) and for this reason it is often

referred to a normalizing selection. Disruptive selection

occurs when two extreme phenotypes are fitter that an inter-

mediate phenotype.

Natural selection does not act with foresight. Rapidly

changing environmental conditions can, and often do, impose

new challenges for a species that result in extinction. In addi-

tion, evolutionary mechanisms, including natural selection, do

not always act to favor the fittest in any population, but instead

may act to favor the more numerous but tolerably fit.

The operation of natural evolutionary mechanisms

exhibited in microorganisms is complicated in humans by geo-

graphic, ethnic, religious, and social groups and customs.

Accordingly, the effects of various evolution mechanisms on

human populations are not as easy to predict. Increasingly

sophisticated statistical studies are carried out by population

geneticists to characterize changes in the human genome,

especially with regard to immunological differences between

populations.

See alsoAntibiotic resistance, tests for; Evolutionary origin of

bacteria and viruses; Extraterrestrial microbiology; Immu-

nogenetics; Miller-Urey experiment; Molecular biology and

molecular genetics; Molecular biology, central dogma of;

Mutants, enhanced tolerance or sensitivity to temperature and

pH ranges; Mutations and mutagenesis; Radiation mutagene-

sis; Radiation resistant bacteria; Rare genotype advantage;

Viral genetics

EVOLUTIONARY ORIGIN OF BACTERIA

AND VIRUSESEvolutionary 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, car-

bon, 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 tem-

perature 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, pHand

other such environmental factors may be examples of the orig-

inal 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 terres-

trial counterparts. In support of the origin of bacteria in the

ancient seas is the discovery of the vast quantities and variety

of virusesin seawater.

The discovery in the 1970s of bacteria thriving at

hydrothermal ventsdeep beneath the surface of the ocean sug-

gests that bacterial life in the ancient oceans was at least cer-

tainly 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 evolu-

tionarily ancient cyanobacteria are photosynthetic microor-

ganisms, 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 meta-

bolic processes and so helped to create an atmosphere contain-

ing 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 pres-

ence 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

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