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The origin of life and the relationships between its major lineages are
controversial. Two main grades may be distinguished, the prokaryotes and
eukaryotes.
origin of life:
Research into the origin of life is a limited field of research despite its profound impact on biology
and human understanding of the natural world. Progress in this field is generally slow and
sporadic, though it still draws the attention of many due to the gravity of the question being
investigated. A few facts give insight into the conditions in which life may have emerged, but the
mechanisms by which non-life became life are still elusive.
For the observed evolution of life on earth, see the timeline of life.
Prokaryotes:
Prokaryotes (from Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, +
suffix -otos, pl. -otes; also spelled "procaryotes") are organisms without a cell nucleus (= karyon),
or indeed any other membrane-bound organelles, in most cases unicellular (in rare cases,
multicellular). This set of characteristics is distinct from eukaryotes (also spelled "eucaryotes"),
organisms that have cell nuclei and may be variously unicellular or multicellular. The difference
between the structure of prokaryotes and eukaryotes is so great that it is considered to be the
most important distinction among groups of organisms. Most prokaryotes are bacteria, and the
two terms are often treated as synonyms. However, Woese has proposed dividing prokaryotes
into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of the
significant genetic differences between the two. This arrangement of Eukaryota, Bacteria, and
Archaea is called the three-domain system. This replaces the two-empire system
Eukaryote:
A eukaryote, is an organism with a complex cell or cells, in which the genetic material is organized
into membrane-bound nucleus/nuclei. Eukaryotes comprise animals, plants, and fungi—which are
mostly multicellular—as well as various other groups that are collectively classified as protists
(many of which are unicellular). In contrast, prokaryotes are organisms, such as bacteria, that lack
nuclei and other complex cell structures. Eukaryotes share a common origin, and are often treated
formally as a superkingdom, empire, or domain. The name comes from the Greek ευ, meaning
good, and κάρυον, meaning nut, in reference to the cell nucleus.
Three-domain system:
The three-domain system is a biological classification introduced by Carl Woese in 1990 that
emphasizes his separation of prokaryotes into two groups, originally called Eubacteria and
Archaebacteria. Woese argued based on differences in 16S rRNA genes that these two groups
and the eukaryotes each arose separately from an ancestral progenote with poorly developed
genetic machinery. To reflect these primary lines of descent, he treated each as a domain, divided
into several different kingdoms. The groups were also renamed the Bacteria, Archaea, and
Eukarya, further emphasizing the separate identity of the two prokaryote groups.
A phylogenetic tree based on rRNA data, showing the separation of bacteria, archaea, and
eukaryotes.Although the three-domain system was quickly adopted by most molecular
systematists, biologists like Mayr criticized him for over-emphasizing the uniqueness of the
archaebacteria and ignoring strong genetic similarities between the groups. Subsequent studies
have confirmed that the archaea are unusual in the composition of their cell membrane and
structure of their flagella. Other significant differences include archaeal systems for DNA
replication and transcription which bear distinct similarity to those found in eukaryotes. For
instance, archaeal RNA polymerase consists of up to 14 subunits, whereas most bacterial RNA
polymerases have only 4 subunits. Analysis of these subunits suggests that they are more closely
related to those found in eukaryotes. Also, the archaea produce a number of DNA-binding
proteins with similarity to eukaryotic histones.
Nevertheless, a minority viewpoint suggests retaining the older two-empire system (Prokaryota
and Eukaryota) and using the word bacterium in its earlier meaning of prokaryote.
Which system is preferable depends partly on the relationships of the organisms in question.
Although the progenote hypothesis is discredited, molecular trees tend to group living things into
the three domains, with the eukaryotes placed beside or within the Archaea and the eubacteria
forming a separate branch. However, it has been suggested this is an artifact of long branch
attraction and that the root may instead belong among the eubacteria, in which case many
eubacterial lines diverged before the archaebacteria did.
In 2006, the discovery of unique properties of the Mimivirus started a discussion about
classification of (certain) viruses as fourth domain of life.
Disciplines of ecology:
Ecology is a broad discipline comprised of many sub-disciplines. A common, broad
classification, moving from lowest to highest complexity, where complexity is defined as the
number of entities and processes in the system under study, is:
Physiological Ecology (or ecophysiology) and Behavioral ecology examine adaptations of the
individual to its environment.
Population ecology (or autecology) studies the dynamics of populations of a single species.
Community ecology (or synecology) focuses on the interactions between species within an
ecological community.
Ecosystem ecology studies the flows of energy and matter through the biotic and abiotic
components of ecosystems.
Landscape ecology examines processes and relationship across multiple ecosystems or very
large geographic areas.
Ecology can also be sub-divided according to the species of interest into fields such as animal
ecology, plant ecology, insect ecology, and so on. Another frequent method of subdivision is by
biome studied, e.g., Arctic ecology (or polar ecology), tropical ecology, desert ecology, etc. The
primary technique used for investigation is often used to subdivide the discipline into groups such
as chemical ecology, genetic ecology, field ecology, statistical ecology, theoretical ecology, and
so forth. Note that these different systems are unrelated and often applied at the same time; one
could be a theoretical plant community ecologist, or a polar ecologist interested in animal genetics.
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