Universality: Biochemistry, Cells and The Genetic Code
There are many universal units and common processes that are fundamental
to the known forms of life. For example all forms of life consist
of cells, which in turn, are based on a common carbon-based biochemistry.
All organisms pass on their heredity via the genetic material
which is based upon the nucleic acid DNA using a universal genetic
code. In development the theme of universal processes is also
present, for example in most metazoan organisms the basic steps
of the early embryo development share similar morphological stages
and include similar genes.
Evolution: the Central Principle of Biology
One of the central, organizing concepts in biology is that
all life has descended from a common origin through a process
of
evolution. Indeed, it is one of the reasons that biological organism
exhibit the striking similarity of units and processes discussed
in the previous section. Charles Darwin established evolution
as a viable theory by articulating its driving force: natural
selection. (Alfred Russell Wallace is commonly recognized as
the co-discoverer of this concept). Genetic drift was embraced
as an additional mechanism in the so-called modern synthesis.
The evolutionary history of a species—which tells the characteristics
of the various species from which it descended—together
with its genealogical relationship to every other species is
called its phylogeny. Widely varied approaches to biology generate
information about phylogeny. These include the comparisons of
DNA sequences conducted within molecular biology or genomics,
and comparisons of fossils or other records of ancient organisms
in paleontology. Biologists organize and analyze evolutionary
relationships through various methods, including phylogenetics,
phenetics, and cladistics. Major events in the evolution of life,
as biologists currently understand them, are summarized on this
evolutionary timeline.
Diversity: the Variety of Living Organisms
A phylogenetic tree of all living things, based on rRNA gene data,
showing the separation of the three domains bacteria, archaea,
and eukaryotes as described initially by Carl Woese. Trees constructed
with other genes are generally similar, although they may place
some early-branching groups very differently, presumably owing
to rapid rRNA evolution. The exact relationships of the three
domains are still being debated.Despite the underlying unity,
life exhibits an astonishing wide diversity in morphology, behavior
and life histories. In order to grapple with this diversity,
biologists attempt to classify all living things. This scientific
classification should reflect the evolutionary trees (phylogenetic
trees) of the different organisms. Such classifications are the
province of the disciplines of systematics and taxonomy. Taxonomy
puts organisms in groups called taxa, while systematics seeks
their relationships.
Traditionally, living things were divided into five
kingdoms:
Monera -- Protista -- Fungi -- Plantae -- Animalia
However, this five-kingdom system is now considered by many to
be outdated. More modern alternatives generally begin with the
three-domain system:
Archaea (originally Archaebacteria) -- Bacteria (originally Eubacteria)
-- Eukaryota
These domains reflect whether cells have nuclei or not as well
as differences in cell exteriors. There is also a series of intracellular "parasites" that
are progressively less alive in terms of being metabolically active:
Viruses -- Viroids -- Prions
A group of organisms is said to have common descent if they
have a common ancestor. All existing organisms on Earth are
descended
from a common ancestor or ancestral gene pool. This "last
universal common ancestor, that is, the most recent common ancestor
of all organisms, is believed to have appeared about 3.5 billion
years ago.
The notion that "all life [is] from [an] egg" (from
the Latin "Omne vivum ex ovo") is a foundational concept
of modern biology, it means that there has been an unbroken continuity
of life from the initial origin of life to the present time. Up
into the 19th century it was commonly believed that life forms
can appear spontaneously under certain conditions. The universality
of the genetic code is generally regarded by biologists as definitive
evidence in favor of the theory of universal common descent (UCD)
for all bacteria, archaea, and eukaryotes.
Homeostasis: Adapting to Change
Homeostasis is the property of an open system to regulate its internal
environment so as to maintain a stable condition, by means of
multiple dynamic equilibrium adjustments controlled by interrelated
regulation mechanisms. All living organisms, whether unicellular
or multicellular exhibit homeostasis. Homeostasis can manifest
itself at the cellular level through the maintenance of a stable
internal acidity (pH); at the organismal level warm-blooded animals
maintain a constant internal body temperature; and at the level
of the ecosystem, for example when atmospheric carbon dioxide
levels rise, plants are able to grow better and thus remove more
carbon dioxide from the atmosphere. Tissues and organs can also
maintain homeostasis.
Interactions: Groups and Environments
Mutual symbiosis between clownfish of the genus Amphiprion that
dwell among the tentacles of tropical sea anemones. The territorial
fish protects the anemone from anemone-eating fish, and in turn
the stinging tentacles of the anemone protects the anemone fish
from its predatorsEvery living thing interacts with other organisms
and its environment. One of the reasons that biological systems
can be difficult to study is that there are so many different
possible interactions with other organisms and the environment.
A microscopic bacterium responding to a local gradient in sugar
is as much responding to its environment as a lion is responding
to its environment when it is searching for food in the African
savannah. Within a particular species behaviors can be co-operative,
aggressive; parasitic or symbiotic. Matters become more complex
still when two or more different species interact in an ecosystem,
and is the province of ecology.
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