Life

Although universal consensus on a definition is lacking, life can be considered the characteristic state of living organisms and individual cells or that quality or property that distinguishes living organisms from dead organisms and inanimate objects.

Although life is a difficult term to define, biological properties common to the known organisms found on Earth (plants, animals, fungi, protists, archaea, and bacteria) are that they are carbon-and-water-based, are cellular with complex organization, use energy and undergo metabolism, possess a capacity to grow, respond to stimuli, reproduce, and have various adaptations to the environment. Beyond these biological manifestations of matter, some philosophical and religious perspectives add that living organisms possess an inner aspect or character that confers the quality of life (see below) (Lee 1973).

An entity with the above properties is considered to be an organism. Some nonliving entities exhibit some of these characterists; for example, a car uses energy, glaciers can grow larger, a television responds to stimuli from a remote, and machines can be built that can reproduce copies of themselves. However, living organisms possess all of these characteristics.

Not every definition of life considers all of these properties to be essential. For example, the capacity for descent with modification is often taken as the essential property of life. This definition notably includes viruses, which do not qualify under narrower definitions as they are acellular and do not metabolise. Broader definitions of life may also include theoretical non-carbon-based life.

Although it cannot be pinpointed exactly, evidence suggests that life on Earth has existed for about 3.7 billion years [1].

Characteristics of life

Living organisms share many universal characteristics, including that they are composed of cells; pass on their heredity using a nearly universal genetic code; need energy from the environment to exist, grow, and reproduce; and maintain their internal environment; and among others. These are the common set of characteristics identified by biologists that distinguish living organisms from nonliving things.

1. Cells. With the exception of viruses, all organisms consist of cells. A cell is the basic unit of life, being the smallest unit that can carry on all the processes of life, including maintenance, growth, and even self-repair. Some simple life forms, such as the paramecium, consist of a single cell throughout their life cycle and are called unicellular organisms. Multicellular organisms, such as a whale or tree, may have trillions of cells, and have differentiated cells that perform specialized functions.
2. Carbon-based biochemistry. Living organisms are characterized by a common carbon-based biochemistry. All organisms pass on their heredity via the genetic material that is based upon nucleic acids such as DNA using a nearly universal genetic code. Every cell, no matter how simple or complex, utilizes nucleic acids for transmitting and storing the information needed for manufacturing proteins.
3. Energy and metabolism. Every living being needs energy from the environment in order to exist, grow, and reproduce. The sun is the main source of energy and is captured through a process of photosynthesis, a process by which plants, algae, and some bacteria harness the energy of sunlight to produce food. Ultimately, nearly all living things depend on energy produced from photosynthesis for their nourishment, making it vital to life on Earth. There are also some bacteria that utilize the oxidation of inorganic compounds such as hydrogen sulfide or ferrous iron as an energy source. An organism that produces organic compounds from carbon dioxide as a carbon source, using either light or reactions of inorganic chemical compounds as a source of energy, is called an autotroph. Other organisms do not make their own food but depend directly or indirectly on autotrophs for their food. These are called heterotrophs. Metabolism is the production of energy by converting nonliving material into cellular components (synthesis) and decomposing organic matter (catalysis). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
4. Development and growth. Living things grow and develop as they age. This involves maintenance of a higher rate of synthesis than catalysis, with growth of the organism taking place through cell enlargement and cell division. A growing organism increases in size in all of its parts, rather than simply accumulating matter. In most metazoan organisms, the basic steps of the early embryo development share similar morphological stages and include similar genes.
5. Homeostasis: All living organisms, whether unicellular or multicellular, exhibit homeostasis. Homeostasis is the property of an open system to regulate its internal environment so as to maintain a stable condition. 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 theoretically able to grow healthier and thus remove more carbon dioxide from the atmosphere. Tissues and organs can also maintain homeostasis.
6. Organization. Living organisms are organized at both the molecular and cellular levels. The energy and materials from the environment are organized, such as the internal structure of a cell, or the organization of multicellular organisms into tissues, organs, adn systems.
7. Response to stimuli. All living organisms respond to the environment. A response to stimuli can take many forms, from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun or an animal chasing its prey. A cockroach may respond to light by running for a dark place. When there is a complex set of response, it is called a behavior. For example, the migration of salmon is a behavioral response.
8. Adaptation. Living organisms have characteristics that give them a survival/reproductive advantage in an environment; that is, they have adapattions to the environment. Living organisms show variabilty in these adaptations, allowing the species to continue in a fluctuating or changing environment.
9. Reproduction. Reproduction is the ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth. Sexual reproduction is a trait that is almost universal, if not universal, among living beings. Asexual reproduction is not uncommon. In fact, it is widespread among fungi and bacteria, many insects reproduce in this manner, and some reptiles and amphibians. Nonetheless, sexual reproduction is also seen in these same organisms, including in bacteria which will exchange genetic material between donors (+ mating type) and recipients (- mating type). Evolutionary biologist and geneticist John Maynard Smith maintained that the perceived advantage for an individual organism to pass only its own entire genome to its offspring is so great that there must an advantage by at least a factor of two to explain why nearly all animal species maintain a male sex.

Dual characteristics

In addition to the above, generally recognized biological principles, two additional qualities of living organisms are , living beings share with all existent beings the quality of dual characteristics or polarity. One of these is the quality of positivity and negativity: Just as sub-atomic particles have positive (electron) and negative (proton) elements that interrelate and form atoms, all living beings exhibit positive and negative characteristics. Most animals reproduce through relationships between male and female, and higher plants likewise have male and female elements, such as the (male) stamen and (female) pistil in flowering plants (angiosperms). Lower plants, fungi, some of the protists, and bacteria likewise exhibit reproductive variances, which are usually symbolized by + and - signs (rather than being called male and female), and referred to as "mating strains" or "reproductive types" or similar appellations.

Another more philosophical concept is the universal dual characteristic of Sung Sang and Hyung Sang, or a relationship within each organism between the internal aspects of function and character and the visible aspects of matter, structure, and shape. For example, an animal will exhibit the internal aspects of life, instinct, and function of its cells, tissues, and organs, which relate with the visible shape made up by those cells, tissues, and organs. (See dual characteristics for a more detailed discussion of this concept.)

Addition of sung sang hyung sang aspect

See UT, p. 49, paragraph 2 and 3 Note from p. 48-52 of UT

Exceptions to the conventional definition

It is important to note that life is a definition that applies primarily at the level of species, so even though many individuals of any given species do not reproduce, possibly because they belong to specialized sterile castes (such as ant workers), these are still considered forms of life. One could say that the property of life is inherited; hence, sterile hybrid species such as the mule are considered life although not themselves capable of reproduction. It is also worth noting that non-reproducing individuals may still help the spread of their genes through such mechanisms as kin selection.

For similar reasons, viruses and aberrant prion proteins are often considered replicators rather than forms of life, a distinction warranted because they cannot reproduce without very specialized substrates such as host cells or proteins, respectively. However, most forms of life rely on foods produced by other species, or at least the specific chemistry of Earth's environment.

Some individuals contest such definitions of life on philosophical grounds, and offer the following as examples of life: viruses which reproduce; storms or flames which "grow"; certain computer software programs which are programmed to mutate and evolve; future software programs which may evince (even high-order) behavior; machines which can move; and some forms of proto-life consisting of metabolizing cells without the ability to reproduce. ^{[citation needed]}

Still, most scientists would not call such phenomena expressive of life. Generally all seven characteristics are required for a population to be considered a life form.

Other definitions

The systemic definition is that living things are self-organizing and autopoietic (self-producing). These objects are not to be confused with dissipative structures (e.g. fire).

Variations of this definition include Stuart Kauffman's definition of life as an autonomous agent or a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle.

Another definition is: "Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation."^{[citation needed]}

Yet another definition: "Life is a self-organizing, cannibalistic system consisting of a population of replicators that are capable of mutation, around most of which homeostatic, metabolizing organisms evolve." This definition does not include flames, but does include worker ants, viruses and mules. Without 'most of', it does not include viruses.

Self reproduction and energy consumption is only one means for a system to promote its own continuation. This explains why bees can be alive and yet commit suicide in defending their hive. In this case the whole colony works as such a living system.

Origin of life

The Grand Prismatic Spring of Yellowstone National Park

There is no truly "standard" model for the origin of life, but most currently accepted scientific models build in one way or another on the following discoveries, which are listed roughly in order of postulated emergence:

1. Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the Miller-Urey experiment.
2. Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
3. Procedures for producing random RNA molecules can produce ribozymes, which are able to produce more of themselves under very specific conditions.

There are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells and metabolism. Many models fall into the "genes-first" category or the "metabolism-first" category, but a recent trend is the emergence of hybrid models that do not fit into either of these categories.^{[citation needed]}

However, add info from from p. 48-52 of UT

The possibility of extraterrestrial life

Main articles: Extraterrestrial life, Astrobiology

Earth is the only planet in the universe known to harbor life. The Drake equation has been used to estimate the probability of life elsewhere, but scientists disagree on many of the values of variables in this equation. Depending on those values, the equation may either suggest that life arises frequently or infrequently.

See also

• Artificial life
• Extraterrestrial life
• Cellular life
• Non-cellular life
• The Game of Life
• Conway's Game of Life
• Biological kingdom
• Origin of life
• Death
• Gaia hypothesis1

References

ISBN links support NWE through referral fees

• Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from [2]
• Guenther Witzany. Life:The Communicative Structure (2000). Libri Books on Demand. ISBN 3-8311-0349-6
• Lynn Margulis and Dorion Sagan - What Is Life? (1995). Simon & Schuster. ISBN 0-684-81087-5
• Erwin Schrodinger - What is Life? (1944 to 2000). Cambridge University Press (Canto). ISBN 0-521-42708-8

External links

• "In the Beginning..." (The Economist)
• "The Adjacent Possible: A Talk with Stuart Kauffman"
• Stanford Encyclopedia of Philosophy entry
• The Biologist: Biology
• Life under extreme conditions An in depth look at how life can form under the most extreme conditions.
• What is Life ? Life is the whole process which exists from birth to death and we have the result in us through what happened in ourselves.

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