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Life can be a property of matter. Matter that is alive, or life forms are organisms of a vast variety. 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, undergo metabolism, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in succeeding generations.
An entity with the above properties is considered to be an organism. However, 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 only 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 and other alternative biology.
Although it cannot be pinpointed exactly, evidence suggests that life on Earth has existed for about 3.7 billion years .
Definition by opinion
A conventional definition
Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life exhibits the following phenomena:
- Homeostasis: Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature.
- Organization: Being composed of one or more cells, which are the basic units of life.
- Metabolism: Consumption of energy by converting nonliving material into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
- Growth: Maintenance of a higher rate of synthesis than catalysis. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish.
- Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present.
- Response to stimuli: A response 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.
- Reproduction: 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.
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 "burn"; 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.
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."
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.
Another attempt at defining the general features and properties of life: "type of organization of matter producing various interacting forms of variable complexity, whose main property is to replicate almost perfectly by using matter and energy available in their environment to which they may adapt."
In this definition "almost perfectly" relates to mutations happening during replication of organisms that may have adaptative benefits.
Origin of life
Main article: Origin of life
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:
- Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the Miller-Urey experiment.
- Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
- 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.
The possibility of extraterrestrial life
- Main articles: Extraterrestrial life, Astrobiology
Earth is the only planet in the universe known to harbour 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 (although strictly speaking Drake equation estimates the number of extraterrestrial civilizations in our galaxy with which we might come in contact - not probability of life elsewhere). Depending on those values, the equation may either suggest that life arises frequently or infrequently.
See also
- Biology - the scientific study of life
- Artificial life
- Extraterrestrial life
- Cellular life
- Non-cellular life
- Extremophile - Organisms that live in so called 'extreme' conditions e.g. hydrothermal vents
- Biological kingdom
- Origin of life
- Death
- Gaia hypothesis
- Taxonomy - the science of describing, categorising and naming organisms
- Phylogenetics - is the study of evolutionary relatedness among species
- Conway's Game of Life - simple mathematical 'cellular automaton' that mimicks the dynamics of an ecosystem.
References
- Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from
- 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 Schrödinger - What is Life? (1944 to 2000). Cambridge University Press (Canto). ISBN 0-521-42708-8
External links
- Wikispecies - a free directory of life
- "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.