Draft:Definition of life

The precise definition of life is a contested aspect of it, and several proposals have been advanced. Biology defines and studies life as we know it, but abiogenesis and astrobiology seek wider and more encompassing definitions. Abiogenesis is the process by which life surges from inorganic materials, so a definition tries to establish the frontier between inorganic matter and the earliest and basest lifeforms. Astrobiology seeks extraterrestrial life, which may differ from Earth's life.

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Common features
Trivial definitions of life, such as those used in dictionaries and science divulgation, rely on several aspects that should take place in it, such as homeostasis, growth, reproduction, and death. Biology, however, provides a more reliable answer: all lifeforms on Earth are composed of cells (both unicellular and multicellular lifeforms), and reproduction replicates information from an ancestor into its offspring with the work of the DNA and the RNA. All lifeforms on Earth have this in common, and nothing that does not live does. It is, thus, a perfect working definition for most sciences. However, it is an incomplete definition for abiogenesis, the science that studies the origin of life. Earth began completely lifeless, and by some unclear chemistry inorganic materials combined themselves and created life. But life as we know it is too complex to appear abruptly, the process must have had steps, and we would require a better definition of life to decide which of those steps can be considered lifeforms, even if more primitive. As for astrobiology, all lifeforms known to us are from a single planet. Life in other planets may have developed in other ways, and we would need a broader definition that would cover such divergent lifeforms as well.

Some common features found in living beings are the following:
 * 1) Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature
 * 2) Organization: being structurally composed of one or more cells – the basic units of life
 * 3) Metabolism: transformation of energy by converting chemicals and energy 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.
 * 4) Growth: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
 * 5) Adaptation: the ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.
 * 6) Response to stimuli: a response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
 * 7) Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms.

Structure
Multicellular lifeforms, such as animals and plants, are composed of cells, and both any individual cell within it and the multicellular lifeform as a whole can be alive or dead. The cell itself is the lowest unit, and none of the organelles within it are independently alive.

Problems
The biggest problem with defining life by a number of characteristics is that it can provide false positives. Long and detailed lists leave potential lifeforms out, and small lists may include things that were not intended to be considered alive. For example, Crystals can grow and tend towards equilibrium, similar to homeostasis, but are not alive. Robert Saphiro and Gerald Feinberg proposed that life is the activity of a biosphere, defining biosphere as "a highly ordered system of matter and energy characterized by complex cycles that maintain or gradually increase the order of the system through the exchange of energy with the environment"; a definition that may be too broad. There are four possible ways to organize a definition. The first one is that there are a number of features and all of them must apply for something to be alive; if something has only some features but not others, then it is not. This is the method used by divulgation outlets. The second is that there is a single necessary and sufficient condition that can define the presence or absence of life. The third is that there are several necessary and sufficient conditions that define life; this is the one used in science. And finally, there may be several types of life without a common characteristic between them all.

Dr. Carol Cleland, a member of the NASA Astrobiology Institute, considers that the problem is caused by the vagueness of spoken language, and that science does not need a definition of life, but rather a general theory of living systems. She compares the problems defining life with the problems defining substances in the middle ages, before the discovery of molecules, and points out that nitric acid was considered a type of water back then because it shared some superficial properties. However, a general theory can not be formulated before a sample of extraterrestrial life can be found and studied. At this point, there is not enough data to formulate such a theory, as it is unknown if life is abundant in the universe or just a rarity exclusive of Earth.

Life on Earth is carbon-based life, and uses water as a solvent. It is often assumed that life in other planets may have a similar composition, disregarding hypothetical types of biochemistry. Scientist Carl Sagan defined it as "carbon chauvinism". However, as those lifeforms are only theoretical, the details of their metabolism are unknown and it would be complex to define what to seek when seeking such lifeforms. However, although it is accepted that life can be composed of substances other than carbon and water, their properties still make them the better ones suited for it.

Adaptation works with natural selection, but it is unclear if human beings are still subject to it. In nature unfit creatures would not reproduce and would not pass their genes to later beings, ensuring that only the best individuals did so, but human beings are capable of compassion and to practice medicine, which may negate the process. And it's hard to test a being and detect if it's capable of evolution, as evolution takes place in the species over time and not in specific individuals.

The ability to reproduce is intrinsic to the species, not the individual. In species with sexual reproduction an individual can not reproduce by itself but requires the intervention of a mate. Besides, a being may suffer infertility but still live. Mules are not a species but a hybrid between two other species, and can never reproduce between themselves.

Viruses
Scientific opinions differ on whether viruses are a form of life or organic structures that interact with living organisms. They have been described as "organisms at the edge of life", since they resemble organisms in that they possess genes, evolve by natural selection, and reproduce by creating multiple copies of themselves through self-assembly. Although they have genes, they do not have a cellular structure, which is often seen as the basic unit of life. Viruses do not have their own metabolism and require a host cell to make new products. They therefore cannot naturally reproduce outside a host cell —although some bacteria such as rickettsia and chlamydia are considered living organisms despite the same limitation. Accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells. They differ from autonomous growth of crystals as they inherit genetic mutations while being subject to natural selection. Virus self-assembly within host cells has implications for the study of the origin of life, as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.