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Life is notoriously difficult to define. Many organisms such as Animals, Plants, Bacteria, Fungi and Protists have been classified as alive, while other entities which have seemed to fit the criteria do not under closer examination. The discussion may have ethical implications in cases such as the eradication of smallpox on the grounds that it may or may not be a living organism.

Viruses

Argument continues over whether viruses are truly alive or not. While scientists have no trouble classifying a horse as living and can see evolutionary relationships between it and other animals, things become complicated as they look at more simple beings such as viruses, viroids and prions. In the case of viruses, they resemble life in that they possess nucleic acid and can respond to their environment in a limited fashion. They can also reproduce by creating multiple copies of themselves through simple self-assembly.

However, unlike all other forms of established lifeforms, they do not possess a cell structure, regarded as the basic unit of life. Viruses are also absent in the fossil record, making phylogenic relationships difficult to infer. Additionally, although they reproduce they do not metabolise on their own and therefore require a host cell to replicate and synthesise new products. However, confounding this previous statement is the fact that bacterial species such as Rickettsia and Chlamydia, while living organisms, are also unable to reproduce outside of a host cell. A powerful argument can be made that all accepted forms of life divide at the cell level via cell division to reproduce, whereas all viruses simply assemble spontaneously within cells. What prevents the comparison to be drawn that viral self-assembly is no different than the autonomous growth of non-living crystals?

Virus self-assembly within host cells also has implications for the study of the origin of life, and if the viral requirement for a host cell was abandoned it could be argued that viruses are indeed alive. If viruses are considered living then the prospect of creating artificial life is enhanced or at least the standards required to call something artificially alive are reduced.

Other questions involve the classification of viruses within the Tree of Life and its implications – if viruses are considered alive, then the criteria specifying life will have been permanently changed, leading scientists to question what the basic prerequisite of life is. Whether or not other infectious particles, such as viroids and prions, would next be considered forms of life could follow if viruses are said to be alive.

According to the United States Code, they are considered to be microorganisms in the sense of biological weaponry and malicious use.

Artificial life

Questions

"What is life?"

"When can we say that a system, or a subsystem, is alive?"

"What is the smallest system that we can consider alive?"

"Why is nature able to achieve an open-ended evolutionary system, while all human models seem to fall short of it?"

"How can we measure evolution?"

"How can we measure emergence?"

"How do virtual simulations of life differ from physically manifested artificial life works?

"How does simulation change the frontiers of science?" (Santa Fe Institute - 1997: John Casti)

"What kind of legal rights should artificial life have?"

Fire

Fire is primarily not classified as a living organism as it is not composed of cells. It does however show many characteristics of life, such as deriving energy from oxidation of fuel and responding to stimuli.

See also

Life

Artificial Life

Artificial consciousness

Virus classification

External links

The Definition of Life

To kill a killer? (decision over killing remaining smallpox virus lab samples)