Acellular

Non-cellular life is life that exists without a cellular structure for at least part of its life cycle. Historically, most (descriptive) definitions of life postulated that a living organism must be composed of one or more cells, but this is no longer considered necessary, and modern criteria allow for forms of life based on other structural arrangements.

The primary candidates for non-cellular life are viruses. A minority of biologists consider viruses to be living organisms, but most do not. Their primary objection is that no known viruses are capable of autopoiesis, which means they cannot reproduce themselves: they must rely on cells to copy them. However, the recent discovery of giant viruses that possess genes for part of the required translation machinery has raised the prospect that they may have had extinct ancestors that could evolve and replicate independently. Most biologists agree that such an ancestor would be a bona fide non-cellular lifeform, but its existence and characteristics are still uncertain.

Engineers sometimes use the term "artificial life" to refer to software and robots inspired by biological processes, but these do not satisfy any biological definition of life.

The nature of viruses was unclear for many years following their discovery as pathogens. They were described as poisons or toxins at first, then as "infectious proteins", but with advances in microbiology it became clear that they also possessed genetic material, a defined structure, and the ability to spontaneously assemble from their constituent parts. This spurred extensive debate as to whether they should be regarded as fundamentally organic or inorganic — as very small biological organisms or very large biochemical molecules — and since the 1950s many scientists have thought of viruses as existing at the border between chemistry and life; a gray area between living and nonliving.

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