Non-cellular life

Life timeline

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water

Single-celled
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photosynthesis

Eukaryotes

Multicellular
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Arthropods       Molluscs

Plants

Dinosaurs    

Mammals

Flowers

Birds

Primates

 

 

 

 

 

 

 

 

 

 

 

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Earth (−4540)

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Earliest water

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Earliest life

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Earliest oxygen

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Atmospheric oxygen

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Oxygen crisis

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Sexual reproduction

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Earliest plants

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Earliest animals

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Ediacara biota

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Cambrian explosion

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Tetrapoda

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Earliest apes

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Pongola

Huronian

Cryogenian

Andean

Karoo

Quaternary

Ice Ages

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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.[6][7][14]

The recent discovery of giant viruses (aka "giruses," nucleocytoplasmic large DNA viruses, NCLDVs) like pandoravirus[15] has reignited this debate, since they are not only physically larger than previously known viruses, but also possess much more extensive genomes, including genes coding for aminoacyl tRNA synthetases, key proteins involved in translation, which were only ever before seen in cellular organisms. Some biologists have hypothesized that the ancestors of giant viruses evolved from cells and engaged in life processes (like self-replication) independent of cells, abilities they eventually lost secondarily.[16] Some have further hypothesized an ancient viral lineage that originated alongside the earliest archaea or before the LUCA. Such a virus would constitute a bona fide lifeform, and its descendants (at least the giant viruses, but possibly all known viruses) could be phylogenetically classified in a fourth domain of life.

Ongoing research is being conducted in this area, using techniques such as phylogenetic bracketing on the giant viruses to infer characteristics of their proposed progenitor.[17] Meanwhile, other phylogenetic analyses have argued that giant viruses are merely highly derived dsDNA viruses.[18]

Viral replication and self-assembly has implications for the study of the origin of life,[19] as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.[20][21]

Viroids are the smallest infectious pathogens known to biologists, consisting solely of short strands of circular, single-stranded RNA without protein coats. They are mostly plant pathogens and some are animal pathogens, from which some are of commercial importance. Viroid genomes are extremely small in size, ranging from 246 to 467 nucleobases. In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2,000 nucleobases in size. Viroids are the first known representatives of a new biological realm of sub-viral pathogens.[22][23]

Viroid RNA does not code for any protein.[24] Its replication mechanism hijacks RNA polymerase II, a host cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes "rolling circle" synthesis of new RNA using the viroid's RNA as a template. Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.[25]

Viroids attained significance beyond plant virology since one possible explanation of their origin is that they represent “living relics” from a hypothetical, ancient, and non-cellular RNA world before the evolution of DNA or protein.[26][27] This view was first proposed in the 1980s,[26] and regained popularity in the 2010s to explain crucial intermediate steps in the evolution of life from inanimate matter (Abiogenesis).[28][29]

In discussing the taxonomic domains of life, the terms "Acytota" or "Aphanobionta" are occasionally used as the name of a viral kingdom, domain, or empire. The corresponding cellular life name would be Cytota. Non-cellular organisms and cellular life would be the two top-level subdivisions of life, whereby life as a whole would be known as organisms, Naturae, or Vitae.[30] The taxon Cytota would include three top-level subdivisions of its own, the domains Bacteria, Archaea, and Eukarya.

• Subviral agent
  • Satellite, a subviral agent requiring coinfection
  • Prion, an infectious protein
  • Defective interfering particle
• Hypothetical types of biochemistry
• Plasmid
• Protocell
• Viral evolution
• Nanobe