Non-cellular life

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

Viruses
Viruses were initially described as poisons or toxins, then as "infectious proteins"; but they possess genetic material, a defined structure, and the ability to spontaneously assemble from their constituent parts. This has 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. Without their hosts, they are not able to perform any of the functions of life, such as respiration, growth, or reproduction. 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.

Viroids
If viruses are borderline cases or nonliving, viroids are further from being living organisms. Viroids are some of the smallest infectious agents, 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 viruses capable of causing an infection are around 2,000 nucleobases in size. Viroid RNA does not code for any protein. 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.

A 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. This view was first proposed in the 1980s, and regained popularity in the 2010s to explain crucial intermediate steps in the evolution of life from inanimate matter (abiogenesis).

Obelisks
In 2024, the possible discovery of viroid-like, but distinct, RNA-based elements called obelisks was announced. Obelisks were found in sequence databases of the human microbiome, and are possibly hosted in gut bacteria. They are different from viroids in that they code for two distinct proteins, called oblins, and for the predicted rod-like secondary structure of their RNA.

First universal common ancestor
The first universal common ancestor is an example of a proposed non-cellular lifeform, as it is the earliest ancestor of the last universal common ancestor, its sister lineages, and every currently living cell.