Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.5–4.1 billion years ago.

[4] When amphiphiles like lipids are placed in water, the hydrophobic tails aggregate to form micelles and vesicles, with the hydrophilic ends facing outwards.

[5] Because of their simplicity and ability to self-assemble in water, it is likely that these simple membranes predated other forms of early biological molecules.

[8] Nutritionally, prokaryotes have the ability to utilize a wide range of organic and inorganic material for use in metabolism, including sulfur, cellulose, ammonia, or nitrite.

[9] Many common bacteria have plasmids, which are short, circular, self-replicating DNA molecules that are separate from the bacterial chromosome.

[11] Plasmids can carry genes responsible for novel abilities, of current critical importance being antibiotic resistance.

[16] Clonal aging occurs naturally in bacteria, and is apparently due to the accumulation of damage that can happen even in the absence of external stressors.

[17] Hydrothermal vents release heat and hydrogen sulfide, allowing extremophiles to survive using chemolithotrophic growth.

[22] Some archaea inhabit the most biologically inhospitable environments on earth, and this is believed to in some ways mimic the early, harsh conditions that life was likely exposed to[citation needed].

[27] This process utilizes hydrogen to reduce carbon dioxide into methane, releasing energy into the usable form of adenosine triphosphate.

[33] However, if the bacteria were capable of respiration, it would have been beneficial for the larger cell to allow the parasite to live in return for energy and detoxification of oxygen.

[citation needed] Protozoa are largely defined by their method of locomotion, including flagella, cilia, and pseudopodia.

[33] Autotrophs like Euglena are capable of producing their energy using photosynthesis, while heterotrophic protozoa consume food by either funneling it through a mouth-like gullet or engulfing it with pseudopods, a form of phagocytosis.

[38] Ciliates are widely abundant in almost all environments where water can be found, and the cilia beat rhythmically in order to propel the organism.

[40] Examples of such ciliates are Paramecium and Tetrahymena that likely employ meiotic recombination for repairing DNA damage acquired under stressful conditions.

[50] Yeasts reproduce through mitosis, and many use a process called budding, where most of the cytoplasm is held by the mother cell.

[50] Saccharomyces cerevisiae ferments carbohydrates into carbon dioxide and alcohol, and is used in the making of beer and bread.

[52][53] Furthermore, research using S. cerevisiae has played a central role in understanding the mechanism of meiotic recombination and the adaptive function of meiosis.