Pre-cell

There are different hypotheses attempting to explain the origin of the three domains of life (Woese et al. 1990)[1] from a last universal common ancestor (LUCA).

Under the RNA world hypothesis (replication-first scenario),[3][4] over a precellular and early-cellular phase,[3][4] the earliest self-replicating biological systems were based on catalytic RNA evolving stage by stage to a nearly complete ancestral cell, the last universal common ancestor (LUCA)[2][3][4] from which the three domains of life[1] emerged.

This ancestral cell (sometimes also called pre-cell or proto-cell), a hypothetical lipid-based structure, could have confined RNA in ancient times.

Under the pre-cell theory (Kandler 1994ff)[6][7][8], based on the Iron-Sulfur world hypothesis (metabolism-first scenario),[9] primordial metabolism led to the early diversification of life through the evolution of a multiphenotypical population of pre-cells, defined by Kandler as metabolizing, replicating loose entities exhibiting many of the basic properties of a cell but no proper cytoplasmic membrane and no stable chromosome, thus allowing frequent mutual exchange of genetic information.

[7][8] A scheme of the pre-cell scenario is presented in the adjacent figure, where essential evolutionary improvements are indicated by numbers: "(1) Reductive formation of organic compounds from CO or CO2 by Me-sulfur coordinative chemistry; (2) tapping of various redox energy sources and formation of primitive enzymes and templates; (3) elements of a transcription and translation apparatus and loose associations; (4) formation of pre-cells; (5) stabilized circular or linear genomes; (6) cytoplasmic membranes; (7) rigid murein cell walls; (8) various non-murein rigid cell walls; (9) glycoproteinaceous cell envelope or glycokalyx; (10) cytoskeleton; (11) complex chromosomes and nuclear membrane; (12) cell organelles via endosymbiosis".