The process by which living organisms developed from inanimate matter, which is generally thought to have occurred on earth between 3500 and 4000 million years ago(mya). The oldest known fossils are stromatolites, rocky masses formed by cyanobacteria as early as 3500 mya. Many hypotheses have been put forward, but it is supposed that the primordial atmosphere contained all the basic constituents of organic matter: ammonia, methane, hydrogen, and water vapour. These underwent a process of chemical evolution using a source of energy (e.g. from the sun and electric storms) to combine into ever more complex molecules, such as amino acids, proteins, and vitamins. For example, the ‘primordial soup’ theory, dating from the 1920s, proposes that in hot pools on oceanic shores, evaporation concentrated monomers such as amino acids to the extent that they underwent polymerization to form proteins. Eventually self-replicating nucleic acids, the basis of all life, could have developed. The very first organisms may have consisted of such molecules bounded by a simple membrane to form protocells. One rival view, proposed in 2010, is that deep-sea hydrothermal vents may have provided the energy and catalytic conditions needed to sustain early life forms. Another alternative theory is that the chemical ‘building blocks’ of life, or even the earliest microorganisms, originated not on earth but were delivered from space by meteorites or other celestial bodies (see panspermia). See also proteinoid.