text: Research into the origin of life is a limited field of research despite its profound impact on biology and human understanding of the natural world. Progress in this field is generally slow and sporadic, though it still draws the attention of many due to the gravity of the question being investigated. A few facts give insight into the conditions in which life may have emerged, but the mechanisms by which non-life became life are still elusive.\n\nFor the observed evolution of life on earth, see the timeline of life.\n\nResearch into the origin of life is the modern incarnation of the ancient concept of abiogenesis. Abiogenesis, in its most general sense, is the generation of life from non-living matter. The term is primarily used in the context of biology and the origin of life. Abiogenesis was long considered to be a very common occurrence until the Law of Biogenesis (omne vivum ex ovo or "all life from an egg") became firmly established in modern biology as a result of the work of Louis Pasteur.\n\nCharles Darwin in a letter to J.D. Hooker of February 1st 1871, made the suggestion that life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed." Thus, it is the presence of life itself, operating in an oxygen rich atmosphere, itself a product of life, which prevents "spontaneous generation" from occurring on Earth today.\n\nThis modern definition of abiogenesis is concerned with the formation of the simplest forms of life from primordial chemicals, in an environment regarded as similar to that at the time shortly after the formation of the Earth. This is significantly different from the concept of Aristotelian abiogenesis, which postulated the formation of complex organisms. This article reviews different hypotheses for modern abiogenetic processes that are currently under debate.\n\nThere is no truly "standard" model of the origin of life, however most currently accepted models build in one way or another upon a number of discoveries concerning the origin of molecular and cellular components for life, which are listed in a rough order of postulated emergence:\n\nPlausible pre-biotic conditions result in the creation of certain basic small molecules (monomers) of life, such as amino acids. This was demonstrated in the Urey-Miller experiment by Stanley L. Miller and Harold C. Urey in 1953. \nPhospholipids (of an appropriate length) can spontaneously form lipid bilayers, one of the two basic components of a cell membrane. \nThe polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis). \nSelection pressures for catalytic efficiency and diversity result in ribozymes which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. Thus the first ribosome is born, and protein synthesis becomes more prevalent. \nProteins outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer. Nucleic acids are restricted to predominantly genomic use. \nThe origin (see Origin of organic molecules) of the basic biomolecules, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic inorganic chemicals from which life was formed are methane (CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2), and phosphate (PO43-). As of 2004, no one has yet synthesized a "protocell" using basic components which has the necessary properties of life (the so-called "bottom-up-approach"). Without such a proof-of-principle, explanations have tended to be short on specifics