Origin of life on Earth Miller-Urey experiment Two approaches: 1953 experiment: • bottom-up - which of the chemical / structural parts mixture of several of modern life could have formed from abiotic simple gases (water, processes on the early Earth? hydrogen, methane • top-down - which of the constituents of current cells and ammonia) was could have been part of earlier, simpler life exposed to sparks forms? (“lightning”) and cycled through a Basic requirements: model of the ocean / atmosphere • genetic information (DNA) • catalytic molecules (proteins) • cell membranes (lipids) Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Result: large number of complex Experiment has been repeated many times: results stand, organic molecules formed in under these conditions amino acids are synthesized easily the experiment, including a number of amino acids Are the conditions a realistic depiction of the early Earth? H2O, H2, CH4 and NH3 are major constituents of the atmosphere of the giant planets. Chemically, this is a reducing atmosphere, very different from the oxidizing conditions on the Earth today. …within weeks of the discovery Early Earth was likely devoid of oxygen, atmospheric of the structure of DNA… composition would have been controlled by volcanic outgassing. Large CO2 concentration (needed for strong greenhouse effect to offset the faint young Sun) leads to less efficient synthesis of organic molecules. Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Oceanic synthesis Meteoritic delivery Amino acids can also be formed Comets or meteorites can also be under conditions similar to a source: Murchison meteorite hydrothermal vents may contain as many as 70 amino acids Protected against impacts, common, thermophiles seem like simple organisms… Synthesis of small molecules: site is very uncertain, However, complex organic but there are several plausible candidates (possibly chemicals are also destroyed the atmosphere, ocean vents, extraterrestrial delivery) by the high temperatures - today water cycles through such systems on a timescale of only ~10 million years Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 1 What about chirality? The RNA-world hypothesis Currently: • DNA encodes information for building proteins • Proteins catalyze the cellular mechanisms that lead to their formation RNA (ribonucleic acid) can fulfill both functions: Both meteoritic amino acids and those synthesized in • carry information that can be copied Miller-Urey type experiments tend to be almost racemic • can catalyze reactions, including the formation mixtures: equal amounts of left-handed and right-handed of proteins (though less efficiently than versions current mechanisms) Additionally, the set of 20 amino acids used in biology today is not particularly favored Hypothesis: first life may have been based around RNA, with the DNA / protein symbiosis evolving later Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Formation of more complex molecules Forming RNA Getting to the RNA-world (or similar schemes based on Constituents of RNA: other molecules) from simple precursors is the hardest step: • sugar (ribose) Proteins • base (adenine, guanine, cytosine, uracil) alanine + glycine " di - peptide + H2O • phosphate Presence of water as a product means that in water, dissociation rather than polymerization is favored… ! Synthesis of the bases (especially adenine) appears to Moreover: more or less random order of the monomers be relatively easy - HCN can yield adenine in water is obtained even under conditions where proteins form when exposed to ultraviolet light Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Polymerization of nucleotides Sugars: pre-biotic synthesis of sugars has been demonstrated experimentally from formaldehyde Neither of the critical steps: (CH2O) in the presence of mineral catalysts • reactions of the bases with ribose BUT - these reactions yield a mix of many sugars • reaction to join in the phosphate (as many as 40) with no chiral preference …have been demonstrated to occur under plausible Chirality of ribose affects the 3D structure of RNA, early-Earth conditions, though there are many upon which the hereditary system rests… how did possible pathways. one form of ribose come to dominate? In particular, water and high temperatures are unfavorable for the survival of RNA… Speculation: perhaps ribose was not part of the first `RNA’-like molecules? Where might early life have started? Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 2 Role of mineral catalysts 1) Organic molecules formed, either in the atmosphere, Often suggested that mineral catalysts such as clays at undersea vents, or via delivery from space may have played a critical role 2) Short strands of RNA formed with the help of catalytic materials (perhaps clays) Early chemistry took place on surfaces 3) RNA became capable of self-replication 4) Membranes formed to enclose RNA Chirality may have been inherited from surface defects 5) Natural selection led to an increase in complexity, until eventually something recognizably living Enclosure within cell membranes came later… formed Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Prospects for progress Clearly this topic involves too much extrapolation from `known’ conditions to allow for robust conclusions. Life started somehow - can we ever know how? • better understanding of the the atmospheres of planets (from observing others) • deeper knowledge of the function of all the molecules involved in life • lab experiments • discovery of life elsewhere Extraterrestrial Life: Spring 2008 3.