128 ORIGIN OF LIFE tion) There may have been a small amount of methane (up to about 100 ppm), making the atmosphere, at best, weakly reducing and allowing the production of small amounts of organic material in this way A possible alternative source of organic molecules is extraterrestrial delivery We know from astronomical observations that interstellar clouds contain at least 100 species of simple organic molecule, while experiments simulating interstellar conditions have shown that more complex molecules, including amino acids, might be produced by photochemistry in ice-coated dust grains The Murchison meteorite, which fell in Murchison, Australia in 1969, has been found to contain a huge range of organic compounds including more than 70 different amino acids During the phase of heavy bombardment in the first few hundred million years after the solar system was formed, the Earth would have been subject to frequent impacts of comets and asteroids (see Earth Structure and Origins) While the heat produced by the impacts of these large bodies may destroy much of the organic material, recent experiments and simulations suggest that at least some organic material may be able to survive these major impacts Organic material can also be delivered to the Earth’s surface by interplanetary dust particles However, current estimates of the amount of organic material that might have been delivered to the early Earth vary widely One intriguing discovery is that at least some of the amino acids in the Murchison meteorite show a small excess of the L or left-handed form This excess is found in amino acids not known to be present in the biosphere, so it cannot be due to contamination, and suggests that a chiral imbalance existed in the organic material of the early Solar System This could be relevant to the origin of biological chirality A possible cause of the original excess of L-amino acids is photolysis by circularly polarized light, which could have occurred in the star-formation region in which the Sun originated Another source that has been suggested for organic compounds is reactions occurring in the superheated water around deep-sea hydrothermal vents (see Tectonics: Hydrothermal Vents At Mid-Ocean Ridges) Reactions involving mineral catalysts could build organic molecules from simple molecules such as carbon dioxide However, some workers argue that the high temperatures in hydrothermal vents are more likely to destroy organic molecules than create them It has been calculated that virtually all the water in the oceans circulates through hydrothermal vents on a timescale of about 107 years, and the temperatures would be high enough to destroy molecules such as amino acids This limits the amount of organic material that can build up in the oceans from any of the above sources Thus, while there are a number of possible sources of prebiotic organic compounds, we are still left with two significant problems First, the total amount of organic material produced by any of these mechanisms seems to be enough to lead to only very weak concentrations in the ocean Second, the molecules that are most easily made not necessarily appear to be the most relevant For example, amino acids are readily produced and are certainly important in modern biology, but the adoption of proteins seems to be a relatively late step in the development of life The building blocks needed for RNA (such as ribose) are more difficult to make, although RNA appears to be essential for early life Where did Life Originate? Given that we not yet understand the chemical processes required for the early stages of life, it is hard to say much about the likely location However, there are some general considerations that can be applied The early Earth would have been subject to bombardment by asteroids at a high but rapidly reducing rate, and some of the largest impacts would have been ‘sterilizing impacts’ capable of vaporizing the oceans and destroying any life Clearly life must have appeared after the last sterilizing impact, which might have been around 4–4.2 Ga ago (There could, of course, have been one or more earlier origins of life that were wiped out in the impacts.) However, somewhat smaller impacts might have destroyed life near the surface while allowing it to be preserved deep in the oceans or within the Earth’s crust This suggests that life might have originated in a deepocean location or even deep within the crust Another important consideration is that life requires an energy source (for modern life this is normally solar energy via photosynthesis) For a deep location the likely energy source is geothermal energy A location on or near the surface would make available other energy sources such as ultraviolet radiation and lightning A surface origin would, however, be much more exposed to impacts, and life would therefore have to have arisen somewhat later, towards the end of the heavy bombardment This leaves a rather small time window between the end of the bombardment and the first evidence of life on Earth However, the timings of both these events are somewhat uncertain Finally, the possibility must be considered that life did not originate on Earth at all More than 20 meteorites have been identified as coming from Mars The most famous is ALH84001, which has been sug-