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GAIA 1, GAIA, G J H McCall, Cirencester, Gloucester, UK, ? 2005, Elsevier Ltd. All Rights Reserved. Introduction The Gaia concept has evolved in the 30 years since it was first introduced by James Lovelock, an independent scientist and inventor. It was initially a rather vague model relating to the climate and diversity of the planet Earth, though living organisms were critical to it. The workings of the model were initially unspecified. However, the concept was one of a ‘superorganism’ operating to ‘regulate’ the planet, especially its surface temperature, yet lacking the ‘foresight’ possessed by intelligent animals. Lovelock updated his work in 2000, publishing The Ages of Gaia. The discussion here is based on this later book, and it is quite unavoidable to echo much of what Lovelock has said because he is the only and definitive source. Gaia in the Twenty-First Century Gaia is essentially about life, because life is seen to combine with inanimate processes on Earth, affecting and even regulating the physical state of the biosphere. Lovelock found it surprisingly difficult to find a good definition of ‘life’. Of the definitions found in Webster – ‘‘the property of plants and animals (ending in death and distinguishing them from organic matter) which makes it possible for them to take in food, get energy from it, grow etc.’’ – and in Oxford – ‘‘the property which differentiates a living animal or plant or a living portion of organic tissue, from dead or non-living matter; the assemblage of the functional activities by which this property is manifested’’ – neither is satisfactory, and the second is tautological in the extreme. To the first might be added, before ‘‘etc.’’, the words ‘‘and move, in the case of animals’’. Lovelock added to his definition of life that ‘‘living things use energy directly from the Sun and indirectly from food’’ (see Origin of Life). There is no difficulty in accepting that advanced living animal organisms, such as humans, are made up of intricate communities of connecting cells, and, as Lynn Margolis has shown, that cells are derived from micro-organisms that once lived free (see Precambrian: Prokaryote Fossils; Eukaryote Fossils). Larger entities, such as ecosystems, are also accepted, and space exploration has contributed to this understanding by allowing the entire planet Earth to be viewed from space. The Gaia concept likewise involves envisaging the entire globe as an integrated system, with the atmosphere, the seas, the rivers, and the rocks interacting to modulate the planet’s physical state and thus the environment in which life can exist, with the presence of life contributing significantly to the interactions. Gaia is thus not a synonym of ‘biosphere’ or ‘biota’: it is a much larger entity. When the Gaia model was originally proposed in the 1970s, it was considered that the atmosphere, oceans, climate, and crust of Earth were regulated to maintain a comfortable state for life to exist by and for the biota. Temperature, oxidation state, acidity, and certain aspects of the rocks and waters were kept constant at any time, maintained in homeostasis by organisms at Earth’s surface. This concept is now seen to have been incorrect because both life, which is continually evolving, and the geological environment are in a state of constant change, and the conditions needed to maintain life change very rapidly with the changing needs of the biota, so homeostasis cannot be maintained for more than very brief periods in Earth history. The Earth is thus seen as being like a helicopter, which unlike a fixed-wing plane, is never in stable flight. The changing and evolving needs of the biota require that the brief periods of homeostasis are quickly overtaken by new requirements. The concept now is of a superorganism in which active feedback processes operate automatically so that solar energy maintains comfortable conditions for life. Molecular Biology: The Laws of the Universe Lovelock regarded the emergence of the science of molecular biology – embodied in the information-processing chemicals that underpin the genetic basis of most life on Earth – as having taken life science out of a routine classificatory and descriptive pursuit into a new and exciting study of how all the components in life are related. Equally important are physiology, the study of organisms seen holistically, and thermodynamics, a branch of physics dealing with time and energy, connecting living processes with universal laws. Two fundamental universal laws of physics are that (1) energy is conserved however much it is dispersed, and (2) energy is always abating. Hot objects cool but cool objects do not heat up spontaneously; water flows downhill but not uphill. Once used, energy cannot be recovered. Natural processes always move towards an increase in disorder, which is measured by entropy; entropy expresses the tendency to burn out. Looking at the relationship between life and entropy, Lovelock referred to Erwin Schrödinger’s conclusion that life has the ability to move upstream against the flow of time, apparently paradoxically and contrary to the second universal law. In fact, what is operating is a tightly coupled system to favour survival; energy is taken in (e.g., oxygen from the atmosphere is breathed), converted (e.g., stored body fats and sugars are transformed), and then excreted (e.g., waste products such as carbon dioxide are released back into the atmosphere). If the entropy of excretion is larger than the entropy of the oxygen consumed, life continues despite the second universal law. The Superorganism Concept There is difficulty in envisaging an eruptive planet with a molten core and other complex inorganic processes as a living superorganism. However, the inspiring ‘whole-planet’ image of Earth as seen from space and the contrast between the environment on Earth and the environments on the moon, Mars, Venus, and Mercury have focused research on considering how significant planetary differences arose, and in particular on the question of how and why atmospheres differ. The atmospheres of the Moon, Mars, Venus, and Mercury are a good starting point for comparisons to Earth because they are the least complex and most accessible of the zones of all these planets; indeed, the atmospheric compositions on other planets were known before space exploration commenced (see Solar System: Mars; Moon; Mercury; Venus). The Earth has an atmosphere of N2 and O2, with traces of carbon dioxide, methane, and nitrous oxide, not in equilibrium, whereas the atmospheres of Mars and Venus are dominated by carbon dioxide and are in equilibrium. If the atmospheres of Mars and Venus were heated, there would be no reaction with the surface materials; whereas heating Earth’s atmosphere would produce reactions leading to a carbon-dioxide-dominated atmosphere. Lovelock concluded that the improbable atmosphere of Earth ‘‘reveals the invisible hand of life’’. The atmosphere contains oxygen and methane, which should react to form water vapour and carbon dioxide: that this does not occur, and that constant atmospheric compositions of these gases are maintained, reveal, Lovelock believes, that there is regulation by life (see Atmosphere Evolution). Scientists as early as Eduard Suess and Vladimir Vernadsky accepted that there was continuous interaction between soils, rocks, oceans, lakes, rivers, the atmosphere, and life. Much later, Stephen Jay Gould stated that ‘‘organisms are not billiard balls, struck in a deterministic fashion and rolling to optimal positions on life’s table’’. Living things influence their own destiny in an interesting and complex but comprehensible way. Thus the sum total of the physical state of a planet with life is a combination of the inanimate processes and the effects of life itself. JZ Young said that the entity that is maintained intact, and of which we all form part, is the whole of life on the planet. This statement really provided the link between theory and consensus, on one hand, and Gaia concept, on the other, expressing as it does the view that the entire spectrum of life on the planet has to be considered alongside the geological and inanimate physical processes if we are to understand how the planet works. This, of course, has led to the present preoccupation in educational circles with ‘Earth System Science’ (see Earth System Science). Gaia goes further than Earth System Science, which is purely a holistic educational approach, in requiring a global system that has the capacity to regulate the temperature and composition of Earth’s surface, hydrosphere, and atmosphere, keeping it comfortable for living organisms. Criticism of the Gaia Concept Criticism of the Gaia concept once advanced was by no means slight, and the Gaia model was not taken seriously by scientists until the early 1970s. Fred Doolittle came out with the belief that ‘‘molecular biology could never lead to altruism on a global scale’’ – altruism by living organisms being apparently inherent to the concept. Richard Dawkins in 1982 supported him: ‘‘the selfish interests of living cells could not be expressed at the distance of the planet.’’ It was also remarked that Gaia lacked a firm theoretical basis. Heinrich D Holland considered that biota simply react to change in the state of Earth’s near-surface environment and processes, geologically produced, and those that adapt better survive: the rest do not. Many scientists saw Gaia as a teleological concept requiring foresight and planning by organisms, something that the model surely never represented. However, a major step was taken at the Chapman conference of the American Geophysical Union in 1988 when numerous papers on Gaia were presented; the question of the scientific testability of the Gaia hypothesis was raised. Holland’s statement was really an oversimplification because the environmental constraints to which Ключевые слова: formed, type, material, sassi, vacuum crystallizers, science, columnar jointing, phase, zn cu, impact, surface, adirondack highlands, san diego, d-day landings, morgan, region, harmon, naqvi, iugs subcommission, almandine pyrope, map, alfred nier, nixon, welsh borderland, buildwas coalbrookdale, semi-precious stones, carbonate, ha, suwannee terrane, pale amber, san francisco, condition, allison, earth, viele, result, volcanic, fossa magna, scale, exploration, classification, mulder efj, level, engineering geologist, temperature, evidence, naga hills, southland syncline, triassic, geological survey, geologist, engineering, biostratigraphie memoire, hawkesworth, hancock, journal, rocks, natural, dorning, nature, vigneresse, cu ag, depth, based, sri lanka, groves, age, briggs deg, iugs sucommission, change, vaal reefs, epicontinental seaway, founding father, process, data, minerals, water, york, kirthar ranges, geological, fume cupboard, study, survey, field, deccan traps, sea, london, small, pressure, late, arakan yoma, bragg, melting, sinton, large, composition, including, nathanail, oldoinyo lengai, sud-ouest, brasier, element, early, environment, chemical, cretaceous, izu-ogasawara islands, case, formation, civil engineers, facies, le, major, admiralty islands, underground excavations, engineering geology, stage, ree, extinction, record, alumina saturation, sinclair, chitinozoan animals, year, barnes, mantle, beche, japanese islands, jura nek, neale, geology, coffin, flow, mineral, alexandre brongniart, robin renaut, purnell, ce la, ma, assemblage, knell, specie, sediment, source, geological engineering, aldridge, marine, mitchell, crystal, crust, zone, mining, history, wa, north, fluid, range, metamorphic, sulphide, crowther, blowes, metamorphism, society, lava, form, europe, include, occur, guth, cary, structure, transantarctic mountains, magma, work, pierre curie, time, sr sr, american, south, sedimentary, site, fossil, thar desert, group, rowe, basin, complex, john saul, kurosegawa belts, freemasons tavern, ore, maurice ewing, university, issued, little men, lagersta tten, high, sibbald tii, aerial photography, igneous, melt, continental, x-ray diffraction, concomitant downwearing, grenvillian orogeny, gold, deposit, area, geological engineer, rock, mid-ocean ridges, scottish highlands, mcgregor, table, press, editorial committee