EARTH AS AN EVOLVING PLANETARY SYSTEM Part 6

Conclusions 221 events, described in Chapter 9, may have profound effects not only on long-term varia-tions in the atmosphere–ocean system but also on life on this planet. Further Reading Frakes, L. A., Francis, J. E., and Syktus, J. L., 1992. Climate Modes of the Phanerozoic. Cambridge University Press, New York, 274 pp. Holland, H. D., 1984. The Chemical Evolution of the Atmosphere and Oceans. Princeton University Press, Princeton, NJ, 581 pp. Huber, B. T., Wing, S. L., and MacLeod, K. G. (eds.), 1999. Warm Climates in Earth History. Cambridge University Press, Cambridge, UK, 480 pp. Kasting, J. F., 1993. Earth’s early atmosphere. Science, 259: 920–926. Wigley, T. M. L., and Schimel, D. S., 2000. The Carbon Cycle. Cambridge University Press, Cambridge, UK, 310 pp. This Page Intentionally Left Blank Living Systems 7 General Features Although the distinction between living and nonliving matter is obvious for most objects, it is not easy to draw this line between some unicellular organisms and large nonliving molecules such as amino acids. It is generally agreed that living matter must be able to reproduce new individuals, it must be capable of growing by using nutrients and energy from its surroundings, and it must respond in some manner to outside stimuli. Another feature of life is its chemical uniformity. Despite the great diversity of living organisms, all life is composed of a few elements (chiefly C, O, H, N, and P) grouped into nucleic acids, proteins, carbohydrates, fats, and a few other minor compounds. This suggests that living organisms are related and that they probably had a common origin. Reproduction is accomplished in living matter at the cellular level by two complex nucleic acids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Genes are portions of DNA molecules that carry specific hereditary information. Three components are necessary for a living system to self-replicate: RNA and DNA molecules, which provide a list of instructions for replication; proteins that promote replication; and a host organ for the RNA–DNA molecules and proteins. The smallest entities capable of replication are amino acids. Origin of Life Perhaps no other subject in geology has been investigated more than the origin of life (Kvenvolden, 1974; Oro, 1994). It has been approached from many points of view. Geologists have searched painstakingly for fossil evidence of the earliest life, and biolo-gists and biochemists have provided a variety of evidence from experiments and models that must be incorporated into any model for the origin of life. 223 224 Living Systems Although numerous models have been proposed for the origin of life, two environ-mental conditions are prerequisites to all models: (1) the elements and catalysts neces-sary for the production of organic molecules must be present, and (2) free oxygen, which would oxidize and destroy organic molecules, must not be present. In the past, the most popular models for the origin of life a involved primordial “soup” rich in carbonaceous compounds produced by inorganic processes. Reactions in this soup promoted by cata-lysts such as lightning or ultraviolet radiation produced organic molecules. Primordial soup models, however, seem unnecessary in rapid degassing of the Earth more than 4 Ga. Rapid recycling of the early oceans through ocean ridges would not allow concentrated “soups” to survive except perhaps locally in evaporite basins less than 4 Ga. Because chances are remote that organic molecules were present in sufficient amounts, in correct proportions, and in the proper arrangement, it would seem that the environment in which life formed would have been widespread in the early Archean. Possibilities include vol-canic environments and hydrothermal vents along ocean ridges. Simple amino acids have been formed in the laboratory under a variety of conditions. The earliest experiments were those of Miller (1953), who sparked a hydrous mixture of H2, CH4, and NH3 to form a variety of organic molecules including 4 of the 20 amino acids composing proteins. Similar experiments, using both sparks and ultraviolet radia-tion in gaseous mixtures of water, CO2, N2, and CO (a composition more in line with that of the Earth’s early degassed atmosphere) also produced amino acids, hydrocyanic acid, and formaldehydes, the latter of which can combine to form sugars. Heat also may pro-mote similar reactions. Role of Impacts As indicated by microfossils, life was in existence by 3.5 Ga; carbon isotope data, although less definitive, suggests that life was present by 3.8 Ga. This being the case, life must have originated during or before the last stage of heavy bombardment of planets in the inner solar system as indicated by the impact craters on the Moon and other terres-trial planets with ancient surfaces. As an example, the impact record on the Moon shows that crater size, and hence impact energy, falls exponentially from 4.5 to about 3.0 Ga, decreasing more gradually thereafter (Sleep et al., 1989; Chyba, 1993). Similarity of crater frequency versus diameter relations for Mercury and Mars implies that planets in the inner solar system underwent a similar early bombardment history, although the Earth’s history has been destroyed by plate tectonics. A decrease in impact energy with time on Earth is likely to be similar to that on the Moon except that less than 3.0 Ga ener-gies were perhaps an order of magnitude higher on the Earth. Because the Earth’s grav-itational attraction is greater than that of the Moon, it should have been hit with more large objects than the Moon before 3.5 Ga. The impact record on the Moon implies that the Earth was subjected to cataclysmic impacts from about 4.0 to 3.8 Ga, preceded by a comparatively quiet period from about 4.4 to 4.0 Ga (Ryder, 2002; Valley et al., 2002) (Fig. 7.1). During the intense impact period, hundreds of impacts large enough to form mare basins (as found on the Moon) must have hit the Earth. Single, large impacts had Origin of Life 225 109 107 105 Heavy bombardment 103 Window for the origin of 10 life ? 1 Figure 7.1 Estimates of the asteroid impact rate for the first 2 Gy of the Earth’s history. Evidence of water comes from oxygen iso-topes in zircons (4.4–4.0 Ga) and sedimentary rocks (Isua, 3.8–3.6 Ga). Modified from Valley et al. (2002). 2.5 3.0 3.5 4.0 4.5 Age (Ga) only a small fraction of the energy necessary to evaporate the Earth’s oceans. Large impactors, sufficient to evaporate the entire ocean, are considered rare or nonexistent less than 4.4 Ga (Zahnle and Sleep, 1997). Although such large impacts mean that life could not form and survive in shallow aqueous environments, it may have survived in the deep ocean around hydrothermal vents. Because it appears that oceans existed on the Earth from at least 4.4 Ga (Chapter 6), life could have formed during the comparative quiet period from 4.4 to 4.0 Ga just before the cataclysmal impacts from 3.9 to 3.8 Ga (Fig. 7.1). As indicated by the oldest fossils, life was advanced by 3.5 Ga. Another intriguing aspect of early impact is the possibility that relatively small impactors introduced volatile elements and small amounts of organic molecules to the Earth’s surface that were used in the origin of life. The idea that organic substances were brought to the Earth by asteroids or comets is not new; it was first suggested in the early part of the 20th century. Lending support to the idea is the recent discovery of in situ organic-rich grains in Halley’s comet, and data suggest that up to 25% organic matter may occur in other comets. Also, many organic compounds found in living organisms are found in carbonaceous meteorites. Some investigators propose that amino acids and other organic compounds important in the formation of life were carried to the Earth by aster-oids or comets rather than formed in situ on the Earth (Cooper et al., 2001). Complex compounds, such as sugars, sugar alcohols, and sugar acids have recently been reported in the Murchison and Murray carbonaceous chondrites in amounts comparable with those found in the amino acids of living organisms. These compounds may have been produced by processes such as photolysis on the surfaces of asteroids or comets. One problem with an extraterrestrial origin for organic compounds on the Earth is how to get these substances to survive impact. Even for small objects (~100 m in radius), ... - tailieumienphi.vn