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Crystals multiply, flames oxidise, emit carbon dioxide, grow, dance and die. Computers mimic every vital function - the brain children of a species which learnt to program the electromagnetic waves. Uniquely living systems metabolise and mutate and are capable of evolving by natural selection. Life transcends the limits of space, scale and time. Living bacteria are found several kilometres deep beneath the surface, the size of the smallest known bacteria is measured in few tens of nanometres, the time of the beginning is not as yet known. The transition between inanimate matter and proteins, the building blocks of life, constitutes the quantum jump not yet bridged by science. In this light, the recent confirmation of the biological origin of 3.45*10^9 years-old microbial stromatolites in the Pilbara, Western Australia, allows an assessment of the environment in which early life forms evolved. Photo synthesising bacteria occupying shallow water colonies needed to reach a balance between exposure to solar radiation and protection from the ultraviolet and cosmic radiation under ozone-less skies. Stromatolites could only survive during brief intermissions in volcanic activity and crustal subsidence, intermittently perturbed by meteoritic impacts - telling a remarkable story of survival against the greatest odds. Historical perspective Intrinsic to Charles Darwin's (1809-1882) theory of evolution is the inquiry into where and when did earliest life forms emerge, an unanswered question to this day. The rise of the uniformitarian paradigm of James Hutton (1726-1797) and Charles Lyell (1797-1875), regarding the present as the key to the past, saw a rift between the early 'Plutonist' and 'Neptunist' schools of thought. The first implied magmatic origin and progressive metamorphic obliteration of the geological record, whereas the second pointed to continuous sedimentation in the seas, including evidence provided by fossils. The uniformitarian paradigm arose against a background of biblical notions such as Noahs flood, as well as the catastrophic school of thought of Cuvier (1769-1832). Nowadays, Lyell's uniformitarian principle is challenged by astronomical, lunar and terrestrial evidence for high incidence of impact by near-Earth asteroids (NEA) and comets, with implications to the survival of early habitats. The advent of isotopic age determination resulted in the surprising realisation that, in places, some of the oldest continental crustal fragments have escaped high grade metamorphism and deformation, containing detailed records of early surface processes, including primitive life forms. The quest for the oldest fossils, pioneered by William Dawson (1820-1899), Charles Walcott (1850-1927), Charles Seward (1863-1941), Stanley Tyler (1906-1963), Elso Barghoorn (1915-1984), Preston Cloud (1912-1991), Vasil Timofeev (1916-1982), Alexander Oparin (1894-1980), and Martin Glaessner (1906-1989), has been recently reviewed by Schopf (1999) and Walter (1999). The most important breakthrough took place when Cryptozoon - micro fossils-bearing stromatolites - was discovered by Tyler and Cloud in the 2.0*10^9 years-old Gunflint chert on an island in Lake Superior. The physical limits of life Both present-day and ancient bacteria tell a story of extraordinary endurance under extreme physical conditions, including temperatures up to about 150oC, the breakdown limit of DNA, such as around hot springs and submarine sulphide-rich "black smoker" fumaroles. Bacteria are found in drill holes several kilometre-deep beneath the surface, under pressures greater than 1 kilobar, and in frozen lakes beneath thousands metres of Antarctic ice. Recently nanometre-scale tubular living cells, "nanobes", were described from both Precambrian sediments (Glikson and Taylor, 2000) and fractures in deep-seated drilled sandstones (Uwins et al., 2000). Micron to sub micron-scale tubes found within iron and magnesium-rich minerals (Bischoff and Coenraads, 1994) testify to the interface between living bacteria and crystal lattices. Prior to the evolution of the ozone layer from photo synthetically released oxygen, fatal ultraviolet and cosmic radiation all but precluded the exposure of life on land surfaces. A low oxygen Archaean atmosphere is attested, for example, by pebbles of pyrite in 2.8*10^9 years-old conglomerates in the Witwatersrand (Transvaal) and the Pilbara, as sulphides do not survive oxidation in present-day waters. The radiation hazard and repeated volcanic and meteoritic impact events (Chyba, 1993), suggest the evolution of photo-synthesising bacteria was likely predated by that of chemotropic bacteria, deriving energy by reduction of volcanic CO2 to CH4 and SO3 to H2S, as around present-day submarine hot springs. These biologic induced fractionations, where lighter isotopes are preferentially partitioned into released gases, are reflected by low 13/12C and low 34/32S values. Nearer to the surface, photo-synthesising bacteria had to strike a balance between their need for solar energy and protection in shallow water from fatal ultraviolet and cosmic radiation. The multiply domed geometry of stromatolite colonies allows maximum protection for a majority of bacterial cells. What is the geological nature of the terrains where early life emerged and survived? Early Pilbara environments In the Pilbara region of Western Australia, as well as in parts of the eastern Transvaal and Zimbabwe, volcanic and sedimentary rocks older than 3.4*10^9 years contain a wealth of well preserved primary textural and compositional features, which allowed detailed information on early submarine and to a lesser extent surface environments. The Pilbara craton has been documented by the pioneering work of Arthur Hickman and his colleagues of the Geological Survey of Western Australia (Hickman, 1983). The sediments which host the stromatolitic colonies are underlain and overlain by thick successions of subaqueous mafic (Mg and Fe-rich) volcanic lavas. The evidence suggests that the extrusions were associated with rapid subsidence of the sea floor, maintaining sub-aqueous conditions despite the great accumulated thicknesses. Volcanic units include Mg-rich 'ultramafic' volcanic lavas - so-called 'komatiites' after the type locality on the Komati River, Barberton Mountain, Transvaal - with bladed crystal textures and globular to tubular structures "lava pillows" formed by rapid cooling. Primary igneous minerals and geochemical patterns allow an insight into the composition of the early mantle and fractionation history of the magmas. Intercalated with the lava flows are silica-rich 'dacite' pyroclastic units and derived detrital sediments, which in places formed submarine topography or exposed islands. Quiescent stable intervals between volcanic eruptions are represented by colloidal silica deposits of chert and/or interbanded silica-ferric iron units - banded iron formations. Thin units of carbonate and barite (barium sulphate) occur, showing bladed crystal growth typical of evaporitic deposition in hypersaline waters. Disruptions of depositional environments by tectonic movements, uplift and denudation are represented by unconformable erosional surfaces. In some instances vertical movements resulted in the emergence at the surface of granitic bodies, locally preserved as buried islands or small continental nuclei - mapped in the Strelley area, central Pilbara (Buick et al., 1995). The fallout from distant meteoritic impacts is recorded by altered glass spherule layers, condensed from impact-generated silicate vapour ('microkrystites'), originally discovered along the Cretaceous-Tertiary extinction boundary in the Apenines (Alvarez, 1980). Similar impact fallout deposits are observed in the Pilbara in 3.45*10^9 year old sediments (Lowe and Byerly, 1986), in the Barberton Mountains in 3.24*10^9 sediments (Lowe et al., 1989) and in the Hamersley region of Western Australia in 2.63, 2.56 and 2.49*10^9 years-old sediments (Simonson, 1972; Simonson and Hassler, 1997). Minimum estimates of the impact incidence by asteroids and comets during the Archaean indicate more that 150 impacts forming craters larger than 100 km, including some 20 craters larger 300 km in diameter (Glikson, 1996, 1999), some of which are observed from impact fallout deposits in South Africa (Lowe et al., 1989). These episodes would have annihilated life over large areas, through a thermal flash, solar clouding effects and acid rain. Remaining bacteria cells must have found new habitats in suitable shallow seas, lagoons or lakes. Considering the combination of volcanic and impact factors, perhaps it is not surprising stromatolites are only rarely found in the many kilometre-thick Archaean sequences. Archaean ecosystems Intercalated with Pilbara chert, carbonate and barite units are undulating to dome-structured, commonly silicified, laminated carbonate sediments, the result of activity by a myriad of prokaryotic (nucleus-free) filamentous blue-green microbes. Although decimated from about 600 million years ago by grazing marine creatures, similar dome-shaped eukaryotic (single celled nuclei-bearing) stromatolite colonies occupy estuaries (Shark Bay, Hamelin Pool) and lagoons along the West Australian coast. Eukaryotes may have only emerged about 2.0*10^9 years ago, with first manifestations of algal sea weed (Grypania) at Gunflint island, Lake Superior. Following the discovery of 3.45*10^9 years-old stromatolites in the Pilbara by John Dunlop and Roger Buick (Walter et al., 1980; Buick et al., 1981) and Don Lowe (Lowe, 1980), doubts lingered regarding their biological origin. Lowe (1994) reinterpreted these structures in terms of deformed laminated sediments. At that stage, the only confident identification of Archaean life hinged on micro fossils, such as 3.45*10^9 years-old filamentous bacteria in cherts intercalated with high-Mg 'komatiite' volcanics in the Marble Bar area in the Pilbara (Schopf, 1993) and ovoid forms in the Barberton Mountains (Muir, 1978). Some twenty years passed before outcrops of cone-shaped carbonate stromatolites, found by Alec Trendall, Arthur Hickman and Kath Grey, all of the Geological Survey of Western Australia, offered new convincing evidence of biogenicity. Morphological analysis leaves little doubt of a biological origin (Hoffman et al., 1999). The stromatolites show similarities to living "pinnacle mat" stromatolites and to fossil Proterozoic Jacutophyton and Thayssagetes, although the latter do not display axial zone elongation. In view of their intra-formational position, branching forms, occurrence of inter-cone detrital deposits, and current-elongation patterns, the individual cone structures are unlikely to have formed by later deformation. The biological significance of banded iron formations remains an enigma. The restriction of this type of sediments to geological systems older that about 1600*10^6 years, about the same time as oxidised 'red bed' sandstones appeared, hints at a relation with the increasing atmospheric oxygen levels. Conceivably, iron oxidising bacteria used the little free oxygen which existed prior to this time to oxidise ferrous into the ferric iron of the banded ironstones. In the absence of micro fossils in banded iron formations the possibility remains unconfirmed. The advent of shallow water bacterial ecosystems is likely to have postdated that of better protected chemotropic bacteria, such as those likely to have been associated with the Cu-Zn sulphide at Sulphur Springs, central Pilbara (Vearncombe et al., 1996). In these environments, evidence of alternations between oxygenated waters and reducing conditions is furnished by the intercalation of barite (BaSO4)-rich evaporitic sediments and the sulphide-rich deposits. Terrestrial versus extraterrestrial origins No reasons have ever been given why the Archaean microbes are anything but original Earthlings. In the fifties Fred Hoyle and his student Chandra Wickramasinghe invoked the spectral signatures of amino acid molecules in interstellar dust and cometary tails as evidence for inter-galactic biological seeding, or 'panspermia'. More recently Paul Davies, physicist-philosopher, considered inter-planetary transport of bacterial spores aboard meteorites (1998, The Fifth Miracle, Penguin Press). The reality of sub-micron-scale microbe-like forms claimed to occur in a Mars-derived Antarctic meteorite ALH84001 has been questioned, among other due to the high temperature origin of the carbonate of which the putative fossils are made. The panspermia hypothesis has to contend with major objections. As the oldest signatures of life occur in 3.8*10^9 years-old rocks, importation of bacteria to Earth would have taken place during the so-called Late Heavy Bombardment (4.2-3.8*10^9 years ago), represented by mare basins on the Moon, when life anywhere in the solar system would have been under enormous stress. It has never been explained how proteins can escape prolonged cosmic radiation without fatal consequences. Bacteria older than the 2.0*10^9 years-old Gunflint chert, Minnesota, are not known to have cell walls or form spores, and may not have been capable of space transport. Viruses, which can occur in a frozen crystallised state, contain DNA or RNA but never contain both, and are thus incapable of reproduction except as parasites within a living host. Despite intensive studies, the essential molecules of life - DNA, RNA, ATP and ADP (adenosine tri- and di-phosphate) are not found in meteorites, whose carbon isotopic composition is heavier than in life remnants represented by kerogen. Amino acids found in carbon-rich chondritic meteorites - isobutaric acid and racemic isovaline, believed to have been shock-modified during deep space impacts, are exceedingly rare on Earth - a key observation militating against 'panspermia'. On the origin of intelligence The cone-shaped and branching algal columns display remarkable regularities. No one yet understands how billions of individual cells communicated without a central nervous system, how each bacterium knew where to position itself relative to its neighbours to ensure the perfectly formed geometrical patterns, or were they merely subjected to environmental controls? At 3.45*10^9 years ago the intelligence that underlies life is already in evidence. In a sense it does not matter where life originated, for wherever it was the enigma remains, how combinations of carbon, oxygen, hydrogen, nitrogen and sulphur evolve all the way into a brain and into technological civilisations! Taylor (1999) believes technological civilisations may be unique in the Universe, which contrasts with estimates arising from the so-called Drake Equation - N = R**pNe*l*i*cL (N - probable number of intelligent civilisations in the Milky Way galaxy capable of radio communications; R - rate of star formation; *p - fraction of stars with planetary systems; Ne - fraction of planets favourable for life; *l - fraction of planets on which life does develop; *i - fraction of planets with intelligent creatures; *c - fraction of planets on which technical civilisations develop; L - longevity of technical civilisation). On this basis between 10 000 and one billion planets with technological civilisation exist at present, depending among other on the L factor and thereby optimism (Shklovskii and Sagan, 1977). To me any restrictive view based on Earthly experience is anathema, the classic "worm in the apple" situation - the worm believes it is the only worm in the only apple in the entire universe. The chance of amino acids combining at random into a protein molecule - the basic molecule of life - is 1 in 10^130 - a larger number than the number of planets in the Universe - 10^20. Life must be written into the laws of nature, arising wherever conditions and sufficient time allow. Intelligence is a subjective value judgement, reflecting species-specific arrogance - there is as much intelligence in a bee dance as in the Swan Lake ballet. Perhaps we are not meant to know the answers to the deepest questions. References - Alvarez, W., 1986. Toward a theory of impact crises. Eos, 67, 649-658. - Bischoff, G.C.O and Coenraads, R.R., 1994. Fossil and recent traces of biodegradation on heavy minerals. N. Jb. Geol. Palaont. Mb., H4, 246-256. - Buick, R., Dunlop, J.S.R. and Groves, D.I., 1981. Stromatolite recognition in ancient rocks: an appraisal of irregularly laminated structures in an Early Archaean chert-barite unit from North Pole, Western Australia. Alcheringa, 5, 161-181. - Buick, R., Thornett, J.R., McNaughton, N.J., Smith, J.B., Barley, M.E. & Savage, M., 1995. Record of emergent continental crust ~3.5 billion years ago in the Pilbara craton of Australia. Nature, 375, 574-577. - Chyba, C.F, 1993. The violent environment of the origin of life: progress and uncertainties. Geochim et Cosmochim. Acta, 57, 3351-3358.. - Glikson, A.Y., 1996. Mega-impacts and mantle melting episodes: tests of possible correlations. AGSO J. Aust. Geol. Geophys., 16, 587-608 - Glikson, A.Y., 1999. Oceanic mega-impacts and crustal evolution. Geology, 27, 387-341. - Glikson, M. and Taylor, D., 2000. Nature of organic matter in the early Proterozoic, earliest life forms and metal associations. In: Organic Matter and Mineralisation, Kluwer Academic Publishers, Dordrecht, pp. 66-101. - Hickman, A.H., 1983. Geology of the Pilbara Block and its Environs. Geological Survey of Western Australia Bulletin, 127. - Hoffmann, H.J., Grey, K., Hickman, A.H., and Thorpe, R.I., 1999. Origin of 3.45 Ga coniform stromatolites in Warrawoona Group, Western Australia. Geol. Soc. Am. Bull., 111, 1256-1262. - Lowe, D.R., 1980. Stromatolites 3400 Myr-old from the Archaean of Western Australia. Nature, 284, 441-443. - Lowe, D.R., 1994. Abiologic origin of described stromatolites older than 3.2 Ga. Geology, 22, 387-390. - Lowe, D.R. and Byerly, G.R., 1986. Early Archaean silicate spherules of possible impact origin.., South Africa and Western Australia. Geology, 14, 83-86. - Lowe, D.R., Byerly, G.R., Asaro, F. & Kyte, F.J., 1989. Geolog-ical and geochemical record of 3400 million year old terrestrial meteorite impacts. Science, 245, 959-962. - Muir, M.D., 1978. Occurrence and potential uses of Archaean microfossils and organic matter. In: J.E. Glover and D.I. Groves (eds.), Univ. W. Aust. Exten. Serv., 2, 11-21. - Schopf, J.W., 1993. Microfossils of the early Archaean Apex chert: new evidence of the antiquity of life. Science, 260, 640-646. - Schopf, J.W., 1999. Cradle of Life, Princeton University Press, Princeton, New Jersey, 367 p. - Shklovskii, I.S. and Sagan, C., 1977. Intelligent Life in the Universe. Picador, 509 pp. - Simonson, B.M., 1992. Geological evidence for an early Precambri-an microtektite strewn field in the Hamersley Basin of Western Australia. Geol. Soc. Am. Bull., 104, 829-839. - Simonson, B.M. & Hassler, S.W., 1997. Revised correlations in the early Precambrian Hamersley Basin based on a horizon of resedimented impact spherules. Aust. J. Earth Sci., 44, 37-48. - Taylor, S.R., 1999. Destiny or Chance: Our Solar System and its Place in the Cosmos. Cambridge University Press. - Vearncombe, E.S., Barley, M.E., Groves, D.I., McNaughton, N.J., Mickucki, E.J., and Vearncombe, J.R., 1995. 3.26 Ga black smoker-type mineralisation in the Strelley belt, Pilbara Craton, Western Australia. J. Geol. Soc. London, 152, 587-590. - Walter, M.R., Buick, R. and Dunlop, J.S.R., 1980. Stromatolites 3400-3500 Myr old from the North Pole area, Western Australia. Nature, 248, 443-445. - Walter, M.R., 1999. The Search for Life on Mars, Allen and Unwin, Sydney, 170 p. - Uwins, P.J.R., Taylor, A.P. and Webb, R.I., 2000. Nannobacteria, fiction or fact? Nature of organic matter in the early Proterozoic, earliest life forms and metal associations. In: Organic Matter and Mineralisation, Kluwer Academic Publishers, Dordrecht, 421-444. Copyright 2000, Andrew Glikson ----------------- CCNet-ESSAY is part of the Cambridge Conference Network. 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