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Life in Pre-Cambrian and Early Cambrian Times

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Life in Pre-Cambrian and Early Cambrian Times Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021 Life in Pre-Cambrian and early Cambrian times JOHN WATSON COWIE CONTENTS 1 Introduction 17 2 The origin of life 18 3 Evidence of life in Pre-Cambrian times . 21 (A) Biogenic materials in ancient rocks 22 (s) Micropalaeontology . 22 (e) Stromatolites 24 (D) Protozoa 25 (~) Metazoa . 25 (F) Trace-fossils 9 9 27 4 Life in early Cambrian times 27 (A) The problem of the base of the Cambrian System 27 (B) The earliest Cambrian fauna 29 5 References. 33 SUMMARY Physical and chemical conditions which could most metazoans are the subject of dispute. have favoured the origin of life on Earth are The remarkable fauna from Ediacara is partly briefly discussed, together with the evidence of metazoan, but the age may be Cambrian or the geological record. Free oxygen may be of Pre-Cambrian. Aspects of the problem of the critical importance as a factor in the early base of the Cambrian System are discussed and evolution of life. Recent studies of the occur- the earliest Cambrian taxa are listed. Selected rence of biogenic materials and the micro- theories which attempt to explain the manner palaeontology of ancient rocks have led to of appearance and character of the early striking advances in knowledge of Pre-Cambrian Cambrian fauna are reviewed in a brief life. Plants and protistids are the oldest known conspectus. fossils. Pre-Cambrian fossil protozoans and 1. Introduction The Pre-Cambrian and Cambrian rocks, and the evidence of life that they con- tain, present two problems of great interest to palaeontologists and non-palaeon- tologists alike" the origin of life and the origin of the early Cambrian fauna. Professor Whittard had to cancel his lecture on these topics to the symposium meeting on 'The Fossil Record' due to the illness which had also prevented him from preparing a manuscript or notes. I was invited by the symposium organizers to prepare a lecture and this article: the sole responsibility for both must remain with me (the note in Proc. geol. Soc. Lond. 1630, p. 40, was incorrect in this respect). It was my hope that Professor Whittard would be able to criticize the manuscript, but this was prevented by his death; I should like to express my gratitude for all the help he gave me during the last twenty years, as he did for so many others. The Fossil Record, pp. 17-35. Geological Society of London, 1967. Printed in Northern Ireland. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021 The Fossil Record, Part I W. T. Dean, S. C. Matthews, J. W. Murray and R.J.G. Savage kindly read and criticized the manuscript. 2. The origin of life A starting point of modern scientific discussion on the origin of life is that it was of necessity preceded by geochemical evolution and that biological and chemical evolution were complementary and interwoven. Extra-terrestrial origin of life is left aside here but may gain further support from continuing research on biological material in meteorites or, conceivably, by direct examina- tion of other parts of the solar system. The possibility that meteorites, or at least some of them, represent returning terrestrial matter cannot be excluded (Urey 1966). The emergence of the biosphere was related to the emergence of appropriate quantities of the right organic compounds in a suitable environ- ment of the lithosphere, hydrosphere and atmosphere. Life may be considered to be an inevitable process; bound to appear wherever and whenever conditions are favourable. Whittard (1953, pp. 299-301) discussed the application of Milne's cosmological theory by Haldane to the enigma of the earliest fossils. Whittard's cautious remark that "the danger lies in the possibility that the model is wrong" was amply justified because later he was informed by Professor M. H. L. Pryce, physicist, of an arithmetical error in Milne's theory which made it untenable (Whittard, verb. comm.). It is nowadays often postulated that the Earth formed initially as a cold body by accretion and that it has subsequently been heating up as a result of internal radioactive decay and gravitational compression. Uffen (1963, pp. 143-4) suggested that the thermal history of the Earth has determined the origin and development of the Earth's core which could have been a major factor in biological evolution through its control of the main geomagnetic field and consequently of the charged particles which have been able to reach the Earth. If the magnetic field intensity was reduced to zero during geological history then the 'solar wind' of ionizing radiation would have been able to bathe the Earth, exerting a major influence in the evolution of life. Another limiting environmental factor for the origin and suvival of life is the temperature range. Variations in solar heat radiation may have occurred in the past. Cyclical changes in the sun may have caused fluctuations and an overall rise in the mean earth temperature so that refrigeration prevented life processes before 3000 m.y. ago (Opik 1958). The Sun's radiation could also have been reduced, and cloud cover increased, by volcanic dust (Wexler 1952). Geologists commonly assume little variation in the mean temperature at the surface or outer part of the earth in the last 3000 m.y. (Rutten 1962, 120-122). The geo- logical record is equivocal and there seems little over-all support for colder con- ditions in the earlier part of the Pre-Cambrian than in the later. The evidence of palaeomagnetic observations, suggests, however, that the Varangian (late Pre-Cambrian) glaciation extended into equatorial latitudes and may have been more severe than later ice ages (Harland & Rudwick 1964; C)pik 1958). The complex heat balance of the whole earth, including the atmosphere and hydro- sphere, has various feed-back mechanisms (Rutten 1962, pp. 122-124) making 18 Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021 Life in Pre-Cambrian and early Cambrian times for stability and perhaps there has been little change in the average surface temperature since well before the origin of life. The earliest atmosphere probably escaped in part from the Earth's gravitational field and this is indicated by the impoverishment of the proportions of the inert gases (Brown 1952; Fesenkov 1960). Slightly later, major contributions to the primeval atmosphere came ultimately from within the Earth--the phenomenon of 'out-gassing'. Thus the atmosphere and oceanic water may have escaped from the Earth's interior. Theory suggests that the primeval atmosphere contained methane, ammonia, hydrogen and water vapour, whilst carbon dioxideand nitrogen were also present in increasing percentages as time passed. An analogy can be made with the present atmosphere of the major planets. An indication of the existence of some of these substances in the past is suggested by the con- tents of cavities in crystals. Some of these cavities contain trapped fluids such as water, methane and ethane. The Earth is distinguished from other planets by its content of free oxygen. The absence of free oxygen from volcanic gases today seems to support the theory that the present free oxygen in the atmosphere did not escape from the interior of the Earth and the element was not present in the primeval atmosphere, only being found at that time in a combined state within the Earth. Free oxygen may have begun to be an atmospheric constituent about 3000 m.y. ago and be largely biological in origin, due directly or indirectly to photosynthesis. In the primeval reducing atmosphere ultraviolet radiation may have acted on carbon dioxide to liberate free oxygen and carbon; this abiogenic process is not operative now due to the subsequent development of an ozone envelope shielding the Earth's surface from the solar ultraviolet radiation. Biogenesis is the hypothesis that living matter arises always from living matter (Oxford Dictionary) and biogenic and abiogenic are used here in a similar sense. This is contrary to Rutten's usage of 'biogenesis' (Rutten 1962, p. 127). Bernal (1960, p. 31) favoured the term biopoesis for life-making. Urey has shown that oxygen pro- duction by photodissociation eventually inhibits itself as the oxygen formed prevents penetration of further ultraviolet radiation. This supports the suggestion that free oxygen is largely biogenic. Other sources of free oxygen could have been through the precipitation in water of calcium carbonate and by green plant photosynthesis. Urey (1952B) concluded that if the earth had a primitive re- ducing atmosphere then organic compounds would have been synthesized in this atmosphere by ultraviolet radiation and electrical discharges. Laboratory experiments by Calvin, Miller, Fox and others support Urey's theories. Model primitive atmospheric gas mixes (CH4, NH3, H~ and H~O) were subjected either to electron bombardment in a particle accelerator or to electrical discharges, producing amino-acids and simple proteins. Amino-acids can be polymerized to more complex molecules by ultraviolet radiation on a clay base. [These experiments have been reviewed: (Miller & Urey 1959; Horowitz & Miller 1962).] The first steps necessary for the origin of life are now understood experi- mentally to some degree but many difficult problems remain. The origin of life seems to require a primeval atmosphere with prevailing anoxygenic conditions 19 Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021 The Fossil Record, Part I to avoid dissipation by oxidation of essential life components such as amino-acids and to allow the accumulation of complex organic compounds by abiogenic mechanisms. Evolution from non-living to living is often considered to be inoperative today. Multiple and repeated origination of life in the past is a theoretical possibility and offers alternative starting points for evolution which may then have been partly parallel and not directly related (Whittard 1953, p.
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