The evolution and distribution of life in the Precambrian eon-Global perspective and the Indian record 765 The evolution and distribution of life in the Precambrian eon-Global perspective and the Indian record M SHARMA* and Y SHUKLA Birbal Sahni Institute of Palaeobotany, 53 University Road, Lucknow 226 007, India *Corresponding author (Email, [email protected]) The discovery of Precambrian microfossils in 1954 opened a new vista of investigations in the fi eld of evolution of life. Although the Precambrian encompasses 87% of the earth’s history, the pace of organismal evolution was quite slow. The life forms as categorised today in the three principal domains viz. the Bacteria, the Archaea and the Eucarya evolved during this period. In this paper, we review the advancements made in the Precambrian palaeontology and its contribution in understanding the evolution of life forms on earth. These studies have enriched the data base on the Precambrian life. Most of the direct evidence includes fossil prokaryotes, protists, advanced algal fossils, acritarchs, and the indirect evidence is represented by the stromatolites, trace fossils and geochemical fossils signatures. The Precambrian fossils are preserved in the form of compressions, impressions, and permineralized and biomineralized remains. [Sharma M and Shukla Y 2009 The evolution and distribution of life in the Precambrian eon-Global perspective and the Indian record; J. Biosci. 34 765–776] DOI 10.1007/s12038-009-0065-8 1. Introduction suggested that all the living forms can be grouped into three principal domains viz. the Bacteria, the Archaea, and The sudden appearance and radiation of both skeletal and the Eucarya (Woese 1987, 2002; Woese et al. 1990) /or soft-bodied organisms in the Cambrian and the lack of fossils two empires viz. the Prokaryota and the Eukaryota (Mayr in the rocks of Precambrian Eon puzzled Charles Darwin 1998). Molecular phylogenies further established that these (Darwin 1859, Origin of Species, Chapter X, John Murray, domains diverged from one another during the Archaean London, p. 447). The renowned palaeobotanist Seward, who Eon. was sceptical about discovering older ancestral life forms, In this paper, we review the extensive contributions once stated that “We can’t hope to get any evidence of and important milestones in our understanding early activities of life of this period” (Seward 1931). (Precambrian) life. Signifi cant Indian contributions to Following Darwin, for almost a century, no direct record the study of Precambrian life are also cited. The direct of life was found in strata underlying the Cambrian. In 1954, evidence of Precambrian biota includes fossil prokaryotes, a seminal paper on the 2000 million years (Ma) old Gunfl int protists, advanced algal fossils, acritarchs, and the indirect Chert reporting discovery of well-preserved microbiota evidences are represented by stromatolites, trace fossils and opened a new vista of investigations into the Precambrian geochemical fossils. These evidences show varied types of life (Tyler and Barghoorn 1954). This discovery supported preservation such as compressions, impressions, and the the view that if the sudden radiation of life in the Cambrian permineralized and biomineralized remains. was any pointer, the answer and the root of evolution must be found in the Precambrian strata. Subsequent to the 2. The Precambrian geological timescale fi rst discovery of Precambrian biota, the types of evidence have increased manyfold through out the world, albeit The two broad divisions of the Earth’s geological history gradually. Recent molecular phylogenetic studies have are the Precambrian and the Phanerozoic; the transition Keywords. Archaean; evolution; palaeontology; Precambrian; Proterozoic http://www.ias.ac.in/jbiosci J. Biosci. 34(5), November 2009, 765–776, © IndianJ. Biosci. Academy 34(5), of November Sciences 2009 765 766 M Sharma and Y Shukla from Precambrian into the Phanerozoic is marked at 542 Australia and the Onverwacht and Fig Tree Groups of South Ma (Gradstein et al. 2004). In terms of numerical age the Africa, supposed to have deposited in tectonically separated Phanerozoic encompasses only ~13% of the entire earth’s parts of a single basin. In these successions, carbonaceous history, and comprises three eras, viz. the Palaeozoic, the cherts occurring predominantly in volcanogenic packages Mesozoic, and the Cenozoic. The fl oral and faunal diversity have yielded enigmatic carbonaceous microstructures which we see today on the earth is the result of evolution of indeterminable affi nity and biogenicity. Some better which took place mainly during the Phanerozoic. The preserved microstructures plausibly may have biogenic oldest rock (4280 Ma) has been found in the Nuvvagittuq origin (Muir and Grant 1976; Knoll and Barghoorn 1977; Greenstone belt in northern Quebec, Canada represented Walsh and Lowe 1985; Schopf and Packer 1987; Schopf by the rock “faux amphibolite” that formed shortly after 1993; Rasmussen 2000; McKeegan et al. 2007). Presently, the earth formation and may even be the oldest preserved eight micro-fossiliferous units from the Archaean satisfy nine crustal section on earth (O’Neil et al. 2008). The pace of varied criteria for biogenicity and criteria of provenance, evolution of life indubitably had its root in the Precambrian. age indigenousness and syngenicity and should be Such a large time period has been further divided into the considered bona fi de early Archaean microfossils (Schopf Archaean Eon (4280 to 2500 Ma) and the Proterozoic 2004). However, these early Archaean fossils at best Eon (2500 to 542 Ma). The Proterozoic is further sub- manifest only the presence of a biosphere; their taxonomic divided into the Palaeoproterozoic (2500 Ma-1600 Ma), the position or physiological adaptations can be construed in Mesoproterozoic (1600 to 1000 Ma) and the Neoproterozoic many ways. (1000 to 542 Ma) (Gradestein et al. 2004). Recently a new Another evidence of early Archaean biosphere is period named ‘Ediacaran’ has been added in the geological represented by stratiform, domal, conical and columnar timescale (Knoll et al. 2006). It is chronostratigraphically structures – stromatolites – preserved in carbonate rocks of established. For the sake of the convenience all the relevant this age. Such structures have been reported from Onverwacht fossil records are being presented under these time divisions and Warrawoona Groups (fi gure 1a) (Lowe 1980; Walter and fi nally Indian status of palaeobiological records is et al. 1980; Byerly et al. 1986). Not all stromatolites are discussed in global perspective at the end of each section. believed to be of biogenic origin. Some forms are believed to have been supposedly formed by abiogenic processes 3. Fossils in the Precambrian (precipitation) or soft sediment deformation or tectonic processes (Buick et al. 1981). Microbial origins of stratiform 3.1 Archaean fossil record stromatolites are rather convincing and comparison of these stromatolites with modern stromatolites suggests that Early Archaean (3800–3000 Ma) rocks were mostly photoautotrophic organisms might have been involved in igneous in origin. Whatever little sedimentary rocks were their formation. But whether or not cyanobacteria were deposited have undergone geological recycling. Subsequent actively involved in their formation remains uncertain. metamorphism of these rocks makes them virtually Advent and evolution of oxygen in the atmosphere is unsuitable for palaeontological investigations though another aspect of Precambrian palaeobiology. Towe (1990) some of these rocks do hold promise for microbial fossil suggested that the early Archaean atmosphere must have investigations. Microorganisms of this period were minute, contained oxygen that was suffi ciently conducive for aerobic poorly preserved, and constituted of simple morphologies respiration ~1–2% of present day levels (PAL). Occurrence that could have been produced even by non-biologic of palaeosols and detrital uraninite in alluvial deposits of processes like mineralic microstructures that make it diffi cult early Archaean also support that the oxygen levels could not to differentiate between true fossils and pseudofossils. have been more than this. Subsequent higher oxygen level in Hence, Early Archaean palaeontological records are atmosphere probably was the result of photosynthetic activity always viewed with scepticism (Brasier et al. 2002; Dalton of cyanobacteria. Extensive Banded Iron Formations (BIFs) 2002; Van Zuilen et al. 2002). Even those recorded from in the Archaean are presented as important evidence for the the Warrawoona Group of Western Australia have been cyanobacterial oxygen production. However, the possibility challenged for their antiquity and biogenicity (Awramik et that BIFs could have also formed by photo-oxidation of al. 1983; Buick 1984; Kaźmierczak and Barbara 2002). ferrous iron dissolved in anoxic Archaean oceans can not be Known Archaean fossil forms can be grouped into isolated entirely ruled out. single cells, paired probably dividing cells, ensheathed All these evidences from the early Archaean collectively colonies of coccoidal cells, and cylindrical and cylindrical suggest that microbial life was present (Figure 1e, e’, f septate-fi laments. Evidences for Archaean biosphere have f’), fl ourishing and widespread by the close of 3500 Ma been gathered from fossils, organic matter and isotopic ago (Schopf et al. 2002). The ecosystem graduated to signatures mainly from the Warrawoona