Keywords

6.1 Introduction

Evidence for the first life on Earth has been extensively researched by biologists, paleontologists and scientists from various disciplines to understand the co-evolution of biological and geological processes and to link the evolution of complex life forms, the prebiotic production of organic molecules (such as amino acids, nucleotides) and the rise of oxygen by correlating geochemical and morphological fossil evidences. Life emerged on this planet as early as 3.8 Ga, and microbial organisms evolved 3 billion years ago (Schopf 1999). The Archean time, 4 billion to 2.5-billion-year interval in the earth’s history, has been the subject of extensive geobiological research. Archean hydrothermal deposits provide clues to understanding the earth’s earliest biosphere (Reysenbach and Cady 2001), and microorganisms from modern hydrothermal systems provide analogies for interpreting the geological record as well as morphological features and isotopic signatures. Lipid biomarkers or chemical molecular fossils, which are natural biosynthetic products (often hydrocarbons) of specific biological origin, were reported in 2.7-billion-year-old samples from Western Australia (Brocks et al. 1999). This discovery suggests greater biological activity during the Archean than earlier thought and provides a separate geochemical means of assessment. A suite of lipid biomarkers, such as hopanes and steranes, indicates diagenetic molecular remnants of cyanobacterial membrane (organisms represent oxygen-producing photosynthesis) and molecular fossils of eukaryotic origin. This discovery pushed back the time of the origin of these biological taxa by at least 700 million years (Fig. 6.1). However, the 2.7 Ga sterane biomarkers, reported by Brocks et al. (1999, 2003), have subsequently been shown to be in error due to contamination (Rasmussen et al. 2008; French et al. 2015), or research suggesting noneukaryotic source of the putative Archean steranes that were taken as proxies for eukaryotic membrane sterols (Kirschvink and Kopp 2008).

Fig. 6.1
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Column on the right denotes the geological time scale in Ga (numbers denote billions of years). The major events in evolution of life from paleontological evidence are presented against the time scale, including the rise of oxygen

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The Proterozoic earth evidenced rapid biotic transition from a microbially dominated biosphere to the evolution of morphologically complex multicellular fauna and flora (Narbonne 2005; Erwin 2015). Fossil traces of mesoscopic and megascopic organisms started appearing during the Mesoproterozoic (Knoll and Carroll 1999; Martin et al. 2000; Knoll et al. 2004). The Ediacaran Period (terminal Proterozoic) constitutes an interval of time bound by decay of Marinoan ice sheets and the beginning of the Cambrian biological radiation (Fig. 6.2). Present evidence suggests that the Ediacara biota (575–542 Ma) marks the first appearance of large, architecturally complex organisms in the earth’s history and includes stem- and crown-group radial animals, stem-group bilaterian animals and representatives of other eukaryotic kingdoms (Fig. 6.2; Narbonne 2005).

Fig. 6.2
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Stratigraphic setting of the Ediacara biota in relation to Neoproterozoic global change underpinning carbon isotope excursions. Data and graph adapted and influenced from Narbonne (2005) for synthesizing this figure

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The Proterozoic sedimentary basins of India are shown in Fig. 6.3. They host thick, relatively undeformed and unmetamorphozed sandstone, shale and limestone rock sequences providing rich evidence of fossil records of Precambrian life. Some of the first observations of fossils were described in the memoirs of the Geological Survey of India. Jones (1909), for example, reported circular discs from Suket shale of the Vindhyan Supergroup where earlier geological reports failed to link these structures to primitive biological entities. The carbonaceous remains reported by Jones (1909) were later accepted as fossil impressions of the soft-bodied animal Chuaria circularis (Sharma et al. 2012). One of the earliest fossil evidences from the Precambrian of India was reported from the Daitari Greenstone Belt of the Singhbhum Craton (Jodder and Hofmann 2016).

Fig. 6.3
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Distribution of different Proterozoic basins in India. Featured in Sharma et al. (2012) after Raha and Shastry (1982). Reused with permission from Springer for publication in this Springer volume: Sharma et al. (2012)

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6.2 History and Background of Research on Earliest Fossil Records in India

Stromatolites are one of the oldest recognizable organosedimentary structures that represent the existence of life in the Achaean and Proterozoic successions (Figs. 6.4 and 6.5). In India, the earliest evidence of stromatolites was documented as ring-like structures by McClelland (1834). Soon, similar-looking features were reported from the limestones of Cuddapah Supergroup and described as ‘Laminated and Segregated in peculiar Way’ (King 1872). Later Auden (1933) observed spheroidal body-like structures in Vindhyan limestones. The biogenic origin of such structures was not understood by Charles Oldham, William King, Robert Bruce Foote, McClelland and H.C. Jones (King 1872; Jones 1909; Auden 1933). The taxonomy of such form and their evolutionary significance and biostratigraphy was not fully understood until the 1960s when stromatolites in most Precambrian rocks of India were systematically documented. Research on fossil evidence of Precambrian life in Indian basins, by studying trace, micro and mega fossils, found momentum following discoveries made by Tyler and Barghoorn (1954). This developed a platform for more recent research work (e.g. Viswanathiah and Venkatachalapathy 1980; Sharma and Shukla 2009a; Guhey et al. 2011; Sharma et al. 2012). Stromatolite-like features in siliciclastic rocks were documented in recent years in Late Paleoproterozoic Chaibasa Formation of Singhbhum Craton (Loon van and Mazumder 2013). Such forms, known as Chaibasa laminated structures, were inferred as formed by microbial activity most likely due to accumulation of fine siliciclastic particles on top of biofilm (Sharma et al. 2016). Pandey (2014) reported clotted fabric of thrombolite in Upper Vindhyans.

Fig. 6.4
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(a) Archaean domal stromatolite from Warrawoona Group, Australia; (b) Batiola indica—Archaean stromatolite from Chitradurga Group, Karnataka, India; (c, d) Archaean stromatolite Marikanwe region Chitradurga Group, Karnataka, India; (e, f) Primaevifi lum conicoterminatum, Apex chert 3465 Ma, Western Australia; (g, h) Phormidella sandurense; (i) Phormidella tenue (scale for e, f = 10 μm; for g–i = 50 μm; hammer for scale in b, d; pen for scale in c). Reused with communication and permission from Springer: Sharma and Shukla (2009a, b)

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Fig. 6.5
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An assemblage of akinetes of Nostocalean cyanobacteria recorded from the stromatolitic cherts of the Salkhan limestone, Semri Group. (a) A low magnification view of the cluster of different species of akinetes Archaeoellipsoides; (b) another cluster of Archaeoellipsoides at low magnification; (c, d) gradual high magnification view of (a); (e–h, k) Archaeoellipsoides minor; (i, j) Archaeoellipsoides major. Scale bar = 500 μm for (a); 100 μm for (b–d); 10 μm for (e–k). Reused with permission from Springer for publication in this Springer volume: Sharma and Shukla (2009a, b)

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6.3 Mesoproterozoic Record

Mesoproterozoic era witnessed the last common ancestor of modern eukaryotes (Bengtson et al. 2017). Oldest morphologically complex organic-walled eukaryotic microfossils were documented in the Paleoproterozoic–Early Mesoproterozoic Chitrakut Formation (>1600 Ma old) of the Vindhyan Supergroup (Singh and Sharma 2014). This report predates the oldest (~1500 Ma) reported eukaryotic fossils (Javaux et al. 2003). Acritarchs, which are small microfossils of unknown biological affinities (Evitt 1963), were reported from the permineralized cherts of Infra-Krol, Deoban and Buxa Limestones in Eastern Himalaya (Sharma et al. 2012). Shukla and Tiwari (2014) documented well-developed and diverse assemblage of large Ediacaran acanthomorphic acritarchs in cherts of Krol Belt of Lesser Himalaya although the occurrence of acanthomorphic acritarchs was documented infrequently in the Mesoproterozoic (Hofmann and Schopf 1983). Well-preserved fossils of cyanobacteria, interpreted as crown-group rhodophytes, were reported in Tirohan Dolomite of the Semri Group, Lower Vindhyan in Jankikund, Chitrakoot by Bengtson et al. (2017). Tirohan Dolomite is exposed north of the Son River Valley in the Chitrakoot region in Uttar Pradesh and Madhya Pradesh (Bengtson et al. 2017). The Jankikund rocks are stromatolitic carbonates, primarily dolostones, with phosphorite bands within and capping the stromatolites and intraclasts in the intercolumnar matrix (Bengtson et al. 2017). The authors reported filamentous and lobate organisms with features similar to modern eukaryotic algae. This finding pushed the appearance of the earliest accepted red algae about 400 million years earlier. The age of the Semri Group was assigned as Paleo-Mesoproterozoic by Sarangi et al. (2004), Ray (2003, 2006) and Basu and Bickford (2014), although based on fossil assemblages, an Ediacaran-Cambrian age was suggested by Azmi et al. (2008) and Prasad and Asher (2016). Both Ray (2006) and Azmi et al. (2008) drew support from published radiometric dates for their argument. Bengtson et al. (2017) questioned the inference drawn by Azmi et al. (2008) and assigned Paleoproterozoic to early Mesoproterozoic to Lower Vindhyan age based on Pb/Pb isochron of the fossiliferous phosphorite from Jankikund and other geochronologic evidence.

Megascopic carbonaceous microfossils, with wide varieties of forms and outlines (e.g. spiral Grypania, circular Chuaria, wide bands with rounded ends of Tawuia), started appearing in early Mesoproterozoic in the areno-argillaceous successions of the Vindhyan, Kurnool, Bhima and Chhattisgarh basins (Kumar 1995; Srivastava 2002; Sharma 2006b; Srivastava and Bali 2006; Sharma et al. 2009; Sharma and Shukla 2009a, b, Fig. 6.6). In particular, the Chuaria-Tawuia assemblage constitutes the most significant megascopic carbonaceous remains. Description of early Mesoproterozoic microfossils includes pre-Ediacaran large filamentous forms that consist of both prokaryotes and eukaryotes (Sharma and Shukla 2012a, b). The carbonaceous fossils, especially Chuaria-Tawuia assemblage, in the Chorhat Sandstone (Kheinjua Formation, Semri Group) represent the oldest dated fossils in the Vindhyan Supergroup and indicate an evolutionary phase from micro- to megascopic organisms (Srivastava and Bali 2006). Megascopic microfossils, preserved in cherts; hosted in a thick, unmetamorphosed sequence of sandstone, shale, carbonate and volcanoclastic rocks; and primarily formed in the peritidal to tidal environments, were reported from Deoban Limestone, Infra-Krol and Krol Formations in the Lesser Himalaya and Vindhyan Supergroup (Srivastava 2006, 2009; Sharma 2006a; Bengtson et al. 2017).

Fig. 6.6
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Varied carbonaceous remains recovered from the Proterozoic sedimentary basins of peninsular India. (a, b) Chuaria circularis showing thick rim on the periphery of specimen noted in the Halkal Formation of the Bhima basin; (c) Tawuia dalensis recorded from the Halkal Formation of the Bhima basin; (d) Katnia singhii preserved on the parting surfaces of grey shale from the Rohtas Formation, Vindhyan basin; (e, f) Protoarenicola baiguashanensis; (g) Sinosabellidites huainanensis; (h) Pararenicola huaiyuanensis all recorded from the Halkal Formation of the Bhima Group. Scale bar for (a) = 1 mm, for (d) = 5 mm and for the rest of the specimen = 2 mm. Reused with permission from Springer for publication in this Springer volume: Sharma et al. (2012)

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Microbial mats in the Precambrian siliciclastic sequences provide records of ancient prokaryote life forms. Microbial mat-related structures (MRS) were reported from the Marwar (Sonia Sandstone), Vindhyan (Chorhat Sandstone and Koldaha Shale Formations) and Chhattisgarh Supergroups and from the Khariar Group and linked to the Precambrian shallow-marine coastal depositional environments (Ahmad and Kumar 2014; Banerjee and Jeevankumar 2005; Banerjee et al. 2006, 2010; Sarkar et al. 2006, 2011, 2014, 2008; Samanta et al. 2015; Sur et al. 2006; Chakraborty et al. 2012). Samanta et al. (2011) observed enhanced preservation of microbial remains in pyrite produced by diagenetic alteration of microbial mat. Banerjee and Jeevankumar (2005) described variation of wrinkle structures within a highstand systems tract in response to continued progradation. Banerjee et al. (2006) linked the occurrence of organic-rich (total organic carbon content >2%) microbial mat-originated laminated pyritic carbonaceous shales to flooding surfaces (Sur et al. 2006; Schieber et al. 2007). The MRS, in the form of discoidal colonies, was reported in the Gulf of Cambay (Banerjee 2012), and Chakraborty and Das (2013) linked varieties of bedding plane structures to microbial mat in Chhattisgarh and Khariar basins. Deb et al. (2007) reported similar microbial mat from the Mesoproterozoic Somanapalli Group in Pranhita-Godavari basin. Banerjee et al. (2010) reported microbial mat with features resembling metazoan burrows, tracks and trails and metazoan body fossil impressions from the shallow-marine sandstones of the Semri Group of the Vindhyan basin. Sarkar et al. (2014) observed features such as petee ridges, sandcracks, gas domes, multidirected ripples, reticulate surfaces and sieve-like surfaces to shallow parts of the marine basins, in upper intertidal to supratidal conditions by drawing inference from their modern analogues, whereas the wrinkle structures, roll-up structures and patchy ripples were linked to broader range of paleogeographic settings ranging from the supratidal to subtidal conditions. Such megascopic carbonaceous microfossils continue till the Late Neoproterozoic. The soft-bodied organisms grew on the mat-infested Proterozoic seafloor producing close association between the traces of such organisms and mat-related structures. The microbial mat colonies provided favourable habitat conditions to the soft-bodied organisms. The unique Neoproterozoic substrate conditions preserved the microbial mat colonies along with the soft-bodied organisms including Ediacaran fauna (Gingras et al. 2011).

6.4 Neoproterozoic Record

Multicellular eukaryotes became predominant around the Proterozoic–Phanerozoic transition. The Neoproterozoic successions in India provide records of one of the most remarkable periods in earth’s history owing to the emergence of macroscopic animals (e.g., Knoll 2000) and the evolution of Precambrian life forms. Sphaeromorphs and morphologically complex acanthomorphs, herkomorphs, netromorphs, prismatomorphs and pteromorphs were common Proterozoic acritarchs. Some of these microfossils had close association with fossils of extant Chlorophyceae, and many large sphaeromorphs had close affinity with prasinophytes. Megasphaeromorphs (~200 μm) show significant occurrence in the terminal Proterozoic (Tappan 1980), and acanthomorphic acritarchs were well documented in Neoproterozoic (Schopf 1992). Acritarchs were reported from the Gangolihat Formation (Tiwari and Pant 2009) and Vindhyan Supergroup, which contain several taxonomically and biostratigraphically important acritarch genera (Prasad and Asher 2001; Prasad et al. 2005, 2010).

The microfossils in the chert nodules of Infra-Krol deposits were attributed to non-septate and septate cyanobacterial filaments, coiled filaments, coccoids, large sphaeromorphs, small and large acanthomorphic acritarchs and vase-shaped microfossils (VSMs). The cyanobacterial filaments and coccoids were identified as Siphonophycus typicum, Siphonophycus robustum, S. kestron, S. inornatum, S. punctatum, Polytrichoides bipartitus, P. lineatus, Obruchevella parva, O. magna, Oscillatoriopsis media, Oscillatoriopsis sp., Eomicrocoleus crassus, Polybessurus crassus, Salome hubeiensis, Myxococcoides sp., Sphaerophycus parvum, S. medium, Huroniospora psilata, Palaeoanacystis vulgaris, Tetraphycus sp., Wengania globosa, Bavlinella faveolata and Melanocyrillium horodyskii (Sharma et al. 2012). Large sphaeromorphic and acanthomorphic acritarchs were identified as Leiosphaeridia crassa, Ericiasphaera spjeldnaesii, Echinospheridium maximum, Asterocapsoides sinensis, Asterocapsoides sp. and Cymatiosphaeroides yinii.

Neoproterozoic successions reveal the presence of relatively simple forms of eukaryotes in primitive oceans that contain much less oxygen compared to oxygen levels in modern ocean (Knoll and Carroll 1999; Knoll 2000). The relatively smaller forms of organisms were likely driven by low dissolved oxygen content in the ancient ocean water. The marine ecosystems were inhabited by a variety of oxygenic and anoxygenic prokaryotes including regular chlorophyll-containing algae. However, the exact time of appearance of green algae is debated. Evolution of multicellular animals is considered the most remarkable event of the terminal Neoproterozoic (Narbonne 2005; Shu et al. 2014) that leads to the Cambrian explosion of life.

6.4.1 Ediacaran Trace Fossil

The appearance of soft-bodied animals on earth during Ediacaran Period, following the Neoproterozoic glacial event, marks an important episode in the evolution of life on Earth. The Vendian Period marks the rise and fall of soft-bodied, megascopic metazoans preserved in unique taphonomic conditions. The Vendian organisms are represented by Neoproterozoic Ediacara faunas. The appearance of multicellular Ediacaran life is consistent with the hypothesis that increase in atmospheric oxygen level played a significant role in metazoan evolution. During this time, the evolution of megascopic organisms is considered to follow the Neoproterozoic glaciations at 750–635 Ma, with maximum biological diversity found during 560–540 Ma (Waggoner 2003; Meert et al. 2011). However, bioturbation evidence of such soft-bodied organisms is sporadic in geological record and primarily recorded in siliciclastic settings (Hofmann and Mountjoy 2001). High benthic population, attached forms, overall absence of scavengers and predators, absence of post-burial disturbances by deposit feeders, shallow water habitat, rapid burial and sediment stabilization by algal and cyanobacterial mat were some of the controlling factors that promoted preservation of the soft-bodied biota. The Ediacaran assemblages, including Pteridinium simplex, Charniodiscus, Zolotystia, Beltanelliformis, Kimberella, Conomedusites sp., Cyclomedusa davidi, Sekwia and Irridinites (Mathur 2008; Maithy and Kumar 2007), are best exposed in Krol successions and the Spiti sections in the Himalaya. In the Peninsular India, Twitya discs, Cyclomedusa and Obruchevella burrow structures in the Kurnool Group represent Ediacaran trace fossils (Sharma 2008; Sharma and Shukla 2012b). Soft-bodied megascopic metazoans showing faunistic, ecological and taphonomic affinities to known Ediacaran fossil assemblage, represented by coelenterate (Tribachidium, Eoporita, Kaisalia, Cyclomedusa, Ediacaria, Nimbia, Paliella, Medusinites and Hiemalora) and arthropod (Spriggina) genera, were reported from the Vindhyan basin (De 2006). Seilacher et al. (1998) reported old triploblastic metazoan traces from the Lower Vindhyan sequence. De (2006) further reported forms (Ediacaria and Hiemalora) similar to Ediacaran assemblages from the Late Neoproterozoic Bhander Group of the Upper Vindhyan. Interestingly, many Ediacaran fossils reported earlier in the Vindhyan Supergroup have now been discarded as artefacts (Kumar and Sharma 2012) although Prasad and Asher (2016) reported abundant and well-preserved Ediacaran complex acanthomorph acritarchs in drill cores through allegedly Lower Vindhyan sedimentary rocks in the western Vindhyan sub-basin, Chambal Valley, Rajasthan.

It is necessary to mention that uncertainties in age calibration based on occurrences and events in fossil record affect the linear divergence of the phylogenic trees. A fossil only gives a minimum age for the lineage in the fossil record where preservation potential of the fossilized organism along with the availability of a suitable depositional environment plays crucial role. Scarce occurrences of fossiliferous deposits, low preservation and the controversial nature of the fossils increase the uncertainty of calibration intervals in Proterozoic (Bengtson et al. 2017).

6.4.2 Biogeochemical Records

Stable isotopes and organic geochemistry-based chemostratigraphy have been used as a tool to trace the earliest evidence of life on earth, but such records from Indian Proterozoic successions are relatively less in number. Major paleobiological and carbon (C)-stable isotope (δ13C) records reflecting rapid evolution and diversification of multicellular life during terminal Neoproterozoic were reported in the Indian Lesser Himalaya (Tewari 2003). The Neoproterozoic–Early Cambrian chemostratigraphy of the Blaini–Krol–Tal succession reflects the Precambrian–Cambrian transition in the Lower Tal Formation (δ13C = −4‰). The Krol belt in the Lesser Himalaya is characterized by positive δ13C value (1–6‰), and the pink cap carbonate of the Blaini Formation, the base of the terminal Proterozoic System in the Lesser Himalaya, shows negative δ13C value (−3‰) (Tiwari and Sial 2007). Positive δ13C values (3.7–5.4‰) were also reported from the terminal Neoproterozoic Buxa Dolomite in Lesser Himalaya (Shen and Schidlowski 2000; Tewari 2003; Tiwari and Sial 2007). The oxygen isotope record (δ18O = −2.2‰) in the Upper Krol is consistent with the increase in atmospheric oxygen that played a significant role in metazoan evolution and Cambrian explosion (Tiwari and Sial 2007). Studied phosphate content and stable isotopes of C and nitrogen (δ15N) in Paleoproterozoic carbonates of Aravalli Supergroup and linked cyanobacterial blooms and Great Oxidation Event to the increased phosphate availability during Paleoproterozoic Lomagundi–Jatuli event and the Neoproterozoic Cryogenian and Ediacaran periods. Studied δ13C and oxygen (O) isotope (δ18O) in Neoproterozoic carbonates of Sirohi Group and linked the variations in δ18O and δ13C to contact metamorphism and changing C cycles, respectively. Studied C, O and strontium (Sr) isotopes in carbonate rocks of Proterozoic Bhima basin and postulated effect of diagenetic alterations on δ18O and Sr isotope (87Sr/86Sr) signatures. Studied δ18O and δ13C and Sr, barium (Ba) and rare earth elements (REE) composition in Paleoproterozoic cap-carbonate unit of Paleoproterozoic Sausar Group in central India and drew parallel of the lithological association and chemostratigraphy with Neoproterozoic cap carbonates in Canada, the USA, South Africa and Australia.

Organic geochemical study of Proterozoic successions in India is primarily limited to hydrocarbon exploration. For example, organic geochemical analysis of shale from Proterozoic Sirban Formation and Vindhyan successions was performed for estimating hydrocarbon prospects (Prasanna et al. 2013; Mani et al. 2014), and lipid biomarkers were documented from crude oils and rock extracts recovered from the Marwar Supergroup. Compared lipid biomarker and δ13C signatures of oil from Bikaner-Nagaur basin of Rajasthan in western India with oils from Oman and eastern Siberia. Raju et al. (2014) inferred oil generation under anoxic hypersaline conditions from marine clastic source rock and biodegradation of the oil using aromatic hydrocarbons, isoprenoid alkane ratio and sterane and hopane ratio. Steroids derived from squalene precursor are an important constituent of membrane lipids of all eukaryotes (Summons et al. 2006). Reported significant presence of C29 and C30 steranes in Neoproterozoic–Early Cambrian samples from the Bikaner-Nagaur basin and inferred significant contribution from both eukaryotes (demosponges) and prokaryotes (green algae). However, migration of oil makes it difficult to establish the exact age of the biomarkers as the source of the oil is not known. Dayal et al. (2014) inferred conducive pathways for migration of the hydrocarbons towards the surface soil by using compound-specific δ13C of desorbed gaseous hydrocarbon (methane and ethane) in outcrop shale samples of Semri and Kaimur Groups in Vindhyan basin. Nonetheless, the biggest analytical challenge for Precambrian organic geochemical studies is contamination from hydrocarbons of a different age in the samples being studied. Precambrian rock samples may contain significant amount of organic C by weight, and biomarker compounds are present in these rocks only at trace amount (parts per billion or less), making contamination from other petroleum sources a possibility (Waldbauer et al. 2009). Despite the challenges, Patranabis-Deb et al. (2016) inferred rare algal presence in bacteria-dominated Proterozoic ocean from the absence of steroids and abundance of hopanes in the late Mesoproterozoic carbonates in Chhattisgarh. The study indicated that Proterozoic basin had little eukaryotic abundance relative to prokaryotic bacteria. Hoshino et al. (2017) reported C27–C29 steranes in pyritic black shale from the Krol Group in the Mussoorie syncline and linked this to the rise of green algae to global dominance of marine ecosystems.