It was extremely fortunate for the stability of life that despite the planet active dynamics over geologic time, surface temperatures were always sufficiently moderate to maintain liquid water in the oceans and, therefore, sustain the continuous biological evolution since at least early Archean time. The first oceans were accumulated via mantle degassing and/or cometary bombardment much earlier, during the Hadean Eon. Within this realm, life appeared on Earth at least by about 3.8 billion yeats ago, the age of supracrustal rocks of southwest Greenland bearing biogenic carbon isotopes of graphite, or possibly as early as 4.1 billion years ago, the age of similar evidence more recently reported from a Western Australian zircon grain. For the following three billion years or so, microorganisms were the only life forms in the primitive oceans, and their fossil evidence attests to a very slow evolutionary rate of morphological change. The first organisms were Prokaryotes, such as Bacteria and Archaea, which continue microscopic and morphologically simple, yet ubiquitous and extremely important, even today. Later, during Proterozoic time, environmental pressures and opportunities led to evolutionary developments that eventually produced the Eukarya, initially and for a very long time solely microscopic, which have lived side-by side with Prokaryotes ever since.
During a relatively long span of geologic time from the Middle Neoproterozoic to Late Cambrian (roughly750–500 million years ago), important innovations in the fossil record register the birth and proliferation of more complex macroscopic forms of life, especially in the Ediacaran Period (635–541 million years ago), while the geologic record reveals rapid changes in continental configurations related to the Gondwana supercontinent formation, extreme climatic variations, including the most severe glacial episodes in Earth history and the oxygenation of oceans and atmosphere, all of which surely exerted great selection pressures on the evolutionary processes responsible for these innovations. In the final part of this time-frame, during the Tommotian–Botomian interval (535–513 million years ago), the 'Cambrian Explosion' of life took place, as evidenced by the variety and amount of shells, carapaces, and other fossil remains representing all known modern phyla conserved worldwide in Cambrian strata.
The planet Earth started hot, and has continuously cooled since then. Plate tectonics is the main mechanism by which the planet loses its internal heat, with geologic evidence indicating that it started functioning gradually in Archean times. Lithosphere subduction is the plate tectonics hallmark, and in 2008 Michael Brown of the University of Maryland, after analysing Earth’s metamorphic record over geologic time, was able to define a 'Proterozoic plate tectonic regime' characterized by oceanic lithosphere subduction. Continental lithosphere remained stable, however, with large areas of granitic-type crust providing the nuclei of major continental masses and several supercontinents during this period. Meanwhile, as Earth cooled slowly during the Archean and most of the Proterozoic, life remained microscopic.
However, in the Neoproterozoic, important modifications occurred both in the planet dynamics and in the life and biosphere complexities. Due to continuous heat loss, the Earth cooled to a point in which, relatively quickly, basaltic-type oceanic lithosphere could become negatively buoyant. As a result, basalts could be transformed into even denser eclogites in deep subduction slabs. This greater negative buoyancy led to the 'slab-pull type' driving the force and appearance of 'modern-type subduction zones' and 'subduction-to-collision' orogenic belts that would become widespread in the Phanerozoic. For the first time in Earth’s history, ultra-high pressure metamorphic terrains bearing coesite or diamond appeared on Earth, indicating subduction of continental crust to depths greater than 100 km, followed by rapid exhumation. The oldest ultra-high pressure eclogites described so far are Ediacaran in age as shown by robust uranium-lead zircon ages of about 610 million years (as zircon forms it can incorporate a variety of different elements into its crystal matrix, including uranium but not lead; this is useful as over time uranium decays to form lead, so any lead in a zircon mineral must be the result of the decay of uranium). It has been further demonstrated that these eclogites are located along the Transbrasiliano-Kandi megashear, which is a huge tectonic lineament, more than 5000 km long, linking South America and Africa, interpreted as the possible site of a collisional suture associated with the West Gondwana Orogeny that produced Earth’s first Himalayan-type mega-mountains.
In a paper published in the Brazilian Journal of Geology on 22 May 2020, Umberto Cordani, Thomas Fairchild, Carlos Ganade, Marly Babinski, and Juliana de Moraes Leme, of the Universidade de São Paulo, suggest that the practically simultaneous appearance of the first very large high-relief mountain chains on the Earth’s surface, similar in magnitude to the Himalayas, and the radiation of macroscopic Metazoans in the Ediacaran are probably intimately related. Other authors have alluded to the influence of Gondwana mountain upon Ediacaran evolution, but Cordani et al. claim to have identified just when and where it began.
The Neoproterozoic (1000–541 million years ago) was a time of important modifications on the planet dynamics, including especially the onset of modern-type plate tectonics that prevails until the present. Such a regime is characterized by subduction-to-collision orogenic belts and disappearance of large oceanic domains. The Neoproterozoic hosted a specific and long-term episode involving the planet lithosphere, when continental masses of all sizes underwent an extended fragmentation period and new assembly with the disruption of the supercontinent Rodinia into many fragments, starting around 900 million years ago, and Gondwana amalgamation that ended around 500 million years ago. Moreover, at the end of the Neoproterozoic, in the Ediacaran, with Gondwana assembly in full progress, marked changes occurred in relief, climate, oceans and atmosphere, while complex forms of life, including the first macroscopic Metazoans appeared, proliferated, and spread quickly over the Earth.
Rodinia was assembled in the late Mesoproterozoic by a series of orogenic pulses usually included in the Grenvillian Orogenic Cycle, roughly between 1300 and 1000 million years ago, the last of which the Rigolet Orogeny dated to between 1010 and 980 million years ago. Rodinia was a long-lived supercontinent centered around Laurentia and comprised the existing continental masses, which by 1000 million years ago occupied a significant part of the planet surface. It remained tectonically stable for more than 200 million years, but at about 750 million years ago a series of breakup events produced rift-related basins, including large passive margins on both sides of Laurentia.
Fragmentation terminated around 600 million years ago, when Laurentia rifted away from Amazonia, thereby opening the Iapetus Ocean. By that time, Gondwana had become almost completely assembled out of various cratonic fragments that had previously rifted away from Rodinia.
Gondwana was made up of several older continental masses, such as Amazonia, West Africa, Sahara, São Francisco Congo, Kalahari, Arabia, India, Madagascar, Australia, Antarctica and a few minor fragments, but it remained for a long time independent of Laurentia, Siberia, and China. Its amalgamation began about 650 million years ago and was complete at about 500 million years agp. About 200 million years later, the latter three continental masses were reunited with Gondwana to form the Pangea supercontinent. In the transition from Rodinia to Gondwana, two very large oceans were consumed by means of subduction-to-collision roughly between 900 and 500 million years ago, with closure of the Goiás-Pharusian Ocean in the West, and the Mozambique Ocean in the East. The former is now the West Gondwana Orogeny, and the latter is the site of the East African Orogen.
Reconstruction of the former supercontinent Gondwana showing the distribution of ancient cratonic nuclei within modern continents and their position regarding important orogenetic features (broken lines), with emphasis on the West Gondwana and East African orogens. Cordani et al. (2020).
The Gondwana supercontinent was formed as a succession of three first-order orogenic events that belong to the Brasiliano–Pan-African orogenic cycle, each occupying a specific space and time. This began with the convergence between the São Francisco-Congo and Amazonia-West Africa-Rio de la Plata cratonic masses that closed the Goiás-Pharusian Ocean and produced the West Gondwana Orogeny (650–600 million years ago). The other two orogenic episodes were responsible for the closure of the Mozambique Ocean and, consequently, the formation of the East African Orogen, which is the world’s largest Neoproterozoic to Cambrian orogenic complex. This involved a collision between Congo and India-Madagascar during the East African Orogen (600–550 million years ago), followed by collision of the rest of Gondwana and Australia-Antarctica in the Kuunga Orogeny (550–500 million years ago).
It has been argued that deeply subducted rocks, specifically the eclogites with coesite exposed in the West Gondwana Orogeny, comprise the earliest evidence of large-scale deep continental subduction on Earth. This linear belt, more than 5000 km long and including the oldest known ultra-high pressure eclogites in the world, cuts across South America and extends into present-day North-West Africa. It represents a series of sutures associated with final convergence between the conjoined Amazonian and West African cratons, on one side, and the Saharan and Congo-São Francisco cratons, on the other, close to the present site of the Transbrasiliano-Kandi mega-shear zone.
The long-term convergent plate motion and evolution of eastward subduction within the Goiás-Pharusian Ocean started at about 900 million years ago. Successive 'soft collisions' occurred between about 800 and 650 million years ago and culminated in terminal continental collision roughly between 650 and 600 million years ago, when the Pharusian-Araguaia-Paraguay belts were closed. The Goiás magmatic arc is a major area of soft collision and accretion of the Amazonian Craton margin made up essentially by Neoproterozoic granitoids with juvenile signatures that indicate they represent the roots of a series of intra-oceanic island arcs produced by consumption of oceanic lithosphere. The predominantly calc-alkaline chemistry of these magmatic rocks indicates persistent, subduction-related and active margin processes.
Graphic representation of the history of West Gondwana assembly and the formation and denudation of Earth’s oldest known Himalaya-style mega-mountains. Cordani et al. (2020).
Along the West Gondwana Orogeny, ages of ultra-high pressure coesite-bearing eclogitic rocks marking the suture zones between colliding cratons in Mali, Togo and northeast Brazil were obtained using coupled uranium-thorium-lead and rare earth element zircon analyses together with geothermal-barometric constraints. The deep continental collision timing was determined from uranium-lead sensitive high-resolution ion microprobe measurements on overgrowth rims of zircon in the eclogites, as follows: 611.3 million years ago for Mali, 608.7 million years ago for Togo and 616.0 million years ago for northeast Brazil. Within experimental error, these results indicate Ediacaran age for the ultra-high pressure metamorphism and simultaneous subduction of at least three fragments of continental margin along the West Gondwana Orogeny. Moreover, pressure-tempreature determinations indicate subduction of this continental margin to depths conducive to ultra-high pressure metamorphic conditions over a distance of at least 2500 km within the West Gondwana Orogeny, which is comparable to the extent of the present Himalayas. Such extreme metamorphism occurs along sutures due to very high temperatures and pressures and the internal rearrangement of material within subducted continental lithosphere, such as at depths of greater than 100 km, followed by rapid exhumation.
All presently known locations of ultra-high pressure terrains exposing eclogites that contain coesite or diamond and their respective peak metamorphic ages in millions of years. Most of them are Phanerozoic in age and commonly related to major orogenetic cycles (i.e., Caledonian, Variscan, Alpine-Himalayan etc.). The youngest, at 4-million-years-old, is from Papua, New Guinea. The oldest are those related to the West Gondwana Orogen in Brazil and northwest Africa, with ages greater than 600 million years. The occurrence marked 'Minas Gerais, 630 million years' is still under suspicion due to its uncertain location and lack of direct age dating of the sample. Cordani et al. (2020).
The East African Orogen, in turn, is a collage of continental cratonic nuclei, such as the Saharan, Congo, Kalahari, India, Madagascar, and Australia-Antarctica. Its northern part, the Arabian-Nubian Shield, represents an oceanic domain formed by island arc terranes with juvenile crust. South of the Arabian-Nubian Shield lies the Mozambique Belt, the main orogenic component of the East African Orogen, with a complicated tectonic history involving continent-continent collisions among several cratonic masses with continental crust that are now located in Kenya, Tanzania, Madagascar, India, Sri Lanka, Zambia, Malawi, Mozambique, and Antarctica. Along the collisional sutures of the Southern part of the East African Orogen, high mountains also resulted from crustal duplication produced by convergence and compressional orogenic deformation of continental crust.
The oldest ultra-high pressure terrains, in which eclogites with coesite or diamond and their respective peak metamorphic ages have been identified are those related to the West Gondwana Orogeny in Brazil and northwest Africa with ages greater than 600 million years. All others are Phanerozoic in age, some of them related to the Caledonian and Variscan belt in Greenland and Europe, and others related to the closure of the Paleo-Tethys and Neo-Tethys oceans to form Eurasia, when the East Gondwana sector of Pangea began to break up, disperse, and amalgamate. Particularly noteworthy are the Meso-Cenozoic ultra-high pressure terrains linked to the Alpine-Himalayan collisional chain and the very recent age, 4 million years, of the youngest known eclogites with coesite, from Papua New Guinea. Geothermal-barometric evidence indicates that coesite rocks can be exhumed from depths exceeding 100 km, which means that crustal material produced by subduction of felsic continental crust less dense than the mantle may be subject to rapid isostatic rebound and exhumation to give rise to significant topographic relief, as in the Alps and Himalayas. Hence, these arguments indicate that the West Gondwana Orogeny registers the oldest known continent-continent collision, resulting in Himalayan-style mountain-building in Earth history. The West Gondwana Orogeny mega-mountains were higher than any previous Neoproterozoic mountain belts and, therefore, a source of vast amounts of erosional sediments, part of which is preserved in the Ediacaran foreland basins and epicontinental seas of Gurma, Taoudeni, and Volta in Africa and Parecis, Paraguay, and Bambui in South America.
Furthermore, within the same Neoproterozoic time frame, at least three great Neoproterozoic glaciations took place, namely, the Sturtian (roughly 720–660 million years ago) and Marinoan (roughly 650–636 million years ago) glaciations, both considered as global in extent, or nearly so, and the more restricted Gaskiers glaciation (582 million years ago). The Snowball Earth hypothesis alleges that in each of these episodes the Earth was covered, or nearly so, by ice at least for a few million years. All continents contain evidence of this phase of Earth history, although the number of glaciations, their duration, and extent are still the subject of debate. Similarly, the importance of these drastic climatic episodes upon microscopic life is unquestioned, although not much is known about just how they provoked or affected evolutionary changes.
It has been suggested that the increased rate of sediment accumulation resulting from erosion of the high relief produced during the amalgamation of Gondwana could have influenced life radiation on Earth. This was supported other studies which argued that massive sediment supply related to Gondwana assembly would have provided abundant nutrients for Algae and Cyanobacteria, and, consequently, a major stimulus for oxygen production by photosynthetic. The subsequent increase of O₂ in the atmosphere and oceans would have been an important factor in the radiation of Late Neoproterozoic, and especially Ediacaran, life. In our view, the full implications of this innovation within the context of Rodinia breakup and Gondwana amalgamation have not still been unfathomed. However, the appearance of very high, open-air, ice-free mountains between about 620 and 570 million years ago in the aftermath of drastic Snowball Earth scenarios must have exerted a dramatic influence on climate, weathering, erosion, nutrient fluxes and carbon burial, with important consequences for the Earth system and especially as triggers for the evolution of Ediacaran life.
Uplift and denudation of Earth’s first very high Himalayan-style mountains along the Transbrasiliano-Kandi alignment began at about 615 to 610 million years ago and putatively continued for the following 40 to 50 million years during a very significant moment on Earth, and especially biosphere, history. Earlier to this event, life was pre-eminently microscopic, yet by the time it was over, macroscopic organisms, albeit many of them of enigmatic biological affinities, appeared and began to diversify en route to the crown group Metazoans that we recognise today.
Claims of the most ancient fossil evidence of life include carbon isotope signatures, microbialites, and microfossils with ages extending back to around 4 billion years ago, near the base of the known geological record. All, however, have been subject to considerable contention. Consequently, present consensus places the oldest reliable evidence of life on Earth from rocks about 3.45 billion years ago in age from the Warrawoona Group of Northwest Australia, in the form of putative stromatolites, microfossils and associated carbon and sulphur isotope data. This means that, independently of the biogenicity of older fossils, Prokaryotic life proliferated and rose to dominance in benthic and planktic environments by mid-Archean and continued that dominance, certainly in benthic settings, even after the rise of unicellular Eukaryotes (Microalgae and other Protists) in the later Palaeoproterozoic. In fact, it was these ancient Prokaryotes, specifically the Cyanobacteria, that furnished oxygen to the atmosphere via photosynthesis beginning no later than 2.7 billion years ago and were responsible for the Great Oxygenation Event in the early Palaeoproterozoic around 2.4 billion years ago. A postulate of modern biology is that stable oxygen in the atmosphere provided an environmental stimulus that favored the appearance of unicellular Eukaryotes, practically all of which are obligate aerobes that depend upon oxygen, later in the Palaeoproterozoic, by at least 1.7–1.9 billion years ago.
Consensual geological evidence for the next great innovation in biological history, multicellular but still microscopic Eukaryotes, dates from the mid-Mesoproterozoic, about 500 million years later. It consists of permineralized microscopic filamentous Algae about 1200-million-years-old, which are very similar, in fact, to modern crown-group Bangiophycean Red Algae. The origin of multicellularity certainly must have originated much earlier. Indeed, decimeter-scale organic compressions have been found in the Gaoyuzhuang Formation of North China just above putative evidence for a global oceanic oxygenation event. If their interpretation as Macroalgae is correct, macroscopic multicellular Eukaryotic Algae may have originated a little after 1600 million years ago. As such, this coincides with increased abundance and diversity of Eukaryotic Microalgae observed in other stratigraphic units in China, Siberia, and Australia.
After the appearance of Macroscopic Algae, the dominance of marine Prokaryotes, especially in benthic settings, began to falter, first, through competition for space and light and, nearly a billion years later, beginning about 600 million years ago, as a consequence of the radiation of complex sessile and vagile macroscopic multicellular Eukaryotes, including not only heterotrophic and autotrophic Metazoans, but also other diverse organisms apparently unrelated to modern crown groups, known as Vendobionts. Moreover, phylogenomic analyses have demonstrated that by 730 million years ago, in middle Neoproterozoic time, just prior to the near-global Cryogenian 'Snowball Earth' glaciations, many groups of Amoeboid heterotrophic Protists among unicellular Eukaryotes had already attained crown-group status. This is evidenced by late Tonian (800–720 million years ago) vase-shaped microfossils, representative of at least two very separate groups of Testate Amoebae, as well as by other scale-bearing Protists (from about 811 million years ago onwards). Younger agglutinated Protists interpreted as Foraminifera are reported from the mid-Cryogenian, but evidence of Metazoan body fossils older than 600 million years is limited to controversial tiny Sponges and supposedly corroborative coeval or older Sponge biomarkers.
Surprisingly large Vendobionts nearly 580 million years old effectively mark the advent of continuous geological record of macroscopic Metazoan and other Animal-like body fossils. However, two exceptionally preserved fossil biotas (Konservat-Lagerstätten) in post-glacial (i.e. post-Marinoan) formations in China provide permissive evidence of a probably older Ediacaran record. The well-known Weng’an Biota from phosphorites in the upper half of Doushantuo Formation could be as old as about 600 million year, then the Lantian biota, from stratigraphically deeper pelites of the Lantian Formation, would have to be older than this. Precise ages of these assemblages have yet to be established.
Main occurrences of macroscopic Ediacaran metazoans. The classic localities of the soft-bodied Avalon biota occur in Newfoundland and the UK. Those of the White Sea biota in northwest Russia and Norway; and those of the Nama biota, in Namibia and Australia. The localities of the older Weng’an and Lantian biotas of South China are indicated by W and L, respectively. Cordani et al. (2020).
The Weng’an Biota consists of phosphatised and silicified microfossils so well preserved three-dimensionally as to allow claims (not all without dispute) that they may represent Animal embryos in various ontogenetic stages, Bilaterian Animals, Red and Green Algae, Acritarchs, and a Ctenophore. In addition, two-dimensional carbonaceous compressions within the formation have been attributed to Macroalgae. A tuff bed overlying the fossil-bearing units has been dated to 609 million years ago, and Acritarchs in the formation are similar to immediately post-Marinoan (i.e. less than 635 million years old) forms elsewhere make the Weng’an assemblage a serious candidate for housing the oldest fossil evidence of embryonic, larval and adult Eumetazoans, which are even older than the classical soft-bodied macroscopic Avalon, White Sea, and Nama Ediacaran biotas. However, other researchers have reviewed the evidence and arguments regarding biological affinities of the Weng’an fossils and concluded that, although some might represent animals, none can yet be confidently identified as stem- or crown-group Metazoans.
Representative fossils of the Weng’an biota (A)–(F) and Lantian biota (G)–(I) of South China. These biotas are older than the middle to late Ediacaran soft-bodied Avalon, White Sea and Nama macrobiotas. (A)–(F) Scanning electron microscope images of selected fossils of the Weng’an biota preserved in three dimensions. (A)–(C) Embryo-like fossil Tianzhushania exhibiting various stages of division, from a few cells (A) to many hundreds of cells (C). (D) Spiralicellula. (E) Archaeophycus, a putative Red Alga. (F) Eocyathispongia, suggested to be a Sponge. (G)–(I) Representative fossils of the Lantian biota of South China preserved as compressions. (G) The Macroalga Flabellophyton, with a fan-shaped or conical thallus. Scale is 5 mm. (H) Lantianella, a putative Scyphozoan Cnidarian, approximately 36 mm long. (I) Xiuningella, a Scalidophoran introvert(?), approximately 18 mm long. Scale bar in (A) 265 μm, (B) 200 μm, (C) 280 μm, (D) 380 μm, (E) 255 μm, (F) 415 μm. Cordani et al. (2020).
The Lantian Biota consists of carbonaceous compression fossils preserved in place in carbonaceous pelites that were deposited below storm-wave base, yet still within the photic zone, judging from the alga-like morphologies of some of the fossils. The biota represents about 15 morphospecies exhibiting complex morphological differentiation and reaching up to several centimeters in maximum dimension. The fossiliferous member of Lantian Formation may be correlated with strata in the Doushantuo Formation older than that containing the Weng’an biota, thus making the Lantian biota the earliest known well-preserved assemblage of varied macroscopic multicellular Eukaryotes. Although many probably represent Macroalgae, few may arguably be Cnidarians or Worms. however, once again, this requires corroboration. The Lantian Formation is assuredly Ediacaran in age and possibly as old as 635 million years; however, from a conservative point of view, all that can be said is that they are most likely older than 551 million years.
The so-called Vendobionts, represented by dozens of taxa of fossilized soft-bodied, but firm macroscopic organisms, appear at about 580 million years ago and continue as the predominant elements of Ediacaran biotas worldwide till the end of the period at 541 million years ago. Three distinct, bio-stratigraphically useful assemblages are recognised, each drawing its name from its most representative area of occurrence. The oldest, the Avalon assemblage (579 to about 560 million years ago), known from Newfoundland and England, has been characterized by modular organisms built from repetitively branched ('fractal') units comprising the Rangeomorpha. Potential macroscopic sponges have also been recognised. The widespread White Sea assemblage (roughly 560 to roughly 550 million years ago) is more than three times more diverse in genera than the Avalon assemblage. It also includes diverse trace fossils, indicating increased eco-space occupation and behavioral complexity. The vagile Mollusk-like soft-bodied Kimberella, that first appears in this assemblage is considered by some to be a total-group Bilaterian (the group which includes all extant Animals except Sponges, Cnidarians, Ctenophores and Placozoans), as is Dickinsonia. The Nama assemblage is the youngest, at about 550 to 541 million years old, and dominated by soft-bodied Erniettomorpha. However, it also includes the oldest biomineralized macroscopic fossils and fossils such as Corumbella and Paraconularia which some palaeontologists consider to be crowngroup Metazoans (Scyphozoa). Holes in some biomineralised exoskeletons of the emblematic Nama fossil Cloudina have been interpreted as evidence of the predation that seems to have been fundamentally important in the subsequent radiation of biomineralising Metazoans marking the beginning of the Phanerozoic Eon and characterising the Cambrian Explosion of marine Invertebrates.
Representative taxa of middle to late Ediacaran macrobiotas. (A) Soft-bodied, frondose members of the Avalon macrobiota of Newfoundland, Canada, buried in place with holdfasts, stipes and fronds. Scale bar is 4 cm. (B) Ediacaran Arborea with associated trace fossil; Flinders Ranges, South Australia. Smooth central part is 12.5 cm long. (C) Ediacaran crown-group Metazoan (Scyphozoan Cnidarian) Corumbella, an organic-walled tubular fossil from Corumbá, Brazil (total length of larger individual, 20 mm). (D) Probable Bilaterian Eumetazoan Dickinsonia from the Flinders Range, Australia (length, 14 mm). (E) Plausible stem-group Ctenophoran Eoandromeda from South China (diameter about 14 mm). Cordani et al. (2020).
In light of the incompleteness regarding the early record of macroscopic fossils, evolutionary biologists have developed models, molecular clocks, that attempt to establish the divergence timing of evolutionary lineages in the Metazoa, from stem groups to crown groups, based on the comparative analysis of amino acid sequences in proteins common to the biological groups under consideration. Molecular clocks depend upon estimates of molecular substitution rates, detection and correction of heterogeneities in these rates, choice of calibration points in the fossil record, choice of calibration strategy, and proper consideration of uncertainties in these parameters. They date from the 1980s and figure importantly in the current understanding of early Metazoan evolution in the Neoproterozoic. The crucial point in the configuration is the choice of calibration points from the geological record of body fossils, trace fossils, and biomarkers (geologically stable organic compounds diagenetically derived solely from known biological precursors) for the nodes that mark major phylogenomic divergences, such as the appearance of the Eumetazoa, Bilateria, Deuterostomia/
Protostomia, and so on.
Protostomia, and so on.
Molecular clock data for main events in the early Animal evolution together with relevant fossil evidence, Neoproterozoic oxygenation history, and record of Neoproterozoic glaciations. Cordani et al. (2020).
Much of the molecular genetic toolkit required for Animal development originated deeply in eukaryote evolutionary history. Hence, the appearance of complex multicellular Animals in the geologic record may have been limited or triggered by such factors as biosphere oxygenation, scarcity of trace metal micronutrients, pulse of continental weathering in nutrient flows to the oceans, and environmental restrictions imposed by extreme Cryogenian and Ediacaran icehouse scenarios, among others. Uncertainties regarding the nature of ancient fossils together with violations of the molecular clock models are such that it is not possible to accurately pinpoint early divergence events in metazoan evolution from the available fossil data and molecular clock models.
However, critical reexamination of the fossil evidence reveals a much less bleak picture for the early Metazoan evolution. The most widely cited evidence for Metazoans in rocks predating accepted Ediacaran body fossils is the biomarker 24-isopropylcholesterol, attributed to Demosponges, in rocks about 635-million-years-old from Oman and Cryogenian Sponge bioclasts in the Trezona Formation of Australia, yet they, too, are subject to debate. Nevertheless, these and other key findings provide a pattern of fossil evidence that is consistent with the molecular clock model. Together, they suggest that the Metazoa originated no later than 635 million years ago; and the divergence and initial diversification of Bilaterians likely occurred prior to about 560 million years ago. A closer fit between the fossil record and molecular clocks is hampered by problems of preservation and identification of biological affinities of these and older possible animal fossils, given that they must include stem-group organisms with unfamiliar and commonly poorly preserved character sets. All would agree, however, that the major diversification of the Metazoa that sets the stage for Phanerozoic animal evolution was indeed an Ediacaran phenomenon.
At the end of the 'boring billion' years in the history of life (between about 1800 and 800 million years ago), a succession of Earth-changing events involving crustal, atmospheric and hydrosphere dynamics and chemistry impinged itself upon life systems. This new complex and dynamic environment effectively transformed the long-reigning microbe-dominated ecosystems into a rapidly evolving macroscopic biosphere capable of ever more complex interactions and presenting greater physical presence within the Earth system.
Primary among the events affecting the biosphere was the oxygen increase in the atmosphere and oceans beginning in mid-Neoproterozoic and perhaps reaching 40% of the present atmospheric level by 550 million years ago. It is now a consensus that physical and chemical processes of carbon recycling, acting during the latter half of the Neoproterozoic, beginning about 800 million years ago, were largely responsible for elevating the oxygen level in the atmosphere and oceans to evolutionarily significant thresholds over the remainder of the terminal Neoproterozoic. Whether this phenomenon, dubbed the Neoproterozoic Oxygenation Event, was episodic or continuous is still under debate, but most researchers accept it as a fundamental factor in the expansion of multicellular eukaryotes. Indeed, by the very early Palaeozoic, increased oxygen availability allowed the introduction of complex, active and multicellular macroscopic Eukaryotes into the biosphere. This is a level of complexity that it sustains to the present day.
Of the various reasoning lines offered as evidence for this event, the most important in the present context are those that demonstrate a continuous increase in strontium⁸⁷/strontium⁸⁶ values in seawater and generally high relative Carbon¹³ values in carbonates in the Neoproterozoic after about 800 million years ago. These tendencies may be explained, respectively, by the increasing rates of continental weathering as responsible for radiogenic strontium input into the oceans and by relatively high sustained rates of organic matter burial (derived from mostly unicellular Prokaryotic and Eukaryotic micro-phytoplankton). The sequestration of organic carbon by burial, together with the withdrawal of atmospheric CO₂ by continental weathering, would have: liberated O₂ that otherwise would have been consumed in the oxidation of the sequestered organic matter; increased the flow of nutrients to photoautrophs in the oceans (favoring high levels of primary production); and reduced the participation of CO₂ in the greenhouse effect, leading to a cooler atmosphere.
Just as the earlier Paleoproterozoic Great Oxidation Event between 2.4 and 2.0 billion years ago, favored the emergence and radiation of the unicellular Eukaryotes, not only capable of O₂-powered metabolism but, in fact, also dependent upon oxygen, the Neoproterozoic Oxygenation Event, more than a billion years later, elevated oxygen levels in the atmosphere and oceans sufficiently to allow heterotrophic Eukaryotes to exploit pluricellular body plans on a macroscopic scale. This was something that putative decimetric Eukaryotic photoautotrophs (Macroalgae) from China had apparently achieved, under less oxygenic conditions, by about 1500 million years ago. They, however, could produce oxygen to their own system through photosynthesis; and their increased size was likely advantageous in terms of photosynthetic area and competition for space on the sea floor. The soft-bodied macroscopic forms that appeared in the Ediacaran much later are interpreted as heterotrophic pluricellular organisms, dependent upon higher levels of ambient oxygen. Initially, these included the enigmatic Vendobionts as well as difficult-to-classify stemgroup Metazoans, but by 560 million years ago Eumetazoans were also included. How these animals lived has not been always clear, and some may even have been sessile osmotrophs, but several Bilaterians are now recognized, such as Kimberella and Dickinsonia. Clearly, from what we know of how Eukaryotes function and reproduce, the Neoproterozoic Oxygenation Event afforded unprecedented opportunities for Eukaryote evolution at macroscopic pluricellular levels and a new level of ecospace opportunities to exploit novel body plans, physiologies, and growth and feeding strategies.
The appearance of multicellular Eukaryotes, even prior to attaining macroscopic size, could have also influenced the atmosphere oxygenation and ventilation of the oceans. For example, the advent of fecal pellets and greater body size in Eukaryotic organisms would have increased sinking rates of organic carbon, thus shortening both residence time in the water column and exposure to microbial decomposers. This would have facilitated carbon incorporation (sequestration) within sediments, thereby favoring oxygen accumulation. By the same token, Neoproterozoic colonisation of the surface of the continents by microorganisms could have promoted CO₂ drawdown from the atmosphere and more efficient weathering of silicates and micronutrient delivery to the oceans. Finally, in the latter half of the Ediacaran, the appearance of infaunal organisms capable of intensely utilising the substrate prompted nothing less than a 'revolution' in redox conditions in near-surface sediments and ecospace exploitation.
Three extreme palaeoclimatic changes, the Sturtian, Marinoan and Gaskiers glacial events, the severest ever registered in the geologic record, also occurred concomitantly with the breakup of Rodinia, Gondwana amalgamation, and early Metazoan evolution. The Snowball Earth hypothesis put forward to explain these events asserts that the Earth was totally covered, or nearly so, by ice, several times between 720 and 580 million years ago. Rocks on all continents record one or more of these glaciations, yet the precise age and temporal equivalence of glacial events have been difficult to be established because their most emblematic sedimentary signature, diamictites, cannot be directly dated, unless intercalated by contemporaneous beds of volcanic materials, which is rare. Much more commonly, maximum depositional ages have been deduced from uranium-lead ages of the youngest detrital zircons within the diamictites, whereas minimum ages have been furnished by lead-lead age-determinations for immediately post-glacial cap carbonates, when present.
Current evidence indicates that the Sturtian event started at about 717 million years ago. The best available ages for Sturtian rocks have been obtained in ash beds intercalated within diamictites from Oman, which yielded uranium-lead zircon ages of 723 million years and 711.5 million years. Ash beds of the Mount Harper Group from Canada, dated by the chemical abrasion, isotope dilution, thermal ionisation mass spectrometer method, yielded a precise age of 716.33 million years. Rhenium-osmium ages of about 659 million years, from the youngest cap carbonates associated with this glaciation suggest that Sturtian glacial events took place over nearly 60 million years, from about 717 to 660 million years ago. Some authors argue that Sturtian glaciation encompasses several shorter glacial episodes. In Brazil, a lead-lead isochron age of 740 million years, obtained on cap carbonates from Sete Lagoas Formation is consistent with their correlation with the Sturtian event.
The best age for the Marinoan glacial event is 636 million years, as indicated by the uranium-lead age of 635.5 million years from zircons from ash beds interlayered in diamictites of the Ghaub Formation, Namibia. A nearly identical age of 636.3 million years was obtained on zircon grains from an ash bed in Nantuo Formation, China. Post-Marinoan cap carbonates have yielded very similar ages, for example a chemical abrasion, isotope dilution, thermal ionisation mass spectrometer age of 635.2 million year was obtained from zircons from an ash bed intercalated within cap carbonates overlying the Nantuo tillite on Yangtze Platform, China. Lead-lead isochrons on post-Marinoan cap carbonates at the base of the Araras Group in Brazil yielded ages of about 633 million years.
Finally, diamictites representing the much more restricted mid-Ediacaran Gaskiers Glaciation in Eastern Canada contain many ash beds in the type section that have been precisely dated within the interval from 581 to 579 millione yeats ago. It is noteworthy that the same region is the site of the classical localities of the soft-bodied Avalon fossil macrobiota.
The extreme climates associated with the initiation, maintenance, and termination of these worldwide glacial events must have had a significant effect on the biosphere, but not much is known about how they provoked or affected subsequent evolutionary changes, either as a bottleneck and/or as a trigger for adaptive innovations. As evident from the fossil record, microscopic Prokaryotic and Eukaryotic lineages, as well as putative macroscopic Algae and possibly Sponges, obviously survived the snowball scenarios of the Cryogenian (720 to 635 million years ago). A pronounced spike in marine phosphorous, an essential nutrient, during this period suggests that massive amounts of nutrients (including phosphorus) may have been delivered to the seas as a product of glacial erosion during deglaciation following individual glacial episodes at this time. Together with concomitant eustatic sea level rise, this certainly must have favored colonisation of benthic and planktonic habitats in the vast shallow seas that spread over low-lying, glacially planed continental margins. Their resultant increase in primary production, oxygenation, and habitat diversification undoubtedly transformed the biosphere and presented new evolutionary options. As a possible example of this, we see relatively complex and diverse Eukaryotic biotas for the first time at Lantian and Weng’an in the interval between the Marinoan and Gaskiers glaciations (635 to 580 million years ago). Moreover, typical, large soft-bodied elements of the Ediacaran macrobiota, the oldest known Vendobionts, date from just after the Gaskiers event at 579 million years ago in rocks from the classical region for that event. Shortly thereafter, Bilaterians, as inferred from trace fossils, appeared between 570 and 566 million years ago and bona fide Bilaterian Metazoan fossils showed up at 558 million years ago.
How and when the Earth moved beyond the ‘boring billion’, in phase with extreme glaciations, increased oxygenation, and ultimately the emergence of animals, remain among the crucial questions in the history of Earth-life co-evolution. In Cordani et al.'s view, 'How' remains an open question currently within the domains of molecular and developmental biology, organic chemistry, and geochemistry, but 'When', on the other hand, is an issue that they address using biological, climatic, and tectonic lines of evidence. Although recognizing the relevance of the oxygen rise in the oceans and climatic extremes in this discussion, they argue specifically that the main trigger for the emergence (and maintenance) of modern ecosystems dominated by macroscopic organisms was tectonic, related to the first appearance of high surface relief involving Himalayantype mega-mountains within the West Gondwana Orogeny.
The Transbrasiliano-Kandi lineament, stretching 5000 km from Brazil to Africa and marking the site of the West Gondwana Orogeny, records the oldest evidence of a continent-continent collision capable of producing very high mountain chains. Such mountains were probably comparable to the present Himalayas and may have been at least as long-lived. The central Tibetan plateau, for example, has maintained elevations greater than 5000 m, with many peaks surpassing 8000 m, since the Eocene. By analogy, then, mega-mountains in Gondwana may also have sustained similar high relief for at least 40 million years after the original collision responsible for their formation..
Therefore, if the West Gondwana Orogeny originated near 610 million years ago in association with high Himalayan-like mountains, it could have persisted at least until 580 million years ago. During this time and through the period of erosion until 540 million years ago, as estimated from the age of several post-collisional granitoids, they have most certainly served as a major source of sediments and nutrients for contemporaneous seas. Just as the Tibetan plateau exerts a profound influence on modern climate, sedimentation, tectonics and biology, the same was probably true for the West Gondwana Orogeny during the Ediacaran as well. The chronology of these events corresponds closely with important events in the evolution of macroscopic Eukaryotes.
Several places in the West Gondwana Orogeny expose low or ultra-high pressure metamorphic rocks typical of the diagnostic prototypes that characterise deep continental subduction, such as observed in modern plate tectonics. Ultra-high pressure rocks in Mali dated at about 620 million years ago thus comprise the earliest evidence not only of large-scale deep-continental subduction, but consequently also of Himalayan-type mountains. Hence, the uplift and subsequent erosion of these mountains in the Late Ediacaran provided massive amounts of sediments and nutrients throughout the most important phase in the emergence and early diversification of megascopic Metazoans on Earth.
Thus, Cordani et al. consider that the West Gondwana Orogeny, harboring as it does the oldest evidence of Himalayan-style relief resulting from continent–continent collisions, is one of the features that changed the way evolution proceeded on Earth in the Ediacaran. The distribution and ages of all known localities of known ultra-high pressure metamorphic rocks shows that none of them is older than those of the West Gondwana Orogeny. Ultra-high pressure metamorphic rocks indicating deep subduction occur within collisional belts associated with globally important tectonic cycles throughout the Phanerozoic, as in the Caledonian, Hercynian, and Alpine cycles. The high mountain ranges they represent are, in fact, a characteristic of Phanerozoic times. The implication is that they have furnished nutrients to the seas through weathering and erosion sufficient enough to sustain intense Eukaryotic evolution and permit an increasingly complex exploitation of ecospace within the biosphere throughout the Phanerozoic.
Cordani et al. expect that similar work on the East African Orogen will add further support for the relevance of Gondwana amalgamation as an important influence upon the acceleration of biological evolution at the Neoproterozoic end. For instance, they expect that investigation of high-pressure metamorphic rocks already known in Tanzania and Mozambique and related to the collisions of India, Madagascar and Australia-Antarctica with the Central African Block, may well confirm the suspicion that mega-mountains were likewise formed in the Southern part of the East African Orogen. It will also be important to test the hypothesis presented by Cordani et al. by means of source-to-sink investigations of delivery rates of nutrients and their bioavailability in basins fed by the erosion of mega-mountains. Regarding the West Gondwana Orogeny, answers to these questions may be found in basins related to the dissection of high mountains associated, for example, with the Voltaian Basin in Ghana or the Parecis Basin in Brazil. As in all of Geology, time will tell.
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