Among the strange early animals of the latest Precambrian, known as the Ediacaran fauna, is the slug-like Kimberella. Unlike most of its cohort, which are impressions in sediment or trace fossils, Kimberella is a body fossil in which can be seen signs of a front and back, i.e. mouth and anus (See also: A lowly worm from the Ediacaran?). In that respect they are the same as us: bilaterians both. Indeed, Kimberella may be one of the oldest of our broad kind that we will ever be able to see. Rare examples have fans of grooves radiating from their ‘front’. It may have grated its food, a bit like a slug does, but drew it in to its mouth. Some enthusiasts have likened the little beasty to a JCB digger, able to rotate and rake stuff into its mouth. In that case, Kimberella would have moved ‘backwards’ while feeding. If it can be likened to any modern animals, it may be a simple mollusc.
Other Ediacaran animals show no such mouth-gut-anus symmetry. Some have tops and bases, but most show no symmetry at all, being flaccid bag-like creatures. Palaeontologists provisionally suggest that they are primitive sponges, ctenophores, placozoans and cnidarians. Such animals excrete through pores on their surfaces and draw food in either through a simple mouth or their skins. The early bilaterians probably ‘grazed’ on bacterial or algal mats, but until now that has been conjectural. Ilya Bobrovskiy of the Australian National University and colleagues from Russia and Australia have managed to extract and analyse biomarker chemicals contained in well-preserved specimens of three Ediacaran animals from strata on the White Sea coast of Russia (Bobrovskiy, I. et al. 2022. Guts, gut contents, and feeding strategies of Ediacaran animals. Current Biology, v. 32, ; DOI: 10.1016/j.cub.2022.10.051). Biomarkers are molecules, such as fatty acids, phospholipids, triglycerides, hopanes and steranes, that definitively indicate metabolic processes of once living organisms, sometimes referred to as ‘molecular fossils’. Their varying proportions relative to one another are key to recognising the presence of different groups of organisms.
Specifically, hopane molecules are the best indicators of the former metabolism of bacteria whereas steranes (based on linked chains of carbon atoms bonded in rings) are typical products of degradation of sterols in eukaryotes. One sterane group involving 27 carbon atoms (C27 steranes) are typically formed when and animal dies and decays. C28 and C29 steranes likely form when algae decay, as when they are digested in the gut of a herbivore. Specimens of one of the Ediacaran animals analysed by the team – Dickinsonia – contained far more C27 steranes than C28 and C29, a sign of biomarkers associated with its decay. It probably absorbed food, weirdly, through its skin. Kimberella and a worm-like animal – Calyptrina – had sterane proportions which suggested that they digested algae or bacteria in a gut, as befits bilaterians. Simple as they may appear, these are among the earliest ancestors of modern animals, including us: of course!
Every organism that you can easily see is a eukaryote, the vast majority of which depend on the availability of oxygen molecules. The range of genetic variation in a wide variety of eukaryotes suggests, using a molecular ‘clock’, that the first of them arose between 2000 to 1000 Ma ago. It possibly originated as a symbiotic assemblage of earlier prokaryote cells ‘bagged-up’ within a single cell wall: Lynn Margulis’s hypothesis of endosymbiosis. It had to have happened after the Great Oxygenation Event (GOE 2.4 to 2.2 Ga), before which free oxygen was present in the seas and atmosphere only at vanishingly small concentrations. Various single-celled fossil possibilities have been suggested to be the oldest members of the Eukarya but are not especially prepossessing, except for one bizarre assemblage in Gabon. The first inescapable sign that eukaryotes were around is the appearance of distinctive organic biomarkers in sediments about 720 Ma old. The Neoproterozoic is famous for its Snowball Earth episodes and the associated multiplicity of large though primitive animals during the Ediacaran Period (see: The rise of the eukaryotes; December 2017).
The records of carbon- and sulfur isotopes in Neo- and Mesoproterozoic sedimentary rocks are more or less flat lines after a mighty hiccup in the carbon and sulfur cycles that followed the GOE and the earliest recorded major glaciation of the Earth. The time between 2.0 and 1.0 Ga has been dubbed ‘the Boring Billion’. At about 900 Ma, both records run riot. Sulfur isotopes in sediments reveal the variations of sulfides and sulfates on the seafloor, which signify reducing and oxidising conditions respectively. The δ13C record charts the burial of organic carbon and its release from marine sediments related to reducing and oxidising conditions in deep water. There were four major ‘excursions’ of δ13C during the Neoproterozoic, which became increasingly extreme. From constant anoxic, reducing conditions throughout the Boring Billion the Late Neoproterozoic ocean-floor experienced repeated cycles of low and high oxygenation reflected in sulfide and sulfate precipitation and by fluctuations in trace elements whose precipitation depends on redox conditions. By the end of the Cambrian, when marine animals were burgeoning, deep-water oxic-anoxic cycles had been smoothed out, though throughout the Phanerozoic eon anoxic events crop up from time to time.
The Late Neoproterozoic redox cycles suggest that oxygen levels in the oceans may have fluctuated too. But there are few reliable proxies for free oxygen. Until recently, individual proxies could only suggest broad, stepwise changes in the availability of oxygen: around 0.1% of modern abundance after the GOE until about 800 Ma; a steady rise to about 10% during the Late Neoproterozoic; a sharp rise to an average of roughly 80% at during the Silurian attributed to increased photosynthesis by land plants. But over the last few decades geochemists have devised a new approach based on variations on carbon and sulfur isotope data from which powerful software modelling can make plausible inferences about varying oxygen levels. Results from the latest version have just been published (Krause, A.J. et al. 2022. Extreme variability in atmospheric oxygen levels in the late Precambrian. Science Advances, v. 8, article 8191; DOI: 10.1126/sciadv.abm8191).
Alexander Krause of Leeds University, UK, and colleagues from University College London, the University of Exeter, UK and the Univerisité Claude Bernard, Lyon, France show that atmospheric oxygen oscillated between ~1 and 50 % of modern levels during the critical 740 to 540 Ma period for the origin and initial diversification of animals. Each major glaciation was associated with a rapid decline, whereas oxygen levels rebounded during the largely ice-free episodes. By the end of the Cambrian Period (485 Ma), by which time the majority of animal phyla had emerged, there appear to have been six such extreme cycles.
Entirely dependent on oxygen for their metabolism, the early animals faced periodic life-threatening stresses. In terms of oxygen availability the fluctuations are almost two orders of magnitude greater than those that animal life faced through most of the Phanerozoic. Able to thrive and diversify during the peaks, most animals of those times faced annihilation as O2 levels plummeted. These would have been periods when natural selection was at its most ruthless in the history of metazoan life on Earth. Its survival repeatedly faced termination, later mass extinctions being only partial threats. Each of those Phanerozoic events was followed by massive diversification and re-occupation of abandoned and new ecological niches. So too those Neoproterozoic organism that survived each massive environmental threat may have undergone adaptive radiation involving extreme changes in their form and function. The Ediacaran fauna was one that teemed on the sea floor, but with oxygen able to seep into the subsurface other faunas may have been evolving there exploiting dead organic matter. The only signs of that wholly new ecosystem are the burrows that first appear in the earliest Cambrian rocks. Evolution there would have ben rife but only expressed by those phyla that left it during the Cambrian Explosion.
There is a clear, empirical link between redox shifts and very large-scale glacial and deglaciation events. Seeking a cause for the dramatic cycles of climate, oxygen and life is not easy. The main drivers of the greenhouse effect CO2 and methane had to have been involved, i.e. the global carbon cycle. But what triggered the instability after the ‘Boring Billion’? The modelled oxygen record first shows a sudden rise to above 10% of modern levels at about 900 Ma, with a short-lived tenfold decline at 800 Ma. Could the onset have had something to do with a hidden major development in the biosphere: extinction of prokaryote methane generators; explosion of reef-building and oxygen-generating stromatolites? How about a tectonic driver, such as the break-up of the Rodinia supercontinent? Then there are large extraterrestrial events … Maybe the details provided by Krause et al. will spur others to imaginative solutions. See also: How fluctuating oxygen levels may have accelerated animal evolution. Science Daily, 14 October 2022
Humans are more or less symmetrical, our left and right sides closely resembling each other. That is not so comprehensive for our innards, except for testes and ovaries, kidneys, lungs, arteries and veins, lymph and nervous systems. We have front- and rear ends, top and bottom, input and output orifices. All that we share with almost all other animals from mammals to worms, particularly at the earliest, embryonic stage of development. We are bilaterians, whereas sponges, ctenophores, placozoans and cnidarians are not – having either no symmetry at all, or just a bottom and a top – and are in a minority. Fossil collections from Cambrian times also reveal bilaterians in the majority, at least insofar as preservation allows us to tell. Before 541 Ma ago, in the Precambrian, there are few signs of such symmetry and faunas are dominated by the flaccid, bag like creatures that form much of the Ediacaran Fauna, although there are traces of creatures that could move and graze, and had a rudimentary sense of direction (see: Burrowers: knowing front from back, July 2012 and Something large moved 2 billion years ago). Unsurprisingly, palaeobiologists would like to know when ‘our lot’ arose. One route is via comparative genetics among living animals, using DNA differences and the ‘molecular clock’ approach to estimate the age of evolutionary separation between ‘us’ and ‘them’. But the spread of estimated ages is so broad as to render them almost meaningless. And the better constrained ages of very old trace fossils rely on accepting an assumption that they were, indeed, formed by bilaterians. Yet ingenuity may have revealed an actual early bilaterian from such traces.
Palaeobiologists from the US and Australia have scoured the famous Ediacara Hills of South Australia for traces of burrowing and signs of the animal that did it (Evans, S.D. et al. 2020. Discovery of the oldest bilaterian from the Ediacaran of South Australia. Proceedings of the National Academy of Sciences, v. 117, online; DOI: 10.1073/pnas.2001045117). One Ediacaran trace fossil, known as Helminthoidichnites is preserved as horizontal trails on the tops and bottoms of thin, discontinuous sand bodies. Luckily, these are sometimes accompanied by elongate ovoids, like large grains of rice. From numerous laser scans of these suspected burrowers, and the traces that they left the authors have reconstructed them as stubby, possibly segmented, worm-like animals that they have called Ikaria wariootia, which may have grazed on algal mats. This name is derived from the local Adnyamathanha people’s word (Ikara or ‘meeting place’) for the locality, a prominent landmark, near Warioota Creek. The age of the sedimentary sequence is between 551 to 560 Ma, and perhaps a little earlier. They could be the earliest-known bilaterians, but the sandy nature of the rocks in which they occur precludes preservation of the necessary detail to be absolutely sure: that would require silt- or. clay-sized granularity
It is easy to think that firm evidence for past glaciations lies in sedimentary strata that contain an unusually wide range of grain size, a jumble of different rock types – including some from far-off outcrops – and a dominance of angular fragments: similar to the boulder clay or till on which modern glaciers sit. In fact such evidence, in the absence of other signs, could have formed by a variety of other means. To main a semblance of hesitancy, rocks of that kind are now generally referred to as diamictites in the absence of other evidence that ice masses were involved in their deposition. Among the best is the discovery that diamictites rest on a surface that has been scored by the passage of rock-armoured ice – a striated pavement and, best of all, that the diamictites contain fragments that bear flat surfaces that are also scratched. The Carboniferous to Permian glaciation of the southern continents and India that helped Alfred Wegener to reconstruct the Pangaea supercontinent was proved by the abundant presence of striated pavements. Indeed, it was the striations themselves that helped clinch his revolutionising concept. On the reconstruction they formed a clear radiating pattern away from what was later to be shown by palaeomagnetic data to be the South Pole of those times.
The multiple glacial epochs of the Precambrian that extended to the Equator during Snowball Earth conditions were identified from diamictites that are globally, roughly coeval, along with other evidence for frigid climates. Some of them contain dropstones that puncture the bedding as a result of having fallen through water, which reinforces a glacial origin. However, Archaean and Neoproterozoic striated pavements are almost vanishingly rare. Most of those that have been found are on a scale of only a few square metres. Diamictites have been reported from the latest Neoproterozoic Ediacaran Period, but are thin and not found in all sequences of that age. They are thought to indicate sudden climate changes linked to the hesitant rise of animal life in the run-up to the Cambrian Explosion. One occurrence, for which palaeomagnetic date suggest tropical latitude, is near Pingdingshan in central China above a local unconformity that is exposed on a series of small plateaus (Le Heron, D.P. and 9 others 2019. Bird’s-eye view of an Ediacaran subglacial landscape. Geology, v. 47, p. 705-709; DOI: 10.1130/G46285.1). To get a synoptic view the authors deployed a camera-carrying drone. The images show an irregular surface rather than one that is flat. It is littered with striations and other sub-glacial structures, such as faceting and fluting, together with other features that indicate plastic deformation of the underling sandstone. The structures suggest basal ice abrasion in the presence of subglacial melt water, beneath a southward flowing ice sheet
The base of the Cambrian has long been defined as the level where abundant shelly fossils and most phyla first occur in the stratigraphic record. That increase in diversity led to the nickname ‘Cambrian Explosion’, despite the fact that sheer numbers and diversity of lesser taxa took a long time to rise to ‘revolutionary’ levels. Yet a great deal of animal evolution was going on during the preceding Proterozoic Era that was revealed once palaeobiological research blossomed in rocks of that age range. Today, the earliest occurrences, or at least hints, of quite a few phyla can be traced to the last 100 Ma of the Precambrian. Clearly, the Cambrian Explosion needs a fresh look now that so many data are in. Any palaeontologist would benefit from reading a Perspective article in the latest issue of Nature Ecology & Evolution (Wood, R. and 8 others 2019. Integrated records of environmental change and evolution challenge the Cambrian Explosion. Nature Ecology & Evolution, v. 3, online publication; DOI: 10.1038/s41559-019-0821-6)
Rachel Wood of Edinburgh University and co-authors working elsewhere in Britain, Canada, Japan and Finland sift the growing wealth of fossil and trace-fossil evidence that predate the start of the Cambrian. They also consider the geochemical events that stand out in the Ediacaran Period that succeeds the Snowball Earth events of the Cryogenian. Their account recognises that the geochemical changes – principally a series of carbon-isotope (δ13C) excursions – may have resulted from tectonic changes. The carbon-isotope data mark a series of short-lived penetrations of oxygen-rich conditions deep into the ocean water column and longer periods of oxygen-starved deep water. Such perturbations in oceanic redox conditions ‘speed-up’ thorough the late-Ediacaran into the Cambrian: a profound and protracted transition from the Neoproterozoic world to that of the Phanerozoic. Over the same time span there is a ‘progressive addition of biological novelty’ in the form and function of the evolving biota, so that each successive assemblage builds on the earlier advances.
The fossil evidence suggests that the earliest Ediacaran fauna was metazoan but with no sign of bilaterian affinities (i.e. having ‘heads’ and ‘tails’). The rise of bilaterians of which most animal phyla are members occupied the later Ediacaran , with the first evidence of locomotion – and almost by definition animals with ‘fore’ and ‘aft’ – being around 560 Ma. Each discrete shift from more to less oxic conditions in the oceans seems to have knocked-back animal life, the reverse being accompanied by diversification of survivors. Oxygenation at the very start of the Cambrian marked the beginnings of a diversification clearly manifested by animals capable of biomineralisation and the secretion of hard parts with clear patterns. Such ‘shelly faunas’ are present in the latest Ediacaran sediments but with a multiplicity of seemingly arbitrary forms, although trace fossils suggest soft-bodied animals did have definite morphological pattern.
Adding yet more information to early metazoan history is the recently discovered Cambrian Qingjiang lagerstätte of Hubei Province in southern China dated at 518 Ma; similar in its exquisite preservation to the Burgess (508 Ma) and Chengjiang (518 Ma) biotas (Fu, D. and 14 others 2019. The Qingjiang biota—A Burgess Shale-type fossil Lagerstätte from the early Cambrian of South China. Science, v. 363, p. 1338-1342; DOI: 10.1126/science.aau8800). The two previously discovered Cambrian lagerstättes are notable for their very diverse arthropod and sponge faunas. That at Qingjiang adds an abundance of cnidarians, jellyfish, sea anemones, corals and comb jellies, rare in the other two biotas, plus kinorhynchs or mud dragons – moulting invertebrates known only from Cambrian and modern sediments. The fossils at Qingjiang include only about 8% of the taxa of the same age found at Chengjiang, suggesting different environments
The idea of a sudden, discrete explosive event in the history of life, which coincided with the start of the Cambrian, now seems difficult to support. This should not damage the status of 541 Ma as the start of the Phanerozoic because stratigraphy basically gives form to the passage of time and has done since its emergence in the 19th century, so keeping the names of the divisions is essential to continuity.
To set against five brief episodes of mass extinction – some would count the present as being the beginning of a sixth – is one short period when animals with hard parts appeared for the first time roughly simultaneously across the Earth. Not only was the Cambrian Explosion sudden and pervasive but almost all phyla, the basic morphological divisions of multicellular life, adopted inner or outer skeletons that could survive as fossils. Such an all-pervading evolutionary step has never been repeated, although there have been many bursts in living diversity. Apart from the origin of life and the emergence of its sexual model, the eukaryotes, nothing could be more important in palaeobiology than the events across the Cambrian-Precambrian boundary.
This eminent event has been marked by most of the latest issue of the journal Gondwana Research (volume 25, Issue 3 for April 2014)in a 20-paper series called Beyond the Cambrian Explosion: from galaxy to genome (summarized by Isozaki, Y., Degan, S.., aruyama,, S.. & Santosh, M. 2014. Beyond the Cambrian Explosion: from galaxy to genome.Gondwana Research, v. 25, p. 881-883). Of course, these phenomenal events have been at issue since the 19th century when the division of geological time began to be based on the appearance and vanishing of well preserved and easily distinguished fossils in the stratigraphic column. On this basis roughly the last ninth of the Earth’s history was split on palaeontological grounds into the 3 Eras, 11 Periods, and a great many of the briefer Epochs and Ages that constitute the Phanerozoic. Time that preceded the Cambrian explosion was for a long while somewhat murky mainly because of a lack of means of subdivision and the greater structural and metamorphic damage that had been done to the rocks that had accumulated over 4 billion years since the planet accreted. Detail emerged slowly by increasingly concerted study of the Precambrian, helped since the 1930s by the ability to assign numerical ages to rocks. Signs of life in sediments that had originally been termed the Azoic (Greek for ‘without life’) gradually turned up as far back as 3.5 Ga, but much attention focused on the 400 Ma immediately preceding the start of the Cambrian period once abundant trace fossils had been found in the Ediacaran Hills of South Australia that had been preceded by repeated worldwide glacial epochs. The Ediacaran and Cryogenian Periods (635-541 and 850-635 Ma respectively) of the Neoproterozoic figure prominently in 9 of the papers to investigate or review the ‘back story’ from which the crucial event in the history of life emerged. Six have a mainly Cambrian focus on newly discovered fossils, especially from a sedimentary sequence in southern China that preserves delicate fossils in great detail: the Chengjian Lagerstätte. Others cover geochemical evidence for changes in marine conditions from the Cryogenian to Cambrian and reviews of theories for what triggered the great faunal change.
Since the hard parts that allow fossils to linger are based on calcium-rich compounds, mainly carbonates and phosphates that bind the organic materials in bones and shells, it is important to check for some change in the Ca content of ocean water over the time covered by the discourse. In fact there are signs from Ca-isotopes in carbonates that this did change. A team of Japanese and Chinese geochemists drilled through an almost unbroken sequence of Ediacaran to Lower Cambrian sediments near the Three Gorges Dam across the Yangtse River and analysed for 44Ca and 42Ca (Sawaki, Y. et al. 2014. The anomalous Ca cycle in the Ediacaran ocean: Evidence from Ca isotopes preserved in carbonates in the Three Gorges area, South China.Gondwana Research, v. 25, p. 1070-1089) calibrated to time by U-Pb dating of volcanic ash layers in the sequence (Okada, Y. et al. 2014. New chronological constraints for Cryogenian to Cambrian rocks in the Three Gorges, Weng’an and Chengjiang areas, South China. Gondwana Research, v. 25, p. 1027-1044). They found that there were significant changes in the ratio between the two isotopes. The isotopic ratio underwent a rapid decrease, an equally abrupt increase then a decrease around the start of the Cambrian, which coincided with a major upward ‘spike’ and then a broad increase in the 87Sr/86Sr isotope ratio in the Lower Cambrian. The authors ascribe this to an increasing Ca ion concentration in sea water through the Ediacaran and a major perturbation just before the Cambrian Explosion, which happens to coincide with Sr-isotope evidence for a major influx of isotopically old material derived from erosion of the continental crust. As discussed in Origin of the arms race (May 2012) perhaps the appearance of animals’ hard parts did indeed result from initial secretions of calcium compounds outside cells to protect them from excess calcium’s toxic effects and were then commandeered for protective armour or offensive tools of predation.
Is there is a link between the Cambrian Explosion and the preceding Snowball Earth episodes of the Cryogenian with their associated roller coaster excursions in carbon isotopes? Xingliang Zhang and colleagues at Northwest University in Xian, China (Zhang, X. et al. 2014. Triggers for the Cambrian explosion: Hypotheses and problems. Gondwana Research, v. 25, p. 896-909) propose that fluctuating Cryogenian environmental conditions conspiring with massive nutrient influxes to the oceans and boosts in oxygenation of sea water through the Ediacaran set the scene for early Cambrian biological events. The nutrient boost may have been through increased transfer o f water from mantle to the surface linked to the start of subduction of wet lithosphere and expulsion of fluids from it as a result of the geotherm cooling through a threshold around 600 Ma (Maruyama, S. et al. 2014. Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion.Gondwana Research, v. 25, p. 910-944). Alternatively the nutrient flux may have arisen by increased erosion as a result of plume-driven uplift (Santosh, M. et al. 2014. The Cambrian Explosion: Plume-driven birth of the second ecosystem on Earth. Gondwana Research, v. 25, p. 945-965).
A bolder approach, reflected in the title of the Special Issue, seeks an interstellar trigger (Kataoka, R. et al. 2014. The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution in the late Neoproterozoic and Cambrian periods. Gondwana Research, v. 25, p. 1153-1163). This looks to encounters between the Solar System and dust clouds or supernova remnants as it orbited the galactic centre: a view that surfaces occasionally in several other contexts. Such chance events may have been climatically and biologically catastrophic: a sort of nebular winter, far more pervasive than the once postulated nuclear winter of a 3rd World War. That is perhaps going a little too far beyond the constraints of evidence, for there should be isotopic and other geochemical signs that such an event took place. It also raises yet the issue that life on Earth is and always has been unique in the galaxy and perhaps the known universe due to a concatenation of diverse chance events, without structure in time or order, which pushed living processes to outcomes whose probabilities of repetition are infinitesimally small.
The first macroscopic life forms were the enigmatic bag-like and quilted fossils in sedimentary rocks dating back to 635 Ma in Australia, eastern Canada and NW Europe. They are grouped as the Ediacaran Fauna named after the Ediacara Hills in South Australia where they are most common and diverse. Generally they are not body fossils but impressions of soft-bodied organisms, often in sandstones rather than muds. Some are believed to be animals that absorbed nutrients through their skin, whereas others are subjects of speculation. One thing seems clear; these first metazoans arose because of some kind of trigger provided by the global glacial conditions that preceded their appearance. It has always been assumed that, whatever they were, Ediacaran organisms lived on the sea floor, probably in shallow water. New sedimentological evidence found in the type locality by Gregory Retallack of the University of Oregon seems set to force a complete rethink about these hugely important life forms (Retallack, G.J 2012. Ediacaran life on land. Nature (online), doi:10.1038/nature11777). So momentous are his conclusions that they form the subject of a Nature editorial in the 13 December 2012 issue.
Retallack, a specialist on ancient soils of the Precambrian, examined reddish facies of the Ediacara Member of the Rawnsley Quartzite of South Australia, whose previous interpretation have a somewhat odd background. Originally regarded as non-marine, before their fossils were discovered, when traces of jellyfish-like organisms turned up this view was reversed to marine, the red coloration being ascribed to deep Cretaceous weathering. A range of features, such as clasts of red facies in grey Ediacaran rocks, the presence of feldspar in the red facies – unlikely to have survived deep weathering – bedding surfaces with textures very like those formed by subaerial biofilms, and desiccation cracks, suggest to Retallack that the red facies represents palaeosols in the sedimentary sequence. Moreover, some features indicate a land surface prone to freezing from time to time. The key observation is that this facies contains Ediacaran trace fossils representing many of the forms previously regarded as marine animals of some kind, including Spriggina, Dickinsonia and Charnia on which most palaeontologists would bet good money that they were animals, albeit enigmatic ones.
If Retallack’s sedimentological observations are confirmed then organisms found in the palaeosols cannot have been animals but perhaps akin to lichens or colonial microbes, and survived freezing conditions. As they occur in other facies more likely to be subaqueous, then they were ‘at home’ in a variety of ecosystems. As the Nature editorial reminds us, from the near-certainty that early macroscopic life was marine there is a chance that views will have to revert to a terrestrial emergence first suggested in the 1950s by Jane Grey. Uncomfortable times lie ahead for the palaeontological world.
Palaeobiologists generally believe that without a significant boost to oxygen levels in the oceans macroscopic eukaryotes, animals in particular, could not have evolved. Although the first signs of a rise in atmospheric oxygen enter the stratigraphic record some 2.4 billion years ago and eukaryote microfossils appeared at around 2 Ga, traces of bulky creatures suddenly show up much later at ~610 Ma with possible fossil bilaterian embryos preserved in 630 Ma old sediments. An intriguing feature of this Ediacaran fauna is that it appeared shortly after one of the Neoproterozoic global glaciations, the Marinoan ‘Snowball’ event: a coincidence or was there some connection? It has looked very like happenstance because few if any signs of a tangible post-Marinoan rise in environmental oxygen have been detected. Perhaps the sluggish two billion-year accumulation of free oxygen simply passed the threshold needed for metazoan metabolism. But there are other, proxy means of assessing the oxidation-reduction balance, one of which depends on trace metals whose chemistry hinges on their variable valency. The balance between soluble iron-2 and iron-3 that readily forms insoluble compounds is a model, although iron itself is so common in sediments that its concentration is not much of a guide. Molybdenum, vanadium and uranium, being quite rare, are more likely to chart subtle changes in the redox conditions under which marine sediments were deposited.
Swapan Sahoo of the University of Nevada and colleagues from the USA, China and Canada detected a marked increase in the variability of Mo, V and U content of the basal black shales of the Doushantuo Formation of southern China, which contain the possible eukaryote embryos (Sahoo, S.K and 8 others 2012. Ocean oxygenation in the wake of the Marinoan glaciation. Nature, v. 489, p. 546-549). These rocks occur just above the last member of the Marinoan glacial to post-glacial sedimentary package and are around 632 Ma old. Since the black shales accumulated at depths well below those affected by surface waves that might have permitted local changes in the oxygen content of sea water the geochemistry of their formative environment ought not to have changed if global chemical conditions had been stable: the observed fluctuations may represent secular changes in global redox conditions. The earlier variability settles down to low levels towards the top of the analysed sequence, suggesting stabilised global chemistry.
What this might indicate is quite simple to work out. When the overall chemistry of the oceans is reducing Mo, V and U are more likely to enter sulfides in sediments, thereby forcing down their dissolved concentration in sea water. With a steady supply of those elements, probably by solution from basalt lavas at ocean ridges, sedimentary concentrations should stabilise at high levels in balance with low concentrations in solution. If seawater becomes more oxidising it holds more Mo, V and U in solution and sediment levels decline. So the high concentrations in sediments mark periods of global reducing conditions, whereas low values signal a more oxidising marine environment. Sahoo et al.’s observations suggest that marine geochemistry became unstable immediately after the Marinoan glaciation but settled to a fundamentally more oxidising state than it had been in earlier times, perhaps by tenfold increase in atmospheric oxygen content. So what might have caused this and the attendant potential for animals to get larger in the aftermath of the Snowball Earth event? One possibility is that the long period of glaciers’ grinding down continental crust added nutrients to the oceans. Once warmed and lit by the sun they hosted huge blooms of single-celled phytoplankton whose photosynthesis became an oxygen factory and whose burial in pervasive reducing conditions on the sea bed formed a permanent repository of organic carbon. The outcome an at-first hesitant oxygenation of the planet and then a permanent fixture opening a window of opportunity for the Ediacarans and ultimately life as we know it.
The combination of glaciogenic sediments with palaeomagnetic evidence for their formation at low-latitudes, together with dates that show glacial events were coeval in just two or three Neoproterozoic episodes are the linchpins for the Snowball Earth hypothesis. There is little doubt that the latest Precambrian Era did witness such extraordinary climatic events. Evidence is also accumulating that, in some way, they were instrumental in that stage of biological evolution from which metazoan eukaryotes emerged: the spectacular Ediacaran fossil assemblages follow on the heels of the last such event (see Bigging-up the Ediacaran in Earth Pages for March 2011). One of the difficulties with the ‘hard’ Snowball Earth hypothesis is how the middle-aged planet was able to emerge from a condition of pole-to-pole ice cover; hugely increased reflectivity of that surface should have driven mean global temperature down and down. Clearly the Earth did warm up on each occasion, and the leading model for how that was possible is massive release of greenhouse gases from sea-floor sediments or deep-ocean waters to increase the heat-retaining powers of the atmosphere; sufficiently voluminous release from volcanic action seems less likely as there is little evidence of upsurges in magmatism coinciding with the events. Almost all glaciogenic units from the Neoproterozoic have an overlying cap of carbonate rocks, indicating that hydrogen carbonate (formerly bicarbonate) ions together with those of calcium and magnesium suddenly exceeded their solubilities in the oceans.
To seek out a possible source for sufficient carbon release in gaseous form geochemists have turned to C-isotopes in the cap carbonates. Early studies revealed large deficits in the heavier stable isotope of carbon (13C) that seemed to suggest that the releases were from large reservoirs of carbon formed by burial of dead organisms: photosynthesis and other kinds of autotrophy at the base of the trophic pyramid selectively take up lighter 12C in forming organic tissues compared with inorganic chemical processes). As in the case of the sharp warming event at the Palaeocene-Eocene boundary around 55.8 Ma ago (See The gas-hydrate ‘gun’ in June 2003 Earth Pages), these negative d13C spikes have been interpreted as due to destabilisation of gas hydrates in sea-floor sediments to release organically formed methane gas. This powerful greenhouse gas would have quickly oxidised to CO2 thus acidifying the oceans by jacking up hydrogen carbonate ion concentrations. Detailed carbon-, oxygen- and strontium-isotope work in conjunction with petrographic textures in a Chinese cap carbonate (Bristow, T.F. et al. 2011. A hydrothermal origin for isotopically anomalous cap dolostone cements from south China. Nature, v. 274, p. 68-71) suggests an alternative mechanism to produce the isotopically light carbon signature at the end of Snowball events. The greatest 13C depletion occurs in carbonate veins that cut through the cap rock and formed at temperatures up to 378°C and even the early-formed fine grained carbonate sediment records anomalously high temperatures. So, it seems as if the cap-rock was thoroughly permeated by hydrothermal fluids, more than 1.6 Ma after it formed on the sea floor. This triggered oxidation of methane within the sediments themselves, with little if any need for an atmospheric origin through massive methane release from destabilised gas hydrates elsewhere.