Once they became established organisms, their deaths and funeral rites have been central players in the Earth’s surface evolution. That is so because ultimately they build themselves from carbon dioxide and water. Death and burial intervene in the ‘greenhouse’ effect that would otherwise be controlled by the mantle’s heat production and eruption of gas-laden magmas, together with a variety of inorganic chemical balances that involve air, oceans and exposed rock. The fate of Venus, probably little different in internal composition from the Earth but with surface temperatures sufficient to melt lead, hints at the importance for planetary evolution of a step that involves life. Venus’s ‘greenhouse’ never had a biological thermostat. An extra 400°C of surface temperature feeds down to that planet’s interior, so that for any given depth Venus is hotter than the Earth. Our limited understanding suggests that Venus’s internal heat loss may somehow become blocked, perhaps by sluggish mantle convection, so that Venus undergoes catastrophic mantle upheaval and volcanic repaving every half billion years to release its accumulated radioactive heat. Liquid water cannot exist there, and UV radiation in the upper atmosphere destroys it to shed hydrogen to space while oxygen combines in its surface rocks. Venus is forever a dead planet whose fate may have been set by failure of living processes to emerge there.
Apart from secondary lines of chemical evidence on Earth (from carbon isotopes, the wane of BIFs and the appearance of redbeds) and the products of once-dominant communities of microscopic organism, such as stromatolites, we have few direct signs of life through most of the Precambrian. Yet there are rare finds of microscopic fossils of prokaryote cells, usually from hard, fine grained cherts. The earliest known and widely accepted are 3.4 Ga old sulfur-metabolising bacteria from Western Australia, which are associated with the fine grained iron sulfide that their metabolism produced. The same area and time has also yielded cells likely to have been oxygen-producing cyanobacteria. However, because evidence of any kind is so sparse, we have little choice for understanding Earth-life interactions in the first 3 billion years of the geological record other than to consider life then as a general, widely permeating entity with chemical consequences. Life undoubtedly helped shape the climate and atmospheric composition of the early Earth.
The ‘invention’ by the blue-green cyanobacteria of biologically costly and chemically difficult engineering, which uses the Earth’s vast abundance of liquid water as an electron donor, opened most of the upper ocean to life, where previously it could lurk only in chemically special but simpler niches. That leap by the ‘blue-greens’, centred on efficient use of solar energy that has yet to be bettered, opened new horizons but breathed out electron-hungry oxygen that is highly toxic to prokaryotes (Chapter 14). That such a chemical crisis did not wipe out life, cyanobacteria and all, stands largely at the door of the early mantle and its higher heat production. Continuous basalt outpouring on the ocean floor and its reaction with seawater fed dissolved iron (Fe2+) into the chemically reducing early oceans. Its combination with the blue-greens’ toxic waste, either by direct reaction or some other biogenic process, built vast piles of insoluble ferric oxides in the form of BIFs. Until about 2 Ga ago that kept oxygen’s toxicity at bay. The early superabundance of dissolved iron also allowed cell chemistry to favour iron in critical metabolic functions, for instance through the iron-centred protein Rubisco that is vital to the Calvin cycle and photosynthesis. Such relics of Earth’s early seawater chemistry have survived to the present. At some stage, calcium became a central messenger for ‘communications’ within living cells. A minimum concentration of Ca2+ ions is needed for cells to function. Yet, there is an upper limit, beyond which calcium becomes toxic unless expelled through cell walls to maintain tolerable concentrations within. Such a life-death balance characterises all elements that are essential to life. Within an evolving plethora of chemical opportunity and risk a union of mutual benefit between some prokaryote cells gave rise to a symbiotic ‘community’ locked within an entirely new kind of cell; that of the first eukaryotes. This negated the relationship between living processes and oxygen from one of deadly hazard to a metabolic essential, yet excessive oxygen can still result in cell death.
Two fundamentally new features of eukaryotes gave them decisive advantages over the now lowly prokaryotes. Sexual reproduction, and thereby genetic recombination, together with the vastly greater amount of genetic material in eukaryote cells compared with those of prokaryotes introduced an accelerated pace for evolution. Not only did this pace allow the eukarya quickly to exploit new niches and survive new threats provided by changing environmental processes. It also permitted the same fundamental or stem-cell architecture to take on different roles in multi-celled agglomerations. That is the basis of the metazoa, the teeming macro-organisms, of which we are one, that surround and astonish us today. Only when the eukarya make their fossil appearance does it become possible to move from the general to the specific, to use anatomy, form and function to divide life into the branches, stems and twigs of an evolutionary bush on whose outer layers sit modern life forms. That is the domain of palaeontology, too specialised for much exploration here. The explosion of DNA sequencing has opened the possibility of recognising degrees of relatedness between all living organisms (Figure 14.2). Through its potential for assessing the relative timing of ancestral branching between different taxonomic divisions, genomics sheds light on the fossil record of evolution. In the early 21st century it even became possible to extract and sequence DNA from fossils, albeit only very recent ones, to assess the course of human evolution (Part VII) since a few hundred thousand years ago.
Big, soft things
The earliest visible fossils take the form of coiled ribbons up to half a metre long (Figure 14.3). In rare examples not flattened by sediment load, Grypania proves to have been tube-like. No cell structure survives, so only its bigness and being clearly organic suggests either a metazoan life form or the imprint of one. The age of the earliest example is about 2.1 Ga; not long, in a geological sense, after the Great Oxidation Event. Remarkably, however, in a single occurrence of unmetamorphosed Precambrian sediments in Ontario, Canada, dated at 2.45 Ga, oily material occurs as fluid inclusions within quartz that cements the sediment together. It contains biomarker compounds, barely altered by heat. Among them are ‘signatures’ of blue-green bacteria, but most surprising are steranes, the breakdown products of sterols that are exclusive to eukaryote metabolism. They define a minimum age for the emergence of the eukarya. Yet, for the succeeding one and a half billion years, Grypania is all that metazoan life amounts to in fossil form, and examples are rare.
Then, suddenly, late Precambrian sediments reveal a surprise, albeit little more than a basic playground. Its inhabitants are animals up to a metre in size, though in reality they are mainly just impressions. They subsisted on mats of bacterial biofilms that are also preserved in the sedimentary record. None of these inhabitants of a new world, beginning about 565 Ma ago, had hard parts. It is thought that they had skins leathery enough for their shapes to be preserved and that some of them ingested food and excreted waste through that skin. Best known from late-Precambrian sediments of the Ediacara Hills in South Australia, the trace fossils of the Ediacaran fauna show a wide variety of pancake-, bun-, bag- and pen-like forms, with clearly organised though barely understood segmentation and radial divisions. Controversially, some experts see likenesses in some of the fossils to modern worms, sea-urchins, jellyfish and even naked shrimps. Sediments deposited in many environments, from intertidal to deep water, contain the Ediacaran fauna. It has even been suggested, albeit to an uproar, that some occurrences are in fact palaeosols, implying colonisation of dry land. The fact that such delicate organisms survived death to leave their imprints implies, with little room for doubt, that very few animals then burrowed to devour buried, rotting corpses, although there are signs of scavenging beneath superficial microbial mats. For eukaryotes to burrow requires oxygen to be available in the wet sediments that they invade. The suddenness of the appearance of big animals in the uppermost Precambrian might suggest that animals appeared just at that time, subsequently to radiate into the many basic body plans recorded in Cambrian skeletal fossils and those of today (and probably many groups that did not make it). Comparing genetic material from modern animals suggests otherwise.
The distinctive body plans of most modern animal groups represented by Figure 14.2 appear among fossilised hard parts from 542 Ma onwards. Using the degree of genetic difference between living organisms as a ‘molecular clock’ and estimations of evolutionary rates derived from them – and that means lots of assumptions that are not testable – suggest that the original divergence of multi-celled animals from a common eukaryote ancestor was at least a billion years ago. So why are there no obvious trace fossils before the Ediacaran, if there were no mud-snuffling creatures to destroy the evidence? The simplest explanation is that they were too small to show up, even as trace fossils. If a reasonable size for eukaryote cells is around 50 micrometres, then at 20 to the millimetre metazoans with 8000 cells would be 1mm across – too small to show up recognisable organisms as distinct traces in sediments, but quite large enough for them to have a wide diversity of habits.
Chinese palaeontologists pushed back the evidence for eukaryote diversification to between 580 and 600 Ma through their discovery of exceptionally well-preserved fossils in phosphate deposits of the Doushantuo Formation of southern China. All of them are microscopic yet so well-preserved that their affinities are often clear. They include algae, sponges, comb jellies, corals and animals to whose broad group we belong. ‘Our’ group comprises creatures that have both a mouth and an anus in a body plane that shows some degree of bilateral symmetry either side of a line joining these two orifices; the bilaterians. Their basic architecture encompass the majority of known animal phyla – arthropods, all kinds of worms, molluscs, brachiopods and deuterostomes to which we belong along with sea urchins. The Doushantuo Formation’s biological diversity sets a minimum age for this fourth major point of radiation represented on Figure 14.2, the earliest being the prokaryote-eukaryote closest to LUCA, then plants-fungi and fungi-animals. The Ediacaran fauna seems to represent a time when metazoans simply grew up.
The emergence of the basic bilaterian body plan from which most living animal groups emerged resolved the crucial issue of waste disposal. Cnidaria, which encompass coral polyps, hydra and jellyfish, largely expel waste through their oral opening. This prevents them from eating and defecating at the same time. Porifera (sponges) do not have distinct circulatory, respiratory, digestive, and excretory systems, all these functions being performed by water circulation through their porous and largely shapeless bodies, which results in much the same problem. Evolving a one-way digestive system, with separate inlet and outlet holes linked by a gut, would have enabled continuous eating, digestion and defecation while creating a framework for bilateral symmetry and the ability to add body length to volume. The advantages of such a leap are clear from its resulting diversification shown clearly by Figure 14.2. An escape from the lingering puzzlement about the path from the early metazoa to this revolutionary development may have been resolved by recent observations of comb jellies’ feeding habits using prey organisms genetically engineered to fluoresce. Widely thought to excrete through their mouths it now seems that after circulating food along a network of tubes waste products emerge through pores at the opposite end of comb jellies. Comb jellies’ DNA suggests that they evolved earlier than cnidaria and porifera. Two scenarios present themselves: comb jellies and bilaterians may have had a shared ancestor possessing a rudimentary one-way mouth to anus passage that cnidarians and porifera lost; comb jellies may have evolved through-guts independently over a long period of separation from other early metazoa, leaving the origin of bilaterians still an open question.
You will recall that the late Proterozoic was a time of great geological turmoil (Chapters 6 and 16). The core of the Rodinia supercontinent left on a great excursion, in the manner of a pip from a crushed grape, to take on an approximate outline of today’s North American continent (Figure 6.3). The remainder drifted rapidly to fill the gap, becoming welded together along great tectonic sutures as Gondwanaland. It was also the time of the Earth’s greatest-ever icehouse conditions, notably the Sturtian (717 – 660 Ma) and Marinoan (640 – 635 Ma) Snowball Earth events, when glacial activity reached to the equator. Moreover, BIFs briefly returned; ominous for the eukarya, because BIF formation demands seawater rich in dissolved iron (Fe2+) that consumes dissolved oxygen, on which eukarya depend, to lay down insoluble ferric (Fe3+) oxides. The only conceivable source for the iron is submarine volcanic activity, and the great excursions of the continents at that time probably involved superplumes. While geological reasoning helps in understanding the global processes of the late Proterozoic, it is to geochemical evidence that we must turn in attempting to chart their timing and magnitude, and how planet-wide environmental changes correlate with the scanty record of life. Figure 18.1 shows in detail the changes in strontium and carbon isotopes of the late-Proterozoic to early-Cambrian oceans, as recorded in limestones, in relation to the break-up of Rodinia, glacial epochs, BIFs and the Ediacaran fauna.
Figure 18.1 Strontium and carbon isotope variations in limestones, and therefore seawater, for the last part of the Precambrian and the early Phanerozoic in relation to the Ediacaran fauna. Blue lines show widespread glacial epochs – only the Sturtian and Marinoan glaciations are recorded globally. Cartoons show global tectonic developments schematically. Partly after Halverson et al. (2010).
Because extraction of carbon from the atmosphere and oceans by living processes preferentially removes ‘light’ 12C, the ratio 13C/12C in organic material is significantly lower than that in the rest of nature. It has a negative δ13C value, but this is difficult to measure from the organic material in, for instance, black shales. Instead the more easily acquired δ13C in limestones, which closely reflects that in seawater from which they were extracted, can be broadly interpreted. When life, death and decay are in balance the δ13C value of seawater and limestones precipitated from it remains constant and close to zero, which it has done for a great deal of geological time (Figure 17.2). The 350 Ma time span from about 850 Ma until the start of the Ordovician Period shows a broad rise to positive δ13C values in limestones (Figure 18.1). Yet this period also shows δ13C in seawater plunging several times to values well below zero, successive declines becoming steadily lower. A plausible explanation of these negative ‘excursions’ is that biological production in late-Precambrian surface water and organic burial beneath the ocean floor may have become extremely low. four of the declines coincide with known glacial episodes, including the two Snowball Earth events. Long-lived, extensive sea ice would indeed dramatically reduce the opportunities for photosynthesis by blocking sunlight.
Conversely, the intervening peaks of positive δ13C have been explained by high biological productivity between glacial epochs, combined with efficient burial of organic carbon in ocean-floor sediments; a process demanding low oxygen content in ocean-bottom water. Reducing conditions of various degrees are indicated by measurements of sulfur-isotope fractionation from sulfate and sulfide minerals in ocean-floor sediments throughout the lengthy period. Interestingly, some glaciations also link with short-lived returns of BIFs that indicate more extreme reducing power, not observed since the Great Oxidation Event of ~2.2 Ga. Seawater chemistry seems to have been on a knife edge
Turning to the strontium isotopes, again we see some unexpected features. As you know from Chapter 16, when continents are eroded and weathered they supply dissolved strontium containing more abundant radiogenic 87Sr than that derived from oceanic crust. On the other hand, ocean-floor hydrothermal activity acting on hot new basalt supplies strontium with a distinctly low 87Sr/86Sr ratio inherited from the rubidium-poor mantle. The relative influences of the two processes on seawater chemistry appear in the strontium isotope signature of limestones. When the continental erosion rate is high, up goes the ratio in marine limestones. It goes down either when continental weathering wanes or ocean-basalt eruption increases. At the start of the late-Proterozoic carbon-isotope changes, 87Sr/86Sr in limestone and therefore seawater begins a broad rise from dominance by sea-floor hydrothermal activity. An influence by hot oceanic crust, either from spreading centres or superplume activity, is hardly surprising as a great deal of submarine magma would have driven the break-up of Rodinia and the accretion of the Gondwana supercontinent from its remnants. By Ediacaran times a more continental influence had emerged. A closer look at the two Snowball glaciations that preceded the Ediacaran reveals higher proportions of radiogenic 87Sr directly after the glaciations than immediately beforehand; erosion by vast continental ice caps would have prepared a great deal of ground-up rock to be weathered when the climate sprang back to warmth. That influx of continent-derived strontium is accompanied by deposition of thick limestones directly above every occurrence of Sturtian and Marinoan glacigenic sediments; the ‘cap carbonates’ built from abundant calcium and bicarbonate ions derived from the continents.
How do the geochemical data bear on the climatic record and what we know of metazoan life? Glaciation, particularly that approaching the equator, demands a greatly decreased ‘greenhouse’ effect and therefore a major fall in the CO2 content of the atmosphere. The broad increase in the 13C content of seawater before each glacial episode (Figure 18.1) can explain that by the burial of organic carbon in large amounts to set in motion a negative feedback effect on atmospheric CO2 and thus on climate. But once CO2 fell to levels that triggered such intense global cooling that glacial conditions extended from pole to pole, sea ice shut down the biosphere. This may be signified by the massive declines in δ13C during the Snowball events. Being so reflective, glacial and floating ice further drives down temperatures by reducing solar warming; risking frigidity from which no escape is possible and snuffing out all life. The Earth’s internal heat engine comes to the rescue.
Volcanism associated with plate tectonics continually replenishes atmospheric carbon dioxide. Climate being so cold, little rain would fall, most precipitation being of snow. With snow being incapable of dissolving CO2 from the air, drastically reduced photosynthesis, little burial of organic matter and no limestone deposition to speak of, greenhouse gas would build unhindered for millions of years. Eventually that would have provided an escape from the ‘icehouse’ through a resurrected greenhouse effect. Rain could fall once again, but charged with carbonic acid whose dissociation (Chapter 2) releases hydrogen ions so essential in the chemistry of weathering. Accelerated weathering of ground-up continental rock then worked to deliver calcium and bicarbonate ions (Equation 16.3) to rivers and thence the oceans, building up anomalously high concentrations of both in seawater. The cap carbonates that lie directly on each set of glaciogenic rocks bear witness to that and are thought to represent rapid inorganic and organic precipitation of the new chemical additions to seawater.
Burial of burgeoning organic carbon between glaciations, at a time when the principal means of biomass production had become oxygenic photosynthesis, implies that oxygen levels also grew in the atmosphere. The BIF-forming periods associated with three of the glacial events signify that their ferric oxides would have consumed at least some of the oxygen that might otherwise have helped break down dead organic matter. Yet soluble iron is also a fertiliser, on which depend the Fe-S based proteins inherited from life’s origins and central to eukaryote and much prokaryote metabolism. Sustained increases in dissolved iron where deep water upwelled to the ocean surface would have fuelled vast plankton blooms after the major glacial events.
Clearly, the Earth system in the late Proterozoic was working towards dramatic change through interwoven feedback loops extending from mantle to atmosphere, to a large extent through the intermediary of living processes. Repeated crises of life marked by brief negative slumps in δ13C that give a sign that the biosphere was decimated, tie-in with both BIFs and glaciation, the last and deepest excursion with just a minor glaciation. Following hard on the heels of each crisis the positive δ13C highs signify the opposite; boom time for whatever life forms came through each crisis. Eukaryote life at the close of the Precambrian experienced huge climatic and chemical stress again and again. Chapter 19 deals with mass extinctions and the stresses faced by life of more recent times. The rest of this chapter examines how those early organisms of a new kind of world first emerged
Surviving mass poisoning
Toxicity, at its simplest, implies individual elements and compounds, or a blend of them, rising above or falling below levels where fundamental processes in the cell are chemically possible. For us, slightly too much sodium may bring on high blood pressure. Far too much, resulting from severe dehydration, gives the acute symptoms of heat stroke including coma and death. Insufficient sodium following excessive sweating causes cramp, whereas severe reduction, as through excessive drinking or water retention, may also be fatal. An even surer way to perish is to increase or decrease calcium levels in our cells, for which there is a very narrow ‘window’ of tolerance. Fortunately, our parathyroid glands, with the help of vitamin D, balance calcium nicely. Low calcium diet or, worse, loss of both parathyroid glands inevitably kills us. Calcium transmits information within cells and between them. Balancing its concentration within very narrow limits is therefore vital, otherwise cell processes break down. Cytoplasm in Bacteria and Archaea seems not to need it, and its signalling role evolved with the eukarya, though we have little idea why.
Of all the common metals vital in cell processes (principally Na, K, Fe, Ca, Zn, Cu), calcium forms the most insoluble common salts, such as carbonates and phosphates. Should such salts crystallise within the cell if calcium’s concentration rose above its solubility threshold their crystals would shut down cell processes completely. Eukaryote cells have evolved means of coping with chemical ups and downs in their environment, pumping ions across their walls in a similar fashion to proton pumping, but they have limits. Among all the candidates for environmental stress in the late Precambrian that the isotopic peaks and troughs demonstrate, varying availability of calcium might well have been another. Following each of the δ13C troughs of Figure 18.1 there was a great deal of dissolved calcium flooding from the land to enter seawater.
Natural selection in relation to changing environments and to changes in life itself would have driven evolution, as always. The exchange and recombination of genetic material through sexual reproduction among the eukarya must have accelerated the potential pace of evolution. Leaving aside for a while the episodic increase in calcium content of seawater through the late Proterozoic, another major change offered advantages to animals in particular, although we have to infer that it did happen.
The massive amounts of organic carbon burial would have released oxygen. That follows from its primary autotrophic production by photosynthesisers, both blue-green bacteria and the eukaryote forerunners of algae and plants. Animals, eukaryotes all, need oxygen in their metabolism; they are aerobic heterotrophs. Indeed, it may have been the toxic stress from oxygen generated by blue greens that drove the evolution of the eukaryote cell as a haven for some Archaea and Bacteria in a symbiotic ‘contract’ around the time of the Great Oxidation event. The level of oxygen in the environment is the ultimate control on size of animals, and this is why. The amount of oxygen that an animal uses depends on how big it is; on its volume and mass. Oxygen enters the cell by being pumped through its walls, as are all elements involved in cell metabolism. The ease whereby oxygen can be pumped hangs to a large degree on how much there is around. The total amount that can be taken in is governed by the surface area of cell wall exposed to the environment. Later evolution produced oxygen intakes, such as fishes’ gills and our lungs, with enormous surface areas relative to mass, which enabled higher animals to metabolise and move both efficiently and swiftly. For much simpler animals, the smaller they are the greater their surface area relative to their mass. From the equations for volume and surface area of a sphere – involving the cube and the square of radius respectively – doubling dimensions increases volume and mass by eight times, but increases area only by four. For a particular level of oxygen in the environment, there is a limit on how big a simple animal can become. This helps explain two things. First, the rarity of earlier trace fossils probably links to the tiny size of the first animals. Secondly, if oxygen levels rose in the last part of the Precambrian, then animals could grow bigger, and what we see in the Ediacaran fauna are bag-like creatures up to a metre across. Oxygen had been freed photosynthetically during the 300 Ma of enhanced organic carbon burial before the Ediacaran. So why didn’t animals grow steadily over that time instead of big ones appearing out of the blue so late?
The answer is almost certainly connected with their appearance very shortly after the Marinoan Snowball Earth event, during which life became scarce beneath large areas of thick sea ice. Though there is no tangible fossil record before the Ediacaran, each of the glacial events marked a mass extinction likely to have been most felt by eukaryotes. In each case, sudden climatic warming increased erosion of the continents to release, among many other elements and compounds, massive amounts of calcium and bicarbonate ions to an already decimated biosphere. If sea level rose 130 m about 7 thousand years before the present to flood exposed continents, after a glaciation extending only to 40° south of the North Pole, melting of Snowball Earths would have caused even larger geographic changes. The surviving organisms were presented with a whole range of brand new niches to colonise and exploit. Opportunities and potential hazards each demand appropriate fitness.
Centred on their basic heterotrophic cell design, eukaryote animals do only two things, apart from reproducing sexually – they devour autotrophs or each other. Oxygen pumping into the cell controls this appetite and the energy that it makes available for use in movement and yet more consumption. With oxygen at a premium and a limit on size, a major driver for natural selection would be through ‘experimentation’ with different styles of animal metabolism within the loose constraint of being a eukaryote and within the inorganic chemical and geographic constraints of their continually changing new environment. Exposure of the now liquid sea surface to air containing oxygen, and spreading over the edges of a ground-down land surface as sea level rose, marine niches would be available at various combinations of depth, energy, solar illumination, turbidity etcetera. Eukaryote and prokaryote autotrophic organisms colonizing them would have created a diversity of primary biomass on which heterotrophic animals might subsist if able to evolve and fit such novel ecosystems. Together these opportunities provide a possible engine for fundamental diversification in animal design. The Doushantuo organisms, living 35 Ma after the Marinoan glaciation confirm an existing diversity, possible begun about a billion years ago as shown up in the molecular differences between modern animal groups.
Whatever brought on the Ediacaran trace-fossil bonanza that first appeared 25 Ma later still (575 Ma), animals then became big enough for their imprints to enter the fossil record, but they present a sorry sight. Bags, pancakes, buns and pen-like forms that just sat on the sea floor do not conjure visions of scurrying activity. At best they grazed idly, wobbled feebly in the manner of jellyfish, or waited for food to come their way, perhaps wafting it with moving cilia to an intake orifice or absorbing it directly through leathery skin. None ventured deeper than a few millimetres into mud seeking the dead organic riches there. That potential burrowing niche either had insufficient oxygen or would clog the delicate membranes of large, soft animals. It remained the domain of single-celled archaea and bacteria.
Burrowing properly began at the start of the Phanerozoic. Characteristically, earlier sediments, and their fossil remains are disturbed only by physical forces, such as tidal ebbs and flows, and moving fluids. Precambrian sediments delight their students by the sheer intricacy of preserved, delicate structures that allow them to decipher the minutiae of depositional environments. BIFs record sediment fluctuations perhaps down to the daily level. Finely banded marine silts deposited seasonally during one of the late-Precambrian glacial episodes show full tidal cycles, and even the 11 year cycles of sun-spot activity that subtly affect temperature ranges. Burrowing animals play havoc with such records from later times. Moreover, the very fact that they disturb buried sediments allows oxygen, if available, to penetrate deeper; a self-sustaining attribute of burrowing and its evolution to exploit a large, energy-rich and previously inaccessible ecological niche.
Evolving hard parts: the Cambrian Explosion
Secreting hard parts would have been a novelty for animals and, in a metabolic sense, an expensive one – far more so for those parts composed of organic materials (e.g. chitin) than for materials derived from the environment (calcite, silica and calcium phosphate). Evolving and retaining hard parts needed to confer fitness to stave off extinction and/or provide some ecological advantage.
The most important boundary in the stratigraphic column is that closing the Proterozoic Eon and opening the proliferation of animals with hard parts in the Phanerozoic. Calcium carbonate, silica and calcium phosphate skeletal material preserves extremely well. There is no surprise in finding a marked increase in fossil preservation above that boundary. Precise dating of the onset of shelly faunas, a growing search for the earliest Cambrian animal groups and burgeoning collections of fossil diversity highlight its drama. Life’s Cambrian Explosion took no more than 5 to 6 Ma, and it transformed the world and the evolutionary course of all its surface components. You have seen the broad climatic outcomes of Phanerozoic life’s interplay with geological forces in Chapter 17, and there is more to come in Chapter 20. This is the place to explore how it might have come about. Everything hinges on the incorporation of inorganic mineral materials in animals. Since hard parts can confer both arms and armour, it is easy to regard the transformation as an evolutionary arms race between the consumers and the consumed. That certainly was the general lot of animal life in later times, but a uniformitarian focus on the predator-prey relationship gets us into the old ‘chicken and egg’ dilemma. Which came first, teeth or armour? There is a better metaphorical question. Why did animals cross that particular road?
The youngest Ediacaran and oldest Cambrian strata contain tiny cups, spines, knobs and platelets (the ‘small shelly fauna’) made of calcium carbonate. There can be no doubt that they were hard parts of living organisms. Luckily, some of the bearers of these objects have turned up in many places. For instance, Cloudina from the uppermost Ediacaran, and named after revered US palaeontologist Preston Cloud, consists of irregular calcareous cones nested within one another, easily disarticulated to yield components of the ‘small shelly fauna’. Its mode of life and taxonomic affinities are uncertain and its hard parts defy explanation. Some specimens contain tiny holes (<0.5 mm) that have been interpreted as signs of attack by very small predators. Another (Halkieria) in Greenlandic fine shale, deposited in the early Cambrian under such anoxic conditions that decay was bacterial and slow, was a flat worm-like creature covered with articulated plates and spines. Both animals might have been armoured against predators, but nothing is known in contemporary fossilised form that could have delivered more than a wet-lipped suck. There is another possibility for their producing hard parts, deriving from the great dangers posed by excess calcium in the eukaryote cell. Perhaps the tiny, rough-and-ready carbonate plates originated as means of ridding cells of too much calcium, in roughly the same way as blue-green bacteria had formed the biofilms that characterise stromatolites during the preceding two billion years. Those yet-to-be-discovered animals that earlier still evolved carbonate or phosphate secretion opened a dual potential, only to be realised when oxygen levels rose to permit both larger bodies and speeded-up mobility and metabolism.
Predation, and thereby the origin of prey among metazoan animals, needs more than hard biting, holding and ripping devices. It needs a feeding end and an excreting end, linked by a gut to process food larger than single-celled items; the body plan of bilaterians. Capture and escape among large animals demand efficient locomotion and thereby energy generation. Guts, muscles and rapid locomotion are achievable by entirely soft animals, given enough oxygen to turn fuel into energy. One plausible scenario (among a great many) is that mineral plates proved an excellent defence against soft predators prone to engulfing or sucking. Yet it is but a small step for hard parts around food intakes to evolve into means of offence, thereby not only turning the tables on lightweight, fleshy predators but opening all the selection pressures involved in the arms versus armour dichotomy of the new shelly faunas. Resolving all these problems depends entirely on finding rocks deposited in such quiet and essentially lifeless (in the metazoan sense) environments that any creatures falling in on death are preserved so well that details of hard and soft parts still show up. It is in the nature of Earth’s continually changing surface environments that such mausoleums of exquisite preservation are extremely rare. The earliest containing the necessary diversity formed some 20 Ma after the Cambrian Explosion. Too late, as they witness highly developed predator-prey relationships, but palaeontologists live in hope in their exploration of sedimentary rocks close to the greatest stratigraphic boundary.
Fundamental body plans that emerged perhaps 400 Ma before the start of the Phanerozoic, growing availability of oxygen, increased size limits and a wealth of possibilities for each animal group opened up by hard parts did more than add a zest to evolution with the emergence of an arms race. The oceans and near-shore seas around Cambrian continental fragments (Figure 17.3a) offered countless niches for occupation that earlier life could not fully exploit, partly because of its fragility and lack of mobility. Nature abhors vacuums, and the course of Phanerozoic evolution has been radiation in form and function to fill whatever potential niches there were. Growing diversity, indeed the riot of metazoan achievement itself creates new niches continually; remember the tiny beast, a phylum in its own right, living on the lips of lobsters. The fossil record, albeit imperfect, is a rich source of evidence for charting evolutionary radiations. Equally it provides statistics about other kinds of event; those of mass extinction for which only indirect evidence presents itself in the Precambrian. Such catastrophes partly sterilised the world for later evolutionary developments, and we examine them first.
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