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The Great Dying

John Bannigan

The Worst of Times. How Life on Earth Survived Eighty Million Years of Extinctions, by Paul B Wignall, Princeton University Press, 199 pp. ISBN 978-0691142098

Until the 1980s, earth scientists and evolutionary theorists rejected the possibility of mass extinctions caused by cataclysmic geophysical upheavals or meteor strikes. But now it is accepted that at least twenty such events occurred during the past 542 million years. The Worst of Times, written by the professor of palaeoenvironment at the University of Leeds, is an account of the severest of these, the Permian-Triassic extinction, which began 252 million years ago and resulted in the elimination of ninety-five per cent of plant and animal species. Before discussing the book it is worthwhile to look at the reasons why geologists and biologists rejected the mere possibility of mass extinction until advances in geochemistry and palaeontology made their reality inescapable.

In the eighteenth and early nineteenth centuries the study of geological strata and their contained fossils made it obvious that there was a progression in the presence of simpler to more advanced forms from the older to the younger strata. So striking is this progression that geologists distinguished the main phases of the formation of the lithosphere on the basis of the contained fossil fauna. First they saw layers that were seemingly devoid of fossils. Above these were strata which, as well as containing the remains of shellfish, sponges, arthropods etc, also contained examples of non-mammalian vertebrates, mainly fish, but also early amphibia and reptiles. These layers were ascribed to the Palaeozoic or ancient era. This was followed by the Mesozoic or middle era, when the first mammalian forms appeared. Thirdly, there is the recent or Cenozoic era, during which the main taxonomic groupings of animals and plants evolved into their present-day forms. These eras are further divided into nested sub-intervals (periods, epochs, ages and so on) based on finer details of faunal classification or on geological features. Needless to say, the early palaeontologists reasoned that their failure to find any or only few fossils in pre-Palaeozoic formations was because the fauna existing then did not have body parts which favoured fossilisation, such as bones, teeth or shells.

Another thing that was realised early on was that species disappeared from the fossil record as new forms appeared. At present 1.2 million species are catalogued in the biological databases but many taxonomists think that the real number could be between eight and thirty million. Enormous as these numbers seem, they represents less than one per cent of the total number of species that have ever existed. Clearly, over the long history of life on Earth a constant elimination of and replacement of species has been the norm, and this is a necessary consequence of the theory of evolution by natural selection. Species will either disappear by evolving into descendant species or will lose the capacity to survive competition with a fitter species occupying the same ecological niche. This is the most likely explanation for the disappearance of the majority of species. But sometimes the rate of elimination seems to speed up as if something out of the normal was happening. Around the turn of the eighteenth century Georges Cuvier explained these apparent accelerations in species loss by the occurrence of periodic cataclysms such as massive floods. This theory, the result of Cuvier’s detailed studies of the geological history of the Paris Basin and its fossil fauna, was called catastrophism. Catastrophism was a valid hypothesis if the fossil record was interpreted literally, but in the hands of more theologically minded theorists it allowed the periodic interventions of God in shaping the world and its biota. As Stephen Jay Gould has said:

Their world view extended beyond a simple theory of geologic change; it encompassed a methodological procedure, excluded from the modern definition of science, which permitted direct providential control of earth history.

Such views did not sit well with the spirit of the Enlightenment, which, following Newton, saw the universe as the work of a Great Designer who implanted unchanging laws in his creation and then interfered no more in their operation. There had to be a reaction and catastrophism was replaced by a principle called uniformitarianism, which then dominated geological and evolutionary thinking until near the end of the twentieth century. 

Uniformitarianism, as a programme, took shape in the works of two Scottish scientists who were the founders of modern geology, James Hutton (1726-1797) and Charles Lyell (1797-1875). The principle stated that the same processes that are seen in action in the present formed the geological features of the Earth. The only forces that had ever been at work were those that we see today, such as erosion by wind and water and the accumulations of sediments washing into the seas in one place exerting pressures which deformed the lithosphere to result in the uplifting of mountains elsewhere. Forces other than physical and chemical ones, entelechies and the tinkering interventions of God, apart from the initial intervention that set the whole mechanism in motion, had no part to play in nature. Uniformitarianism also required very long periods of time for geological change to occur. Hutton even discussed quadrillions of years but Lyell settled for hundreds of millions. As a methodology, uniformitarianism is correct in the sense that the laws of physics do not vary over time or place. It simply endorses the inductive method upon which scientific inquiry depends. But Lyell went further and insisted that the intensity and rate of physical processes did not vary during geohistory. Seismic and volcanic phenomena were admitted, but only allowed the power and geographic range of those that are observed in the present. Hence, there are two aspects to uniformitarianism, which Gould has called the methodological and the substantive, and it is the substantive which has had a stifling effect on geological and evolutionary hypothesis formation for nearly two hundred years. 

When Darwin embarked on HMS Beagle in 1831 he took on board, in both senses of the term, Volume I of Lyell’s Principles of Geology, being an Attempt to explain the former changes of the Earth’s Surface by Reference to Causes now in Operation and its methodology. The remaining two volumes were picked up at stopping points during the voyage. His first publications were on the geology of volcanic islands, coral reefs and mountain formation in South America and all were vindications of Lyell’s theories. In the working out of his theory of evolution he repeatedly emphasises that extinction is a slow process and caused only by the operation of natural selection. The apparent rapidity with which some species loss occurred was due only to imperfections in the fossil record. Everything always was as it is now. Nature has always done, albeit without consciousness or purpose, what agriculturists and stockbreeders have been doing with purposeful artificial selection. Again and again in his writings he emphasises the slow gradual nature of selection, even though he was aware that the disappearance of species from the fossil record could at times seem “wonderfully sudden”. On page 393 of the fifth edition of Origin of Species (1869), he sums it up:

I may repeat what I published in 1845, namely, that to admit that species generally become rare before they become extinct ‑ to feel no surprise at the rarity of a species, and yet to marvel greatly when the species ceases to exist, is much the same as to admit that sickness in the individual is the forerunner of death – to feel no surprise at sickness, but when the sick man dies, to wonder and to suspect that he died by some deed of violence.

It has often been said that the hold of uniformitarianism, other than as a methodology, on the earth and life sciences was due to how well it fitted the political and economic interests of a wealthy middle class, to which both Lyell and Darwin belonged. Stephen Jay Gould, who was a Marxist in outlook, has even argued in The Structure of Evolutionary Theory that Lyell forced the substantive form of uniformitarianism onto its methodological counterpart dishonestly and largely out of an ideology that abhorred revolution but was at the same time committed to progress. Apart from this there may have been other reasons for the neglect of catastrophe explanations.

The fact is that evolutionary theorists, including Darwin himself, had more to worry about than what might, after all, turn out to be only an artifact of the fossil record. The core concept of the theory of evolution by natural selection is very simple. There is a struggle for existence because populations grow at a faster rate than the resources needed to sustain them. But individuals within a population vary from one another in small ways and these variations are heritable. Variations, depending on their type, will make their possessors either more or less successful in the struggle for existence. The more successful will be more likely to live to reproductive age and pass on their characteristics to their descendants. In successive generations, some characteristics will become commoner in a population and others will become rarer and eventually disappear completely. Over extremely long periods the changes in a population may become so great as to constitute a new species. The problem was: where is all this variation coming from? The belief in Darwin’s time was that an organism inherited an average mixture of the characters of its parents. It follows that in each generation about half of variation will be lost and eventually all variation should vanish from a breeding population, leaving nothing for selection to act on. The problem was only solved around 1900 with the rediscovery of Mendelian genetics, which showed that inheritance was not blending but had a particulate nature. This, along with the discovery that genetic mutations occurred spontaneously, opened up whole new research programmes for evolutionary theorists. It also turned out that population genetics gave itself very well to mathematical analysis and soon people like RA Fisher in England and Sewall Wright in America were producing mathematical models of great explanatory power. The emphasis was now on laboratory and computational studies of mutation and the investigation of genetic variation in wild populations. Palaeontology took a very lowly place in evolutionary studies.  

The turning point came in 1980, when Walter and Louis Alvarez showed that the mass extinction marking the boundary between the Cretaceous period of the Mesozoic and the beginning of the Cenozoic (the K-T event) sixty-five million years ago was causally linked to the impact of a very large meteor on the earth. Significantly, the research that led to this discovery was aimed initially at investigating the conjecture that the apparent rapidity of this extinction was an artifact of stratigraphy. In 1988 W Alvarez wrote:

Yet because of the influences of uniformitarianism, many geologists and paleontologists prefer to explain mass extinctions by gradualist mechanisms which require unlikely combinations of unrelated causal events. Earth science is now at a point where it can no longer afford to be shackled by a dogma of the nineteenth century. (Catastrophes and Evolution: Astronomical Foundations)

The use of the word “dogma” in the above is worth pondering because unlike catastrophism, uniformitarianism, in either its true or false aspects, was not a theory. A theory is an explanation that stands or falls or is modified as evidence accumulates for or against it. A principle is a different thing, in that it does not by itself have any explanatory power. It is simply a method, a way of thinking about or doing things. It stands or falls according to its usefulness and usefulness may extend beyond scientific method into economics and ideology.

Catastrophic extinctions like storms or earthquakes vary in scale but of the twenty that have occurred since the beginning of the Palaeozoic five are singled out as massive because the extinction rate exceeded seventy-five per cent. The Worst of Times describes the eighty-million-year time span from the mid-Permian to the mid-Jurassic, during which two massive extinctions occurred as well as four of lesser magnitude. Wignall gives a detailed account of the most massive one of all, in which ninety-five per cent of all life perished 250 million years ago. This was the second in the series and occurred at the Permian-Triassic (P-Tr) boundary. The account is based largely on field studies in which he has had major involvement over twenty-five years.

The main argument is that the extinctions were due to enormous episodes of volcanism and that their severity was intensified by the peculiar geography of the planet at the time. Wignall begins with the geography. In the Palaeozoic nearly all the land of the earth was concentrated in a single super-continent called Pangea. Shaped a bit like an irregular fat letter C, Pangea stretched from pole to pole. The shallow concavity of the C was open to the east and enclosed the Thetys Ocean with the equator on its southern shore. The Panthalassa Ocean occupied the rest of the planet. Pangea was the last of a series of super-continents formed by cycles of coalescence and fragmentation resulting from movements of the earth’s tectonic plates. The volcanism was caused by columns of magma ascending from the core-mantle boundary of the planet. On reaching the crust they cause uplift and then rupture, which pours huge amounts of acidic gas into the atmosphere, followed by flows of magma over the surface. The lava flows covered very large areas, whose presence today is evident in areas of volcanic rock called Large Igneous Provinces. Sometimes these formations have a stepped appearance due to fluctuations in the rate of lava flow and are called Traps, from the Swedish word for stairs. The P-Tr eruption resulted in the Siberian Traps and it was truly massive. We can appreciate its enormity from the two-million-square-kilometre extent of the Siberian traps. Wignall estimates that over five million cubic kilometres of magma were spewed out over one million years and he compares this with the thirty cubic kilometres produced by the eruption of Tambora in 1815, the largest volcanic eruption of the past thousand years.

In developing his argument he proposes that the eruptions began with explosive emissions of gases, mainly carbon dioxide but also sulphur dioxide, chlorine and fluorine, into the atmosphere. Later, during the long periods of lava outflow, there was a baking effect on the underlying rocks as the magma approached the surface. This caused further carbon dioxide to form as the heated rocks were converted to marble. Two spikes of increased CO2 emission can be detected with a 200,000-year interval between them at the P-Tr boundary. (The amounts of atmospheric CO2 in the past can be measured by comparing the ratios of two different carbon isotopes in marine limestones).

The carbon dioxide caused a prolonged period of global warming and the other gases interfered with the ozone shield. In turn, the warming then caused widespread marine deoxygenation, which is known to have occurred because anoxia causes rapid precipitation of trace metals in water and the latter are documented in the chemistry of marine sediments. The mechanisms responsible for the anoxia were physical and biological. The solubility of oxygen in water decreases as temperature rises and water temperatures after the end of the Permian became as great as thirty to forty degrees centigrade. Again water temperatures in the past can be known by the ratios of two oxygen isotopes in sediments. The biological explanation is related to the fact that the degradation of organic material by bacteria requires oxygen and decay is accelerated by increased temperature. Wignall presents convincing arguments against previously held theories that ocean acidification or hyercapnia (excess CO2) played any part in the extinction of sea life. It has always been a bit of a puzzle that whereas cartilaginous fish species were decimated in the P-Tr extinction the ray-finned fish were left unscathed and experienced a remarkable burst of speciation during the Triassic. Their absence from fossil beds ascribable to the equatorial parts of Pangea would suggest that they simply migrated to cooler, higher latitudes. But if the marine anoxia was as profound and as global as Wignall suggests, it is difficult to see how fish with their high metabolic demands could escape. As an embryologist it would seem to me that a possibility worth considering is the extreme sensitivity of the developing organism to increases in temperature. In vertebrate embryos even a one-degree-Celsius increase in temperature can be lethal or result in serious malformations. It is more likely that marine vertebrate reproduction came to a very halt in equatorial regions.

On land the most striking losses were in plant life. Stark evidence for this is the total absence of coal worldwide in early Triassic strata. Wignall puts it as follows: “In short it would have been possible to walk all over the world in the earliest Triassic without encountering a tree.” This, of course, had a big impact on animal life. Insects suffered the greatest extinction in the entire history of their Class with over forty per cent of Families disappearing. Among the vertebrates, about seventy per cent of species vanished, mainly large-sized types of reptile. Significantly, smaller-sized forms did better which could be attributed to the fact that small body size is a good adaptation to warm climate (Bergmann’s rule).

Overall, recovery was extremely slow in comparison to other extinctions before and after the Triassic. There were to be four further extinctions linked to volcanism at intervals of two, twenty-two, twenty-nine and seventeen million years from the end of the Permian to the middle of the Jurassic, the second last of which, at the boundary between the Triassic and Jurassic, almost equalled the end-Permian event in scale. There was nothing out of the ordinary in this as mass extinctions occur with an average periodicity of twenty-seven million years and there were to be about seven over the last 180 million years (mid-Jurassic to the present). But none of these were as severe as the Triassic series, even though the volcanic eruptions associated with them equalled or were greater than the Siberian one. In the final chapter, Wignall proposes an elegant explanation for this lesser impact of volcanism.

The key to his explanation is that the later eruptions occurred when Pangea was breaking up to form the continental configurations we see today and this made for a more efficient removal of carbon dioxide from the biosphere. There is a cycle that operates to maintain equilibrium between carbon dioxide production and removal, which works as follows. As atmospheric carbon dioxide levels increase, several feedback processes operate to remove it. Firstly, rises in temperature will increase rainfall. Rain, with its dissolved carbon dioxide, will form soluble carbonates from rock that will wash into the oceans along with other nutrients. The nutrients, along with carbon dioxide dissolved in seawater, will stimulate the growth of photosynthesising plankton with fixation of carbon. In addition certain types of plankton form calcitic shells using soluble carbonates, with a further drawdown of carbon dioxide. As these organisms die, they carry the sequestered carbon with them to form the limestones of coastal shelves. On land the increase in plant biomass, as a result of increased humidity and atmospheric carbon dioxide, will be a further source in drawdown. But, Wignall contends, this mechanism will not work on a very large continent like Pangea. Firstly, because most of its land area was too distant from the sea to experience enough rainfall. Secondly, the total perimeter of Pangea was of necessity smaller than the sum of the perimeters of the insular masses into which it fragmented which means that Pangea had a smaller surrounding area of shallow coastal waters than its insular progeny. It is in the shallow waters of continental shelves that the carbon-scavenging organisms thrive. For these reasons, in Pangea the normal negative feedback loop became a positive feedback loop where increased CO2 content in the atmosphere (3,000 ppm compared to 407 ppm today) raised temperatures to the extent that the carbon-scavenging abilities of photosynthesis were overcome and a kind of runaway greenhouse effect kicked in.

The Worst of Times is a book of 199 pages, of which twenty-one are notes and bibliography. Although listed by the publishers as popular science, it is written in a very closely argued style that requires constant attention if one is to follow its thread. And its thread is worth following for it is an excellent account of methodologies that allow the reconstruction of ancient climates and environments. It is also a wonderful narrative of the interdependence of living things with each other and with their environments.


John Bannigan is emeritus professor of anatomy at University College Dublin, where he has, among other things, taught courses in palaeoanthropology.

Space to Think, an anthology bringing together more than fifty of the best pieces to have appeared in the Dublin Review of Books since its foundation ten years ago, will be published in October. Selling in the shops at €25, it is available now for pre-order at a special price of €20 (to collect in Dublin) or €20 + post and packing charges as appropriate for shipping to addresses in Ireland and internationally. To buy online, follow the steps from the home page of our website.

One piece featured in Space to Think is Éamon Ó Cléirigh’s essay from 2006 on the lives, loves and eccentricities of Irish filmmaker Brian Desmond Hurst, "Angel of the North". Here is a short extract:

One weekday morning sometime in the 1970s, the distinguished filmmaker Brian Desmond Hurst made his way shortly after opening time into the Turk’s Head in Belgravia. The pub, which was his regular, was empty save for three labourers from a nearby building site, who were seated over pints of Guinness. The drinkers observed the newcomer, a tall distinguished man in his early eighties, with pale blue, ageless eyes and a shock of white flowing hair. He was dressed in a Savile Row jacket of antique cut, grey flannels, chocolate brown suede shoes and with an emerald green tie set against a white shirt.

The three men were in no doubt what to make of this apparition. “Fucking old queen,” one of them commented. The elderly party affected not to hear and proceeded to the bar, where he placed his usual breakfast order of half a glass of fresh orange juice topped up with champagne. He then turned to his fellow drinkers, instructing the barman: “Please ask those three gentlemen if they would like a drink.” The labourers accepted and, when fresh pints had been drawn, each in turn raised his glass to his lips, murmuring somewhat shamefacedly: “Cheers mate!” “Your very good health,” Hurst replied, raising his glass in their direction. “And by the way, gentlemen,” he added, pausing long enough to oblige them to look at him, “I am not an old queen. I am the Empress of Ireland.”