Supercontinent: 10 Billion Years In The Life Of Our Planet

by Ted Nield

  Since then, geologists’ reading of the record has become more sophisticated. Now, instead of just looking for things they can hold in their hands, they can detect fossils and fragments of fossils mere microns across. Using the tools of organic chemistry, they can even pick up the molecules of life, so-called ‘biomarkers’. These chemicals, many of which are quite specific to certain types of organism, are remarkably durable in the fossil record. Now, when apparently barren rocks are ground up, dissolved in organic solvents and passed into a mass spectrometer or a gas chromatograph, these telltale molecules stand out as diagnostic peaks in the instrument’s read-out: the merest whiffs of long-vanished life.

  Darwin often invoked the imperfection of the fossil record to get himself out of such difficulties with evidence, and he was quite right to do so. The circumstances under which a fossil can form are rare. The nineteenth century’s fossils, the hand-specimen or ‘macrofossil’, is a very scarce beast. Microfossils and molecules, on the other hand, have a much higher chance of being preserved, and so provide a much more reliable tool for judging ‘first appearance’.

  Think about it: if an organism evolves at a certain moment, it will take some time for this creature to become common. Yet even its hard parts (its shell, or bones) stand a very slim chance of being preserved as fossils at any time, let alone during that species’ very earliest days on Earth. Pile upon these slim chances the chance of those rare fossils surviving the vicissitudes of all subsequent geological history and then add the further unlikelihood of their being found, and you produce some very unfavourable odds indeed. So, any macroscopic species’ first appearance in the fossil record is bound to post-date its true first appearance on Earth.

  But microfossils are different. Microscopic things are everywhere in the environment – ask a hay-fever sufferer. We are trying to produce an accurate date for the first appearance of complex multicellular animals, when the first actual fossils we may discover will post-date that event quite considerably. It would be very useful if the creature in question produced something durable and microscopic in astronomical numbers. Unfortunately, unlike modern plants with their spores or pollen, animals don’t do this.

  Alternatively, you could test for animals’ first appearance by using some superabundant microfossil as a proxy. It is a reasonable assumption that the appearance of multicelled animals had profound ecological effects, and that these might be visible in the remains of other organisms. It was, after all, the first time any of them had ever been eaten. You would expect this to provoke an evolutionary response. You might therefore be able to detect something both in the appearance of microfossils (a change to the roughness and durability of their armour-plating, for instance) and in the style and pace of their evolution.

  Such a potential proxy group exists in the rather unprepossessing form of tiny organic sacs called acritarchs. Acritarchs are an ancient but artificial grouping of microscopic (20–150 microns across), organic-walled fossils found in nearly all sediments – once you have dissolved away everything else in bath after bath of strong acids. Acritarchs are thought to represent the ‘resting cysts’ of single-celled algae with a many-staged life cycle. They were first discovered in 1862, but the term ‘acritarch’, which just means ‘of uncertain origin’, was only coined in 1963. Acritarchs are useful for correlating sedimentary rocks of Proterozoic and Palaeozoic age mainly because they were the only microfossils around then; but their usefulness as correlation tools increases enormously after the snowballs.

  The oddest thing about acritarchs is that before the younger of the two main snowball events, the Marinoan glaciation, individual acritarch species tended to exist for 1000 million years: something inconceivable in the modern biosphere. But after this period of extreme evolutionary stasis, at about 650 million years ago, everything changes. Thereafter, acritarch species tend to survive only for a few tens of millions of years. Something fundamental in the way the biosphere worked had changed.

  This is thought to be the proxy evidence pinpointing the moment that planktonic animals appeared. For the first time, the acritarch-producing algae found themselves to be prey. Before then, the seas had probably been a saturated algal soup, with nothing more than the availability of nutrients to control runaway growth. No wonder things did not change for a billion years. Why should they?

  But now things were different and soup was on the menu. Life had turned on itself for the first time, and the selection pressure this imposed accelerated the origin and the extinction of new acritarch species. The spiny, heavily armoured and relatively short-lived forms of the post-snowball world are nothing less than the first tooth and claw marks of nature’s new order.

  A little later, at around 580 million years ago, we find the first unequivocal animal fossils, in the form of a beautifully preserved clusters of cells representing embryos of an animal (though we cannot say what sort), in the mid to upper parts of the Doushantuo Formation of southern China. Not long afterwards the first trace fossils – those telltale marks made by animals moving across and through the sediment – first appear, in rocks no younger than 558 million years old, in the Verkhovka Formation of north-west Russia.

  The period we are speaking of is called the Ediacaran, now officially named for the remarkable forms over which Mark McMenamin and many others have been puzzling. The Ediacarans were part of this new evolutionary order, but although some may have evolved into animal forms that are still around us today, others (perhaps, who knows, those photosynthesizing animals in McMenamin’s vision of the Ediacara Garden), after first trying sheer size as an evolutionary refuge against the mounting pressures against them, finally succumbed. They succumbed to the destruction of the algal lasagne they rested on and ended up as lunch for those voracious new predators that, tiring of soup and lasagne, moved on to the meat course.

  Code breakers

  There is one final line of evidence for life’s sudden dash to complexity after the snowballs, suggesting that these climatic crises were indeed what set the process off. That evidence is found inside the molecule of life itself. DNA, the molecule that carries the genetic codes and governs our growth and development by regulating the process whereby amino acids build proteins, is vast; yet it is a lot vaster than it needs to be, since large tracts of every organism’s genome have no known function. Because they are not expressed in the biology of the organism, they are not subject to natural selection like other areas of the molecule.

  Natural selection is often thought of as what ‘changes’ organisms, but this is only true if something in the environment is changing, disturbing the equilibrium. Under stable conditions, natural selection is a strongly conservative force that keeps things the way they are and makes sure that things that aren’t broke don’t get fixed. But those areas of the DNA molecule that are not expressed biologically are able to mutate freely without impairing an organism’s evolutionary ‘fitness’. Biologists have found that, left to its own devices in this way, this so-called ‘junk’ DNA mutates at a rate that (over geologically long periods of time) is constant enough to be used as a clock (though it has to be regulated by reference to a good fossil record).

  Despite what the film Jurassic Park would have you believe, very little DNA is preserved in the older fossil record, and the further back you go, the less survives. So if you want to use DNA to date events that took place more than 500 million years ago, finding DNA from those times is not an option. But fortunately there’s no need; we can look at the DNA of different groups of living animals (sponges, worms, molluscs etc.) instead. Then we can combine what we know about the rate at which the DNA clock ticks, with a statistical expression of the difference between the junk DNA sequences of sponges, worms or molluscs, to work out how far back we would have to extrapolate these in order to make the different groups’DNA look the same. The result should be the point in time at which the groups’ evolutionary paths diverged, and it should be confirmed by the fossil record. On the supercontinent of science, everything must
fit together. The molecular biology of today’s animals confirms the fact that they are all related, that some groups split before others and that every living thing is the product of its unique history: a history that dates back billions of years and ultimately makes us one with the stars.

  To confirm whether the snowballs and the Supercontinent Cycle might have been responsible for creating complex life, the crucial evolutionary divergence that we want to date is the one that gave animals the power to move purposefully, even through sediment (burrowing), to ingest material, process it in a gut and expel excrement. For these were the developments that changed nature for ever and finally dug up the Garden of Ediacara. This evolutionary event was all to do with how the tissues of animals became organized and differentiated in the embryo.

  Sponges are simple multicellular animals, but they are little more than balls of barely differentiated cells. Jellyfish, on the other hand, are two-layered animals. This allows them to move, albeit in a rather undirected way, as they drift with currents or to move away from harm (they hope) when warned by their rudimentary sense organs. They have an outer layer of cells and an inner one, separated by a springy, gelatinous mass. Muscles around a jellyfish bell contract against the springiness of this mass, which bounces back into shape when they relax, allowing the swimming cycle to start again.

  But the next development in the history of embryology was crucial because it created animals with three layers of tissues, the middle layer developing from the jelly-like stuff as it became invaded with cells. Moreover, the embryos of the new three-layered animals developed a method of ‘turning in’ on themselves. First, the embryonic ball of cells became hollow, to create the body cavity. A fold in the outer layer then developed, forming a pouch on the inside. This became budded off along most of its length, but remained connected to the exterior by a single pore that developed into, literally, the fundamental opening: the anus. The mouth developed later, at the opposite end. You can see this evolutionary process re-enacted every time a human embryo develops today, and it is the reason why you have a body cavity filled with tubular guts.

  Three-layered animals were a giant leap. The outer layer gave skin, nerves, ears and eyes. The middle layer gave rise to muscle, bone and the circulatory system; while the inner layer created the digestive, glandular and respiratory systems. Worms, snails and humans all have this same basic organization. Having three layers meant having more cells per unit volume, and separating the digestive tube from the body wall created problems of transport. Oxygen needed to be brought in, and wastes and nutrient carried out. All this required ducts, circulation systems and respiratory mechanisms that simpler animals didn’t need because they could live by absorption alone.

  Above all, three-layered animals were directed by sense organs concentrated at a head end, and their muscles enabled them to move in more or less any direction in active pursuit of food. The mouth, being also at the head end, was the first thing to arrive at the food source, ingesting what it found, filling the gut with a mixture of food particles and sediment. Burrowing and grazing modes of life were born; and as a result, trace fossils started to appear in the fossil record, and the fine, undisturbed laminations of Lasagne World became rare as the Garden of Ediacara went under the plough.

  According to the molecular clock, the first three-layered animals, complete with heads and nerves and muscles and guts, should have appeared about 580 million years ago, which places this evolutionary event right in the middle Ediacaran (630–542 million years); just after the Marinoan snowball had melted, after its cap carbonate had been safely deposited, and the climate had settled down again. What is more, 580 million years also turns out to be the date when fossils of the Ediacaran animals, though already doomed, became abundant enough to be preserved in non-exceptional circumstances. And all this took place barely forty million years after the last of the two major Neoproterozoic white-outs came to an end.

  As scientists are fond of pointing out, correlation need not imply causation; but to many, these convergences are too consistent and too numerous to be meaningless coincidence. If true, the correlation between the origin of complex life and the end of major snowball episodes firmly ties our own origins to the Supercontinent Cycle, because it was the chance siting of continents around the Equator 1000 million years ago that made those all-important snowball events possible.

  Life itself probably owes its origin to the geology of ocean-floor hydrothermal vents. But that part of it that isn’t slime may owe its brief 580-million-year tenure of this planet to nothing more than random turbulence within the Earth’s convecting mantle, which once swept the continents to the tropics. There, far from stopping a runaway snowball, their fragmentation from maximum packing enhanced weathering, created volcanic cooling and multiplied the length of shallow, lasagne-covered sea floor, just at the time when a billion and a half years of photosynthesis and carbon sequestration had already made the Earth System especially vulnerable.

  Out of this catastrophe came glorious, complex life. And now, we are blessed (or cursed) with the brains to work it all out for ourselves. The question then becomes: will we bother, or will we sit, like Albrecht Dürer’s Melancholy, surrounded by the tools that can help us explain the world, but too indolent to use them?

  Melancholia by Albrecht Dürer. Hapless Melancholy lies surrounded by the untouched tools of science and art. Reproduced by permission of the Mary Evans Picture Library.

  EPILOGUE

  LIFE, THE UNIVERSE AND THE PUDDLE

  For nature, heartless, witless nature,

  Will neither care nor know

  A. E. HOUSMAN, LAST POEMS XL

  On the morning of Sunday, 26 December 2004 about 1300 tourists and pilgrims crowded on to Kanyakumari rock, a charnockite islet 200 metres or so off the southernmost tip of India. Geologists call the islet ‘Gondwana junction’ because it marks the 550-million-year-old suture where India, Madagascar, Sri Lanka, East Antarctica and Australia once joined together to build the eastern portion of Suess’s Gondwanaland. But to followers of Vedantist spiritual philosopher Swamy Vivekananda (1863–1902), the rock is remarkable for being the foundation for his memorial, which opened to visitors in 1970. This was the impressive structure that drew them to Kanyakumari as the sun came up over the Bay of Manar that morning.

  Mr G. Ramalingam of the Port Authority remembered afterwards that they had sold 3469 tickets for sailings between 8am and 9.45am, though they had suspended sailings to another outlying rock, Valluvar, at 9am – a decision that probably saved about 500 lives because the monument to the great Tamil poet, a forty-metre-high statue on a square plinth, offers no shelter. For 26 December would prove to be a much more memorable day than anyone expected. Although not one of the tourists and pilgrims would die at Kanyakumari, some 230,000 others around the Indian Ocean would lose their lives before the day was out.

  Perhaps as they were preparing for their expedition, some of those pilgrims may have been aware of a slight earth tremor; but most of them either didn’t notice or slept through it. Yet even as that distant seismic shock rumbled through India and around the world, slower and much more deadly waves began spreading across the Indian Ocean. By the time the pilgrims climbed aboard the ferry of the Poompuhar Shipping Corporation that would take them to the island, tens of thousands were already dead in Indonesia. Thousands more lives were being lost in Sri Lanka, just over the horizon. Soon the wave would turn the corner and sweep up Sri Lanka’s west coast and bear down upon Kanyakumari.

  No surprise

  The deep ocean trench that skirts the Indonesian Archipelago on the other side of the ocean, marks the contact between two of the tectonic plates making up the cracked eggshell of the Earth’s crust. One is the Australian Plate, consisting of Australia and the floor of the Indian Ocean, and the other, to the north, carries Europe and Asia and is called the Eurasian Plate. At this trench the floor of the Indian Ocean is subducting, sinking down into the mantle, beneath the island arc of Indonesia. This
is but one small part of the long process of building the next supercontinent, piece by piece, each fragment edging into place, just as India has already been annealed to Asia in the collision that is today creating the Himalayas and the Tibetan Plateau.

  In many ways the earthquake that caused the 26 December tsunami should have taken nobody by surprise. There are known to have been two great earthquakes of over magnitude 8 along this part of the Indonesian Arc: in 1833 and 1861. The zones of rupture that caused these two events sit along adjoining, non-overlapping parts of the same plate boundary, adjoining the Batu Islands. No quake of similar size happened during the twentieth century, until June 2000, when a 7.9 quake struck near Enggano at the extreme south-eastern end of the 1833 rupture zone. The 26 December event extended movement along the plate boundary from the island of Simeulue, at the other end of the chain, almost to the coast of Myanmar (Burma). This left a gap of a few hundred kilometres between Banyak and Simeulue over which no movement at all had taken place in historical time. The omission was rectified on 28 March 2005, when the last ‘stuck’ part of the fault gave way in an 8.6 magnitude tremor that thankfully produced no very serious widespread tsunami.