Supercontinent: Ten Billion Years in the Life of Our Planet (29 page)

Take, for example, the one big question mark over the whole idea of the snowball model as advocated by Paul Hoffman and his supporters. How could a total white-out, involving a global ice-cover perhaps many kilometres thick, ever have allowed photosynthetic organisms in the oceans to survive? Clearly, life
did
survive successive snowballs and therefore, argue the theory’s critics, each so-called snowball must really have been a ‘slushball’, preserving ice-free refuges at the Equator.

The slushball model, on the other hand, produces a slower deglaciation, with no sudden ‘flip’ from icehouse to greenhouse, and
has atmosphere and hydrosphere remaining in balance throughout. A Slushball Earth might not even have had uniformly anoxic oceans. Can this explain the recurrence of BIFs? Some geologists have found evidence for erosion and deposition, with glacier advance and recession at this time. Those who believe in the Hard Snowball have to say this all took place during the short deglaciation phase, because their model predicts a total shutdown of such hydrologically dependent activity for ten million years. This may be true; but is it?

Could computer models help resolve these issues? They tend to produce a runaway snowball effect when levels of atmospheric carbon dioxide are about the same (or only slightly lower) than today. However, not all climate computer models display this kind of
flip-flop
instability. For example, models in which no heat is transported across the ice-line are less likely to go to total snowballs because the tropics stay warm enough to remain melted. Programs that couple atmosphere and ocean circulations are also resistant to global glaciations, because they allow ocean convection at the ice-free tropics. So climate models, on their own, do not provide conclusive evidence because they can be tweaked, within quite reasonable bounds, to support a number of plausible outcomes.

So although models that result in ‘slushball’ solutions keep biologists happy, they do not appear to explain the geological evidence, especially the ‘cap carbonates’, and the temporary return of BIFs, quite as well. The full snowball model also demands that deglaciation be very rapid, which is consistent with the way cap carbonates seem to have been deposited, without any time gap, on top of glacial deposits.

Resolution of this impasse hinges on one crucial question: exactly how thick did the ice of Snowball Earth get? We know how sea-ice thickness and surface temperature are related in modern oceans, and we know from computer models roughly how cold it would have been
during a snowball episode. Applying these simple formulae in a uniformitarian way suggests that, during a full snowball, ice at the tropics should have exceeded a kilometre in thickness, far too thick for any light to get through. How, under a full snowball, could slimeworld have survived even
one
ten-million-year-long night, let alone two (or more)? Clearly, somehow it did.

In 2000 a new suggestion came along to break the impasse, involving a more biology-friendly ‘thin ice’ model. In this model the ice-cover is total, but just a few metres thick at the tropics: thin enough to allow light through while providing enough of a seal to restrict the hydrological cycle to a minimum and prevent the oceans breathing. The idea came from David Pollard and James Kasting, of the Earth and Environmental Systems Institute at Pennsylvania State University.

With ice, thinness and transparency go hand in hand. Opaque ice is compelled to be thick. You could call this another ‘greenhouse effect’, because transparent ice, on the other hand, traps heat like a greenhouse’s glass panes, melts itself from below, and stays thin.

Today’s sea ice is full of inclusions that scatter the light and make it highly opaque. Critics of the ‘thin ice’ idea were quick to point this out; for if ice at the Neoproterozoic tropics was like modern sea ice, it would have been opaque, hence also thick. However, Pollard and Kasting are not so sure about this uniformitarian approach. According to them, the ice at the tropics would have formed by a combination of two processes: the Equatorward flow of ice from high latitudes, forming ‘sea glaciers’, and water that simply freezes on to the bottom of the sheet.

When sea ice grows in the modern-day Baltic, for example, water freezes to the underside of the ice sheet, trapping pockets of brine that make the ice opaque. But Pollard and Kasting’s model suggests that Neopoterozoic Snowball Earth ice would have formed at a mere
seven millimetres per year – much more slowly than modern sea ice. Such slow freezing would have produced much clearer ice.

As for ‘sea-glacier’ ice, its nearest analogue today is the ice seen on land glaciers, like those in Antarctica, which form by the accumulation of snow. In land-glacier ice the main light-scattering inclusions are bubbles of air, which originally lay between the snowflakes before they were annealed together by pressure. According to Pollard and Kasting, for Neoproterozoic sea-glacier ice to have been clear enough for it to stay thin, it must have had a bubble density of no more than about 0.32 per square millimetre, and that lies well within the range of bubble densities seen in the upper parts of ice cores taken from Antarctic ice sheets today.

So it appears that life could indeed have survived ten million years in the chiller, because when the freezer door closed the light didn’t, for once, go out. Slushballers, on the other hand, regard the thin ice idea as an unnecessary sophistication. They do not believe that total ice cover is required to ensure that the oceans stagnate and so accumulate ferrous iron in solution. Evidence emerged in 2005, from organic-rich rocks dated to 700 million years ago, that suggested to Alison Olcott of the University of Southern California in Los Angeles and her colleagues that not only was photosynthesis operating during the snowball, but it was widespread, tropical and happening in stagnant water. If their interpretation of the biomarkers in these rocks from south central Brazil is correct, even the photic zone of the Neoproterozoic ocean was oxygen-free. Perhaps there was a thin ice cover, as Pollard and Kasting suggest; but perhaps there was simply no ice at all. Perhaps a tropical, ice-free waistband around the Earth was just not enough to break up the stratification of the global ocean.

As I finish writing this book at the beginning of 2006, another international conference, this time in Switzerland, is planned for the
summer. More evidence, from all over the world, will be presented in support of new interpretations of these pages from the greatest palimpsest that may settle the controversy between snowball and slushball. But just as Lemuria finally sank beneath the waves of new knowledge, today’s closest approximation to truth slides into myth as the latest ideas are subjected to the evolutionary pressure exerted by the realities of new evidence.

Can it be entirely coincidental that, after three billion years during which the pinnacle of evolution was green slime, complex life burst into existence just as the last snowball melted away never to return? Could it be that, if a low-latitude supercontinent called Rodinia allowed the snowballs to happen, and if the snowballs somehow gave life the kick in the genes it needed to develop complexity, Rodinia really
was
our motherland? Could the vagaries of the Supercontinent Cycle be the main reason why, in place of universal lasagne, we have in Jacques Prévert’s words, ‘New York passions, Parisian mysteries, the little canal at Ourq, the Great Wall of China, the river at Morlaix, legionnaires, torturers, rulers, bosses, priests, traitors, pretty girls and dirty old men’? Could Rodinia be the reason we have such a diverse world? Could Rodinia be the ultimate reason we are all here today, doing what only humans do: wondering how we got here?

To try to answer this question, we need to know something rather accurately. When exactly did complex life first develop? Only when we know that can we hope to judge whether there is a case here to answer.

On 12 November 1931, three years before his death at the age of seventy-six, Sir Edward Elgar and the London Symphony Orchestra performed the trio from the
Pomp and Circumstance March No. 1
(‘Land of Hope and Glory’) for the opening of EMI’s new recording
studios at 3 Abbey Road, not far from a certain pedestrian crossing later made world famous by the Beatles.

The Abbey Road recording was not Elgar’s first foray into this newfangled technology; he made his first recording in 1914, weeks before the First World War broke out. But despite the poor quality of contemporary reproduction (imagine a wind-up gramophone playing 78rpm shellac discs using a needle and a big horn as acoustic amplifier), these ancient recordings actually contain a wealth of sound detail that was invisible – or rather, inaudible – to the old technology. With digital remastering all kinds of unimagined detail can now be heard. All that information was always there, but only the new tools allow it to be revealed. The same is true of the geological record.

When geologists began looking systematically for fossils for the first time in the nineteenth century, using William Smith’s discovery that you could identify and correlate strata of any given age by the fossils they contained, they noticed that rocks below the Cambrian system were barren. The term they gave to the whole ‘Cambrian and younger’ geological record was ‘Phanerozoic’, which means ‘evident life’. The apparent suddenness of life’s appearance posed a great problem for evolutionary theory because at 542 (plus or minus one) million years ago, and seemingly from nowhere, nearly all the main animal body plans (arthropods, molluscs, echinoderms and so on) seemed to burst on to the scene and hit the ground running, swimming and burrowing like there had been no yesterday. It all gave Darwin sleepless nights.

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?