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SPRING 2003 - The Moving Finger

Science musings from a desktop in West Oxfordshire

Fossil Feuds from the Edge of Time

Mad Max

In a recent article for Chemistry in Britain I addressed one of the biggest controversies to rock the science of palaeontology for some years. It's the debate over whether the Apex Chert fossils really are the oldest fossils on Earth. I also examine the latest evidence that suggests that traces of photosynthesis thought to be left in even older rocks aren't traces of that at all. Here's the text of the article in full:

'My life fades, the vision dims… all that remain are memories. I remember a time of chaos, ruined dreams, this wasted land…' So said an aging Feral 'Kid' at the end of Kennedy Miller's harrowing dystopian tale of a post-apocalyptic Australia where the average traffic speed is at least 200 miles per hour despite a chronic shortage of gasoline.

As a kid myself I can still remember watching the unfolding stunt sequences on the big screens outside the Odeon Leicester Square, as the extras and the stuntmen crashed and tumbled amidst the chaos of the Broken Hill outback painting their vision of a world that was fast losing its dependence on gasoline and descending into terminal decay faster than you could say 'Arnold Schwarzenegger'. And yet there is an irony here, for, as far as the new science of fossils is concerned, Australia is not about dystopian endings at all but rather is about utopian beginnings – indeed the most fundamental beginnings of all, for Australia is the land that has the oldest fossils on Earth – or does it? Far in the northwest of Western Australia there is a landscape that could well have formed the backdrop to the Mad Max movies. It is a land of dust and grass; thick brown dust that rises in choking clouds in the fifty degree heat of the Austral summer or which turns to a viscous mud so dense in the winter rains that it will trap the most nimble 4x4; silica-tipped Spinafex grass that will chew your boots to ribbons as you make the long trek from the town of Marble Bar out to the eroded, desiccated stream bed known as 'Chinaman Creek'. As you walk you begin to see outcrops of glassy looking rock in the edges of the rills and gullies and you know that you are now walking on rock of a singular age – you are walking on some of the most ancient rocks in the world. You are walking on the Apex Chert.

Back in the 1980's a young scientist from the University of California at Los Angeles came to this blighted place in search of fossils. Bill Schopf was interested in the history of Precambrian life, that long era of Earth history spanning the Achaean and Proterozoic - each of which dwarfs the duration of the Phanerozoic that started some 550 million years ago with the rise of conventional fossils.

Schopf was laughed at by his colleagues, for how could you expect to find fossils in rocks that were so old they surely predated the evolution of bacteria? But Schopf was a determined man: he'd been involved in describing the unassailable bacterial fossils of the Gunflint Chert in northern North America and these were a staggering 2 billion years old (2Ga). These organisms had lived near the beginning of the Proterozoic, the younger of the Precambrian's two aeons (see fig 1). And if bacterial fossils of 2 Ga could be found, why not older?

The importance of this question could not be overstated because it brings us nearer to knowing when life on Earth began. Schopf knew well that there were only two places in the world where rocks of this age that might contain fossils were likely to be found; in the rocks of the Australian Pilbara Supergroup or in the rocks of the hills and valleys of the Barbeton Mountain Land of South Africa, the Swaziland Supergroup. Both of these sedimentary terrains have been pressure-cooked to some extent by post-depositional heating and sinking within the Earth's crust but the Barbeton Range has been cooked longer and hotter than the rocks of the Pilbara. This decided Schopf, he would make the best of this bad hand and travel to Australia to look for his fossils.

The outcome of his studies was the publication in Science in 1993 of a description of eleven microfossils that were claimed to be the oldest fossils in the world. They were all thought to be members of the cyanobacteria (the blue-green algae). The news was sensational at the time was because age-dating of the encapsulating rock units (using the very accurate uranium-lead technique, see Searching for Realtime in the January issue of Chemistry in Britain) constrained the age of the fossils to about 3.5 Ga - to put this into perspective remember that the dinosaurs became extinct 65 million years ago! The clear implication of this date was that life must have got started very soon indeed after the Earth formed (at 4.5 Ga) to have produced something as complex as single celled organisms that left morphological traces of themselves in the fossil record.

Because rocks of this Achaean age have all been metamorphosed to some extent, and since cyanobacteria don't have much morphology to start with, experts often disagree whether traces in the rock are truly fossil or simply a mineral artefact. The recent controversy over the 'nanobacteria' found in the Martian meteorite ALH84001 makes this point all too clearly. In the case of the search for life at this extremity of time these morphological clues are already right at the edge of credibility and will not do if we wish to extend the search to older rocks. But chemistry provides an answer that is considered definitive: the presence in the rock of isotopically depleted carbon. This technique relies on the fact that the Rubisco enzyme system of photosynthesis preferentially incorporates the light isotope of carbon (carbon-12) over the heavy isotope (carbon-13). This in turn is a consequence of the fact that carbon-12 is more kinetically active than carbon-13 and so encounters the binding site on the Rubisco enzyme more frequently.

Since photosynthesis is a very ancient biochemical trick that is common to simple unicellular as well as multicellular organisms it is reasonable to assume that if isotopically light carbon is found in ancient rocks then this is evidence for photosynthesis. At first sight is seems disarmingly simple to use something a trivial as a negative carbon isotope ratio to infer something as massive as the presence of life on Earth. But the truth is that there still seems to be no unequivocal way of inorganically mimicking the negative carbon isotope compositions that go hand-in-hand with the presence of life.

There is one place - and one place alone - where there are sedimentary rocks that are older than those of the Pilbara or Barbeton rocks and these are the Isua supercrustal rocks of western Greenland. They are a staggering 3.8 billion years old and naturally have been the target for investigating life's earliest history: in fact their investigation predates that of the Apex Chert fossils. But the Isua supercrustals have one big problem: they have been seriously pressure-cooked. Estimates suggest that within a billion years of deposition these sediments were subjected to temperatures in excess of 500 degrees C and pressures of greater than 5,000 atmospheres!

In short any traces of a bacterium or blue-green algal cell's morphology will have been totally wiped out. All we can do to check for the presence of life is to use the carbon isotope assay. This is truly testing for life at the edge of time for the Isua rocks are almost 400 million years older than the fossils of the Apex Chert; that is about the same amount of time that separates humanity from the doughty lung-fish that decided to make the leap out of the ocean and try their hand at living on land.

The carbon test was applied to the Isua rocks in the late 1970's. It was a bulk isotopic analysis of the carbon in the rock. 'Bulk' simply means that the entire sample was homogenised and analysed, necessary in the days before mass spectrometers became refined enough to work on truly tiny samples. The results were equivocal: only a little more negative than the values that could be caused by inorganic carbon isotope fractionation (see Fig 2), but for true believers they were hailed as evidence that life had existed as early as 3.8 Ga.

But technology moves on and by the mid-1990's it was no longer the case that an entire sample had to be homogenised to get enough carbon to measure isotopically. It was now possible to use an ion-microprobe to direct a beam at specific particles within the rock, ionise discrete carbon inclusions and measure their isotopic composition by magnetic-sector mass spectroscopy. This was the technique used by a team led by Stephen Mojzsis in California in the mid-1990s.

Mojzsis' team focused not merely on the Isua rocks but a specific enclave that crops out on the offshore island of Akilia, where the rocks are a staggering 3.85Ga - fifty million years older than the surrounding rocks. The team made multiple analyses of picogram sized particles of carbon within the rock and discovered that without exception they were all isotopically depleted i.e. enriched in carbon-12. Moreover the ratios were far from the range diagnostic of inorganically derived carbon (fig 2) strengthening the case that they could only be the spoor of ancient photosynthesis. In addition the carbon globules were encased within apatite grains - the mineral building block of bones and teeth. Although apatite can be formed inorganically its presence in the rock with isotopically depleted carbon was doubly suggestive of the presence of life.

The dating of these Akilia Island samples was more controversial than that of the rest of the Isua complex. Even so there was one extraordinary and inescapable finding; it seemed certain that life evolved during, rather than after, the period of Earth history known as the 'late heavy bombardment' period of the Hadean Era, the era immediately prior to the Archaean. The Hadean ('hell-like') is so-called because at this time the Earth and its sister planets - the Moon and Mars - were still being bombarded by debris left over from the formation of the solar system. Curiously enough we know this by dating rocks from lunar craters returned by the Apollo missions - all of the Earth's own craters of Hadean age have long since been subducted and recycled by the motions of its tectonic plates. The late heavy bombardment finished as far as we can tell about 3.8Ga - the same age as the Isua rocks which supposedly already contained good evidence of life. It followed therefore that photosynthesis must have evolved a good while before this; during the late heavy bombardment period.

Chart 1
Chart 2

Life evolving under these circumstances is quite different to the way that Darwin envisaged it happening, in a 'warm little pool' of favourable prebiotic compounds and a few aeons of time for some gentle chemical cookery.

Indeed it seems probable that if these Akilia carbon inclusions are the spoor of life then it evolved somewhere away from the surface bombardment, perhaps in the depths of the ocean near hydrothermal vent systems.

New genetic evidence from contemporary bacteria that inhabit these environments seems to indicate that their biochemistry is very ancient indeed; could these perhaps be our own ultimate ancestors? It has also been suggested that wherever life evolved on this hellish Earth it may have needed several tries to get going, being repeatedly snuffed out by marauding asteroids that made the impact that did for the dinosaurs look as delicate as a falling thistledown.

But there is a new controversy over the rocks of Akilia Island. Central to the Mojzsis team's findings is the 'geological context' from which they took their samples, the certainty that no matter how cooked they are today these rocks were once sedimentary (part of a Banded Iron Formation, BIF). But even this most basic assumption has been challenged. In the 24 May 2002 issue of Nature Christopher Fedo and Martin Whitehouse argue that the rocks that the Mojzsis team took their samples from are not sedimentary BIFs at all but rather are igneous rocks - formed by the action of ancient volcanism. If this is the case then all bets are off and we need to re-examine the certainty that only organic chemical reactions can result in very negative carbon isotope ratios. We are also back to Schopf's bugs as the oldest evidence for life on Earth.

Or are we? The detailed Isua and Akilia isotopic analyses were all inspired by Schopf's finding of the Apex Chert fossils so it is doubly ironic that it is these themselves that are now the subject of renewed controversy. The evidence unseating the Apex Chert's claim to contain our planet's oldest fossils is once again the 'geological context' from which the samples came and the interpretation of carbon-isotope chemistry, albeit investigated by a different analytical technique, laser-Raman spectroscopy.

When a laser beam of a particular wavelength hits a target material, most of the light that bounces off the material remains the same wavelength. However a small proportion of the scattered laser light changes wavelength and the magnitude of the wavelength shift is determined by the molecular composition of the targeted material. This is the Raman effect. It turns out that every type of molecule has a unique signature. So laser-Raman spectroscopy is a step beyond the ion-microprobe. It can yield the molecular composition of materials, including fossils.

Schopt vs. Brasier

Martin Brasier and colleagues at Oxford University recently re-examined the Apex Chert samples and presented evidence suggesting that the structures within are all mineral artefacts. The structures are branched, Brasier and his colleagues insist - a morphology that is inconsistent with Schopf's claim that they are bacteria. Furthermore the Brasier team presents geological evidence showing that the rocks of the Apex Chert are criss-crossed by lava flows of the same age as the supposed sediments, negating Schopf's claim that the rocks were deposited under quiet near-shore conditions.

In the opposite camp, and in the same issue of Nature, the Schopf team presented isotopic evidence (using the laser-Raman technique) that seems to show conclusively that the fossils are all isotopically depleted. But Brasier and colleagues - who also employed Raman spectroscopy - conclude that the carbon is isotopically depleted not as a result of photosynthesis but instead because of a set of chemical reactions known as Fischer-Tropsch synthesis. Here carbon monoxide and hydrogen are catalysed to more complex carbon compounds at high temperatures in the presence of catalysts such as iron oxide.

This reaction may be accompanied by a shift to negative carbon isotope ratios (there is no clear consensus yet) that could mimic the isotopically negative signature supposedly uniquely indicative of life. Indeed there is evidence to suggest that the Fischer-Tropsch type synthesis is responsible for the negative carbon isotope ratios found in certain carbonaceous meterorites, an interesting parallel given that the search for the most ancient life on Earth requires the same types of techniques needed to identify the presence of life on other planets.

At the present time it is hard to disentangle the competing claims and counter claims for true photosynthetic fractionation versus the Fischer-Tropsch reaction in the Apex Chert debate. At first sight it seems strange to invoke a particular type of inorganic chemical reaction when three decades of previous interpretation have all favoured the isotopically depleted carbon -photosynthesis link.

However it is clear that the difficulties involved in unambiguously identifying life at these edges of time and space are extreme. The quality of evidence must be unassailable - this is not now the case with the Isua, Akilia and Apex Chert fossils. Time of course will sort out the Brasier-Schopf and Mojzsis-Fedo controversies and to the victor the spoils. But a more important question than academic bickering remains the origins of life itself. Did it really get snuffed several times before finally catching on? And was there one discrete moment, perhaps captured in the rocks of Akilia Island, where, to paraphrase my favourite Australian road-movie, 'it learned to live again?'

Max

Thanks to Cal for the images of Mad Max (Mel Gibson) used in this article. One of the most extraordinary, moving and spiritual places on the web was once at www.max.org

This link has now been deactivated

Cal - her real name was S. Geoffre - was killed in a road accident in early January 2003. Her site has now shut down and seems to have been replaced by yet another that simply tries to sell things to people. That site has no endorsement from mine. To those of you out there who try to make the web a worthwhile place where you can express yourself with freedom and decency, the message is simple: Don't ever let your domain name go!- even after you die.

Cal was an extraordinary character who helped me greatly with the thoughts -and images - that I needed to write this article. I never knew her properly - indeed I only knew her via the internet - but I do wish I had had a chance to know her as a person. I would have loved to have visited her in the State that she seemed to have adopted as her own - Arizona. A place where the roads are straight and fast - and the spirit of Max, like hers, will live forever.

God Bless You, S. Geoffre. We will miss you.

WINTER 2002
SUMMER 2002

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Richard Corfield 2003 in association with pedalo.co.uk