It is easy to picture a planet without oxygen. It looks like Mars. Our nearest planetary neighbour was probably once a water world too, primed for life to evolve. But it lacked a vital ingredient: a protective shield of ozone derived from oxygen. Without an ozone layer, the sun's rays slowly atomised the Martian water. The hydrogen floated off into space while the oxygen oxidised the iron-rich Martian topsoil, turning it rust-red. Perhaps there is - or was - life on Mars. But if so it never progressed beyond the bacterial stage.
So how did Earth get lucky? Ten years ago, when I was writing my book Oxygen, it didn't seem too big a deal. Photosynthesising bacteria were the magic ingredient. These tiny organisms popped up in Earth's oceans early on, sometime between 4 and 3 billion years ago. In the couple of billion years that followed, their oxygenic exhaust fumes slowly did the job. By 600 million years ago, the air was primed for complex animal and plant life.
Now this cosy story has collapsed. We are no longer so sure how Earth's atmosphere got - and retained - its oxygen-rich atmosphere. "Photosynthesis by itself was not enough," says Graham Shields, a geochemist at University College London. "It was a complex dance between geology and biology."
Uncovering life's earliest origins is never an easy task. There are no large animal or plant fossils to draw on: these only make an appearance starting around 600 million years ago. Yet perhaps remarkably, hints of life's humble beginnings do survive in ancient rocks, crushed by the weight of sediment and time. With ardour, patience and skill, they can be marshalled into a convincing story.
William Schopf had those qualities. Two decades ago he thought he had the story, too. A palaeontologist at the University of California, Los Angeles, he was investigating the Apex cherts of Western Australia, 3.5-billion-year-old rocks that are among the oldest on Earth. In 1993, he announced that they contained 11 different types of "microfossil" that looked for all the world like modern photosynthesising cyanobacteria (Science, vol 260, p 640).
The finding fitted a global pattern. Other 3.5-billion-year-old Australian rocks contained rippling structures that looked like fossil stromatolites. A few examples of these structures, domed edifices up to a metre high built by cyanobacteria, still eke out a marginal existence in salty lagoons on the coast of Western Australia and elsewhere. Meanwhile, 3.8-billion-year-old rocks from Greenland had reduced levels of one of the two stable carbon isotopes, carbon-13, compared with the other, carbon-12 - a chemical signature of photosynthesis. It seemed that life had come early to Earth: astonishingly soon after our planet formed some 4.6 billion years ago, photosynthesising bacteria were widespread.
This emerging consensus lasted only until 2002, when palaeontologist Martin Brasier of the University of Oxford unleashed a barrage of criticisms. The Apex cherts, he claimed, were far from being the tranquil sedimentary basin evoked by Schopf. In fact, they were shot through with hydrothermal veins that were no setting for cyanobacteria. Other evidence that the rocks had undergone convulsions in the past made the rippling stromatolites no more biological in origin than ripples on a sandy beach. As for the microfossils Schopf had identified, they ranged from the "almost plausible to the completely ridiculous".
This very public spat produced no clear outcome, but since then new evidence has been emerging. In 2006, Thomas McCollom of the University of Colorado in Boulder and Jeffrey Seewald of the Woods Hole Oceanographic Institution in Massachusetts found that reactions known as Fischer-Tropsch syntheses can occur in hydrothermal vents, leaving a carbon isotope signature that mimics photosynthesis with no need for a biological explanation. The mere possibility that hot water might have massaged the evidence in Australia and elsewhere was damning enough for the duo. "The possibility must be entertained that complex life was not present on Earth, or at least not widespread, until a much later date," they wrote (Earth and Planetary Science Letters, vol 243, p 74).
That conclusion was supported by a reanalysis of "biomarkers" found in 2.7-billion-year-old Australian shales. These organic molecules had been thought to indicate the presence of cyanobacteria, but in 2008 an Australian team concluded that the shales had been contaminated by ancient oil that had filtered down into the sediments some time after the rocks first formed (Nature, vol 455, p 1101). Even more damningly, in September 2009 a French team discovered living bacteria buried deep down in ancient rocks of a similar age (PLoS One, vol 4, p e5298).
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