Oxygen made us. But what made oxygen, asks Nick Lane Life’s a gas XYGEN is life. That’s true not just for us: rocks, crushed by the weight of sediment and all animals and plants need oxygen to time. With ardour, patience and skill, they can O unleash the energy they scavenge from be marshalled into a convincing story. their environment. Take away oxygen and William Schopf had those qualities. Two organisms cannot produce enough energy to decades ago he thought he had the story, too. support an active lifestyle, or even make them A palaeontologist at the University of California, worth eating. Predation, an essential driver of Los Angeles, he was investigating the Apex evolutionary change, becomes impossible. cherts of Western Australia, 3.5-billion-year- It is easy to picture a planet without oxygen. old rocks that are among the oldest on Earth. It looks like Mars. Our nearest planetary In 1993, he announced that they contained neighbour was probably once a water world 11 different types of “microfossil” that looked too, primed for life to evolve. But it lacked a for all the world like modern photosynthesising vital ingredient: a protective shield of ozone cyanobacteria (Science, vol 260, p 640). derived from oxygen. Without an ozone layer, The finding fitted a global pattern. Other the sun’s rays slowly atomised the Martian 3.5-billion-year-old Australian rocks contained water. The hydrogen floated off into space rippling structures that looked like fossil while the oxygen oxidised the iron-rich stromatolites. A few examples of these Martian topsoil, turning it rust-red. Perhaps structures, domed edifices up to a metre high there is – or was – life on Mars. But if so it never built by cyanobacteria, still eek out a marginal progressed beyond the bacterial stage. existence in salty lagoons on the coast of So how did Earth get lucky? Ten years ago, Western Australia and elsewhere. Meanwhile, when I was writing my book Oxygen, it didn’t 3.8-billion-year-old rocks from Greenland seem too big a deal. Photosynthesising had reduced levels of one of the two stable bacteria were the magic ingredient. These tiny carbon isotopes, carbon-13, compared with organisms popped up in Earth’s oceans early the other, carbon-12 – a chemical signature on, sometime between 4 and 3 billion years of photosynthesis. It seemed that life had ago. In the couple of billion years that come early to Earth: astonishingly soon after followed, their oxygenic exhaust fumes slowly our planet formed some 4.6 billion years ago, did the job. By 600 million years ago, the air photosynthesising bacteria were widespread. was primed for complex animal and plant life. This emerging consensus lasted only until Now this cosy story has collapsed. We are no 2002, when palaeontologist Martin Brasier of longer so sure how Earth’s atmosphere got – the University of Oxford unleashed a barrage and retained – its oxygen-rich atmosphere. of criticisms. The Apex cherts, he claimed, “Photosynthesis by itself was not enough,” were far from being the tranquil sedimentary says Graham Shields, a geochemist at basin evoked by Schopf. In fact, they were shot University College London. “It was a complex through with hydrothermal veins that were no dance between geology and biology.” setting for cyanobacteria. Other evidence that Uncovering life’s earliest origins is never the rocks had undergone convulsions in the an easy task. There are no large animal or past made the rippling stromatolites no more plant fossils to draw on: these only make biological in origin than ripples on a sandy an appearance starting around 600 million beach. As for the microfossils Schopf had years ago. Yet perhaps remarkably, hints of identified, they ranged from the “almost life’s humble beginnings do survive in ancient plausible to the completely ridiculous”. > ANDREW HEN 36 | NewScientist | 6 February 2010 6 February 2010 | NewScientist | 37 ”With all of oxygen’s ups and downs, life on Earth was Did lichens help oxygen to finally break free? even luckier than we thought to get as far as it has” This very public spat produced no clear Our best guess is still that cyanobacteria shows that, by 1.9 billion years ago, levels of outcome, but since then new evidence has were around some time before this event. breathable oxygen in Earth’s atmosphere were been emerging. In 2006, Thomas McCollom Persuasive evidence is converging on a date back down to the merest trace. of the University of Colorado in Boulder and around 2.7 billion years ago (see diagram, We don’t know why. It might have been a Jeffrey Seewald of the Woods Hole right). Research from Linda Godfrey and knock-on effect from a big freeze: if Earth did Oceanographic Institution in Massachusetts Paul Falkowski of Rutgers University in New indeed enter a snowball phase, glaciers would found that reactions known as Fischer- Brunswick, New Jersey, indicates that the have scoured huge amounts of nutrients from Tropsch syntheses can occur in hydrothermal modern nitrogen cycle kicked off around this the underlying rock. When the ice eventually vents, leaving a carbon isotope signature that time. This requires free oxygen to form nitrogen retreated, melted by the build-up of volcanic mimics photosynthesis with no need for a oxides, suggesting that a first whiff of oxygen – greenhouse gases in the atmosphere, those Y RAR biological explanation. The mere possibility not even 1 per cent of today’s levels – had just nutrients would have found their way into the OTO LIB that hot water might have massaged the appeared (Nature Geoscience, vol 2, p 725). oceans. One idea is that they nourished a huge H P E evidence in Australia and elsewhere was That squares with evidence from Robert Frei transient bloom of cyanobacteria that quickly C EN damning enough for the duo. “The possibility of the University of Copenhagen, Denmark, died and rotted, in the process consuming all /SCI E C Mind the gap must be entertained that complex life was not and his colleagues that the oxidative the oxygen they had once produced. EN present on Earth, or at least not widespread, weathering of rocks kicked off around this time OF SCI E Y If, as seems increasingly likely, photosynthesising until a much later date,” they wrote (Earth too. They measured levels of chromium in E cyanobacteria first made an appearance in Earth’s and Planetary Science Letters, vol 243, p 74). ancient marine rock layers known as banded Stinking oceans oceans around 2.7 billion years ago, why did they That conclusion was supported by a iron formations. Exposed to oxygen in the air, Oxygen levels in the atmosphere soon cyanobacteria did produce the first oxygen Earth’s anoxic stasis was broken in the take so long to make a difference to Earth’s air? reanalysis of “biomarkers” found in the metal is weathered from rocks and washed recovered again as rates of photosynthesis and in Earth’s atmosphere, all the evidence is they end by a dramatic series of snowball Earths, One possibility is that the oxygen’s first 2.7-billion-year-old Australian shales. These out to sea, where it reacts immediately with weathering established a new equilibrium, at lacked the oomph to push levels much above indicating bursts of oxygen, beginning about chemical mission was to oxidise all the iron and organic molecules had been thought to iron, settles to the ocean bottom and forms about 10 per cent of present-day levels. But 10 per cent of present levels in the long term. 750 million years ago and recurring over the compounds like hydrogen sulphide in the oceans. indicate the presence of cyanobacteria, but in these layers. The chromium signature in them this was no fresh dawn of a high-octane world: That has led William Martin, an expert in following 100 million years. They broke the Only after it had done that was it free to escape 2008 an Australian team concluded that the suggests there was essentially no oxidative quite the reverse. This time, the oxidative cell evolution at the University of Düsseldorf eternal loop: soon afterwards, oxygen levels into the atmosphere. shales had been contaminated by ancient oil weathering before 2.7 billion years ago, after weathering of sulphides on land filled the in Germany, and others, to come up with a shot up and never looked back. Animal life Perhaps the most persuasive answer, that had filtered down into the sediments which chromium became significantly more oceans with sulphate. That in turn fuelled a controversial theory: that the boring billion soon exploded onto the scene. though, is purely geological. It comes from some time after the rocks first formed (Nature, mobile (Nature, vol 461, p 250). hardy group of bacteria that filled the oceans was anything but boring. In fact, the stinking What made the difference this time? One veteran geologist Heinrich Holland of Harvard vol 455, p 1101). Even more damningly, in If these coordinated changes are the calling with sewer gas – hydrogen sulphide – turning oceans were the true cradle of life. Evidence intriguing possibility is that it was down to the University. He points the finger at gases such September 2009 a French team discovered cards of the first photosynthesising bacteria, them into stinking, stagnant waters almost behind this idea includes the fact that organisms that had evolved in a leisurely way as methane and hydrogen sulphide that are living bacteria buried deep down in ancient there is still a mysterious hiatus of 300 million entirely devoid of oxygen, rather like the mitochondria, the powerhouses of all during the boring billion: terrestrial red and constantly spouted out by volcanoes.