Science

New research has reopened debate about the origins of oxygen, the precursor to complex organisms. By Wendy Zukerman.

When did the Earth get its oxygen?

Volcanic activity may have scrubbed the atmosphere of oxygen at various times in the planet’s history.
Credit: MINT IMAGES

Take a deep breath – but don’t take it for granted. For most of our planet’s history, Earth was asphyxiated, starved of the life-giving gas oxygen. Today, the quest to understand how oxygen finally appeared in our atmosphere is shrouded in mystery, and remarkable controversy.

Not just a simple curiosity, understanding how oxygen came to blanket Earth will give us unprecedented insight into the forces that keep our planet healthy. It could even help find life on other planets.

“Oxygen’s emergence caused the biggest change that Earth has experienced,” says Jochen Brocks, an associate professor at the Australian National University. “It’s such powerful chemistry. If we understand where oxygen was first produced, we can understand a lot.”

Of course, whiffs of gas from billions of years ago don’t leave fossils behind. Instead, they leave indirect, and often confusing, fingerprints in ancient rocks. Interpreting their secrets has led to aggressive disputes between academics. Textbooks have been ripped apart (or at least some pages torn out). But in the aftermath scientists are now piecing together a new vision of oxygen’s origin story.

Our home planet was born in a violent game of meteoric billiards. Four-and-a-half billion years ago space rocks pelted an infantile Earth. Once they stopped, an atmosphere thick with carbon dioxide, carbon monoxide and methane was left behind. There was virtually no oxygen. Complicated life could not survive in this stifling atmosphere. Somehow, oxygen began accumulating in the atmosphere, and eventually, as a result, complex organisms sprouted, including us.

By the turn of the 21st century, a tidy story for oxygen’s emergence had formed. It went something like this: more than 3.5 billion years ago there evolved a new kind of bacteria, dubbed “cyanobacteria”, which could harness the energy of the sun and use it to split water into hydrogen and oxygen through a process called photosynthesis. These new critters started pumping out large quantities of oxygen into the atmosphere – a novel experience for our young planet. Evidence for the tale came from imprints in 3.5 billion-year-old rubble in Western Australia, which J. William Schopf, a palaeontologist at the University of California, said looked like photosynthesising bacteria.

At first the bacteria were so few they couldn’t muster the oxygen production to change Earth’s massive atmosphere. But over time, their numbers soared, along with oxygen levels in the skies. In 1999, Brocks, then a graduate student at the University of Sydney, discovered evidence of complicated creatures that need oxygen, called eukaryotes, which were 2.7 billion years old. He also found cyanobacteria from the same period. This fitted the textbook story well. Growing oxygen levels were triggering complex life.

Three hundred million years later, oxygen levels were booming and ancient iron rocks reacted with the gas, leaving a distinctive rusty red stripe that can be seen today. This moment in time, about 2.4 billion years ago, became known as the Great Oxidation Event, and it was believed that oxygen levels continued to rise from then until they reached today’s levels. So, all sorted then. The only problem? Much of the evidence for this geological narrative recently crumbled.

The rumble started in 1999, when University of Oxford palaeontologist Martin Brasier was updating his microfossil textbook. He decided to reanalyse Schopf’s rocks with marks of 3.5 billion-year-old bacteria. On closer inspection, Brasier didn’t think the patterns in the rocks looked like cyanobacteria at all. He scrutinised them further, even visiting the remote site where they were discovered. Once there, Brasier realised that at the time these so-called bacteria were supposedly pumping out oxygen, their home would have been filled with incredibly hot water – a hell that was no place for cyanobacteria to evolve. He figured that the rock patterns Schopf thought were signs of life were actually created when volcanically heated water snaked through the emerging rocks.

In 2002, the two academics faced off in a packed hall at the Astrobiology Science Conference. They had 15 minutes each to make their case. One who documented the moment described it as a “heavyweight prize fight”. Suffice it to say, Brasier won. Birger Rasmussen, a geologist at Curtin University, confirms that Schopf’s “microfossils are no longer cited as evidence for the presence of cyanobacteria”.  

Six years later Brocks’ discovery of 2.7 billion-year-old life would be discredited too. But in an intriguing twist, Brocks shot down his own work. “I always had a suspicion, because it’s hard to prove or disprove the age of these objects,” he says. It took nine years, but in 2008 Brocks joined forces with Rasmussen and published a study showing that his find was the unfortunate result of contamination.

Earlier this year, another crack formed in the textbook tale. Publishing in The Geological Society of America Bulletin, Rasmussen presented evidence questioning whether oxygen levels even rose 2.4 billion years ago. His study suggests that chemical fingerprints of the gas in today’s rocks would have been contaminated when Earth’s tectonic plates smashed together. Those violent movements forced younger oxygen into ancient rocks, says Rasmussen, mixing up the geological time line. This paper is highly controversial, however, and many academics still believe oxygen levels were high 2.4 billion years ago.

With every broken piece of evidence, scientists learn. They develop new techniques to track oxygen’s movements through history and account for contamination with fresh eyes. Now, a handful of promising new findings have led to a new, and more complex, story of oxygen’s origin.

Rather than continually rising after the Great Oxidation Event, oxygen’s emergence on Earth may have happened in a series of pulses. In a paper published in Nature earlier this year Tim Lyons, of the University of California, Riverside, described it as “more like a roller-coaster ride”. “The Great Oxidation Event really wasn’t that great,” he says.

This month, Quentin Crowley of Trinity College Dublin found an early bump in that ride. Ancient soils from India suggest that “a primitive form of bacteria” squeezed out oxygen more than three billion years ago, says Crowley. However, those bugs couldn’t permanently change Earth’s atmosphere, and oxygen levels later plunged.

Similarly, Lyons says that soon after the so-called Great Oxidation Event, the gas took another nosedive. By analysing ironstones from around the world, Lyons and his Yale University colleague Noah Planavsky believe that this drop left Earth asphyxiated for more than a billion years. The team has new, as yet unpublished, evidence of more yo-yoing oxygen levels. “This is going to be a game changer,” he says.

The next step will be unravelling what caused these booms and busts, and critically, what allowed oxygen to take its final leap into the boom we’re experiencing now. One idea is the remarkable influence that oxygen has on climate. Before oxygen’s heyday, methane hovered in the atmosphere at levels perhaps 1000 times higher than they are today. Methane, a potent greenhouse gas, traps heat. But it’s believed that when oxygen poured into the atmosphere, it reacted violently with methane, ripping it from the skies and triggering a global freeze. These intense environmental shifts may have affected populations of cyanobacteria, causing oxygen levels to change.

Other players are finding their way into oxygen’s origin story. At certain times in Earth’s history, volcanoes would have spewed out toxic gases that reacted with oxygen, potentially wiping it from the atmosphere. Shifting plate tectonics on early Earth may have also released elements, such as nickel, which triggered chemical processes “that would tip the balance” and encourage oxygen to accumulate in the atmosphere, says the University College of London’s Nick Lane. 

Untangling this mess could help us find alien life, or at least explain why we haven’t yet found Martians. It’s likely that Mars once had massive oceans, just like ours, and was ready for life to emerge. But without the remarkable evolution of photosynthesising bacteria, perhaps complex life couldn’t form. And, if oxygen is the ultimate life-giving gas, finding it on other planets could be a smoking gun to discovering extraterrestrial life. “Oxygen may be the least ambiguous symbol for life,” says Lyons, whose work is funded by NASA. 

For Crowley, however, the hunt for oxygen’s origin story is important for other reasons. “It shows how precarious life is,” he says. One change – the emergence of minuscule bacteria – can have major flow-on effects. “Perhaps understanding this will help us take better care of our planet.”

This article was first published in the print edition of The Saturday Paper on Sep 27, 2014 as "Respiration date". Subscribe here.

Wendy Zukerman
is a science journalist and host of the Science Vs. podcast.

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