A New Context for Complex Life

We normally think of the appearance of oxygen on Earth in terms of a ‘great oxygenation event,’ sometimes referred to as the ‘oxygen catastrophe’ or ‘great oxidation.’ Here oxygen begins to emerge in the atmosphere about 2.3 billion years ago as oceanic cyanobacteria produce oxygen by photosynthesis. The actual oxygenation event would be the point when oxygen is not all chemically captured but becomes free to escape into the atmosphere.

It’s a straightforward picture — we move from a lack of oxygen to gradual production through photosynthesis and then a concentration strong enough to destroy many anaerobic organisms, an early and huge extinction event as life on our planet adjusted to the new balance. But a team of researchers led by Michael Kipp (University of Washington) has produced a paper showing a much more complicated emergence of oxygen, one that produced a surge in oxygenation that lasted a quarter of a billion years before easing.

Kipp and team studied oxygen in the Earth’s atmosphere between 2 and 2.4 billion years ago. Their work focuses on the element selenium and its isotopic ratios in sedimentary shale, using mass spectrometry techniques at the University of Washington Isotope Geochemistry Lab. The question: How have isotopic ratios been changed by the presence of oxygen? The reduction of oxidized selenium compounds causes a shift in these ratios which can be measured, and the abundance of selenium itself increases as oxygen levels climb.

What the team found is that oxygen levels were higher far earlier than we’ve believed. Indeed, these levels may have supported complex life, at least for a time. For instead of a gradual and continuing rise, these levels then drop. Roger Buick (UW Astrobiology Program) explains:

“There is fossil evidence of complex cells that go back maybe 1 ¾ billion years. But the oldest fossil is not necessarily the oldest one that ever lived – because the chances of getting preserved as a fossil are pretty low. This research shows that there was enough oxygen in the environment to have allowed complex cells to have evolved, and to have become ecologically important, before there was fossil evidence. That doesn’t mean that they did — but they could have.”

Image: This is a 1.9-billion-year-old stromatolite — or mound made by microbes that lived in shallow water — called the Gunflint Formation in northern Minnesota. The environment of the oxygen “overshoot” described in research by Michael Kipp, Eva Stüeken and Roger Buick may have included this sort of oxygen-rich setting that is suitable for complex life. Credit: Eva Stüeken.

Thus shallow coastal waters may have held the oxygen needed for complex life hundreds of millions of years earlier than thought. The researchers call this event an ‘oxygen overshoot,’ a significant increase in atmospheric oxygen and in the surface ocean, but one that did not affect the deep ocean. Oxygen levels would have risen for a quarter of a billion years before sinking back. That makes the so-called ‘great oxygenation event’ a more complex process than we realized, with a sharp peak in oxygen before a drop to a lower, more stable level.

About this complex process there remain plenty of questions. What caused the elevation of oxygen levels in the first place, and what precipitated its decline? The researchers have no answer, but can only point to a selenium isotope record that clearly sets this period apart. The selenium technique is a potent way to analyze our own planet’s past, but it also reminds us of the need to be cautious in evaluating exoplanet habitability. Says lead author Kipp:

“The recognition of an interval in Earth’s distant past that may have had near-modern oxygen levels, but far different biological inhabitants, could mean that the remote detection of an oxygen-rich world is not necessarily proof of a complex biosphere.”

The paper is Kipp et al., “Selenium isotopes record extensive marine suboxia during the Great Oxidation Event,” published online by Proceedings of the National Academy of Sciences 18 January 2017 (abstract). This UW news release is also helpful.