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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. Such formations provide evidence of oxygen-rich settings on ancient Earth.. (UW Photo / Eva Stüeken)

Researchers say multicellular life could have arisen in Earth’s oceans more than 2 billion years ago, only to fall victim to a drop in oxygen levels.

That scenario is based on a study of concentrations of the element selenium of sedimentary shale, led by researchers at the University of Washington. The findings – published online in the Proceedings of the National Academy of Sciences – shed light not only on the origins of life on Earth, but on the potential for detecting life on distant planets.

Previously analyzed fossils have turned up solid evidence of complex organisms known as eukaryotes going back as far as 1.7 billion years ago. That surge in multicellular life is thought to have been caused by a well-documented rise in atmospheric oxygen levels.

The newly published analysis paints a more complex picture. Because selenium reacts with oxygen in characteristic ways, the balance of different isotopes of the element can serve as an indicator for oxygen levels when the rocks were formed.

UW postdoctoral researcher Michael Kipp and his colleagues focused on rocks that went back to a period between 2 billion and 2.4 billion years ago. The selenium isotopes indicated that there was ample oxygen for a stretch of time lasting about a quarter of a billion years.

“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,” UW astrobiologist Roger Buick, the study’s senior author, said in a news release. “That doesn’t mean that they did – but they could have.”

The geological record shows that the oxygen levels rose, then crashed dramatically, and then rose again to usher in the age of complex life as we know it.

The initial burst of oxygen was apparently moderately significant in the atmosphere and the surface ocean, but not in the deep ocean, said Eva Stüeken, a former UW researcher who is now a faculty member at the University of St. Andrews in Scotland.

Stüeken said the reasons for the rise and fall in oxygen levels are not fully known, although they may relate to rock weathering. “That’s the million-dollar question,” she said.

The researchers said their findings had implications for the search for life on planets beyond our solar system. A future generation of powerful telescopes could analyze the atmospheres of exoplanets to look for traces of oxygen and other gases linked to biological activity. In light of the newly published results, Kipp said those analyses should be done carefully.

“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,” Kipp said.

In addition to Kipp, Stüeken and Buick, the authors of “Selenium Isotopes Record Extensive Marine Suboxia During the Great Oxidation Event” include Andrey Bekker of the University of California at Riverside. Bekker came up with the original hypothesis for the rise and fall in oxygen associated with a phenomenon known as the Lomagundi Event.

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