It seems the history of oxygen on Earth will have to be adjusted, based on a new paper by Kaarel Mänd from the University of Alberta, Canada, and colleagues. They analyzed carbon-rich sedimentary rocks from two billion years ago and found chemical markers—including very high levels of molybdenum, uranium and rhenium—consistent with large amounts of oxygen in the ocean and in Earth’s early atmosphere.
Our planet’s atmosphere underwent a big transformation about 2.45 billion years ago, in what’s known as the Great Oxidation Event. Cyanobacteria in the ocean released oxygen as part of their metabolism, which reacted with methane in the atmosphere to produce carbon dioxide. The Sun was weaker at the time, and combined with the loss of methane—a much stronger greenhouse gas than carbon dioxide—it resulted in the so-called Snowball Earth, when nearly the whole planet was covered with ice.
Then something else strange happened. Known as the Lomagundi-Jatuli Event, dated at about 2.22 to 2.06 billion years ago, the geological record shows a very long positive peak of the carbon 13 isotope (heavy carbon). This is interpreted to be due to the huge production of organic carbon from inorganic carbon, mostly carbon dioxide, and its later burial. As a consequence, a lot of oxygen (from the original carbon dioxide molecules) was set free. This must have extended the Snowball Earth period even more. Until now, many scientists believed that following the Lomagundi-Jatuli Event, oxygen levels in the atmosphere declined, except in very localized areas. Mänd’s results may force us to rethink that. Global oxygen levels appear to have remained high for much longer, probably for many tens of millions of years.
This history is of more than academic interest, because oxygen is believed to have been important for the rise of eukaryotes, the first microbes with a nucleus. The fossil record is unclear, but eukaryotes appear to have originated around that same time, and may have been jump-started by a longer period of high oxygen levels. The transition from prokaryotes (bacteria and archaea) to eukaryotes was a critical step in the increase of complexity of life on Earth, which eventually ended up with a technologically advanced species–us (yes, we’re all eukaryotes).
This brings up the question of how fast evolution can progress toward complexity on a habitable planet. Most scientists believe it takes several billion years, as it did on Earth, because oxygen is needed to fuel the high metabolism of animals. To get high enough oxygen concentrations in the atmosphere, the rocks have to be oxidized first, followed by the oceans. All that takes a very long time. But what if there are environmental mechanisms that keep the oxygen content high early in a planet’s history, allowing the rise of animals much earlier—say a billion years earlier than it did on our own planet? If so, there could be a planet out there with intelligent life that’s a billion years ahead of us.