Life As We Know It, Now With a Date

Early Earth. Credit: Peter Sawyer / Smithsonian Institution

Early Earth. Credit: Peter Sawyer / Smithsonian Institution

Breathe in. Breathe out. A billion years ago, there was very little oxygen on Earth to breathe.

Through much of its existence, Earth’s atmosphere, like the atmospheres of Mars and Venus, was relatively oxygen-free. A new study out of University of California at Riverside (UCR) details when in Earth’s history oxygen may have become abundant.

Tim Lyons of UCR, Noah Planavsky of Yale University and Christopher Reinhard of the Georgia Institute of Technology and their colleagues arrived at this solution using soil chemistry. Their team analyzed sediments from North America, China and Australia for traces of the element Chromium (Cr). Chromium is chemically altered in the presence of O2 – molecular oxygen, the kind that we breathe. Looking at the 1.8 billion to 0.8 billion years before the evolution of animals, the research team didn’t find much in the way of altered Cr.

“Our data, which point to overall low oxygen in the atmosphere, tell us that oxygen was likely very low in the ocean and, most importantly, very low in the surface ocean – where the animals lived prior to the appearance of animals,” said Lyons, “It took the development of the Cr isotope technique to refine our understanding of oxygen in the atmosphere and shallow ocean.”

The Tyrannosaurus rex and other animals arose in the environment of a newly oxygenated Earth. Credit: Wikimedia Commons

The Tyrannosaurus rex and other animals arose in the environment of a newly oxygenated Earth. Credit: Wikimedia Commons

According to the paper, less than 1% of today’s oxygen was available before the mid-Proterozoic, 1.8 to 0.8 billion years ago. Then Earth hit the 800 million year mark. Those various versions of Cr began to show the signs of increasing oxygen in the atmosphere. Fast-forward to today: our atmosphere is around 21% oxygen.

Why oxygen rose 800 millions years ago – remains a mystery. What we do know, thanks to Lyons and his colleagues, is that genetic diversity followed the sudden appearance of high sustained oxygen levels. Furthermore, Lyons and his colleagues have shown us evidence that a planet’s atmosphere can have a fundamental change of chemical heart. This bodes well for the possibility of life arising on other bodies in the Universe – even ones that, by their atmospheric contents, might not have always been suitable.

“New tools for exploring planetary bodies (and moons) within and outside our solar system are evolving quickly.” Lyons told Astrobio.net, “Detection capabilities (even for exoplanets) for the oxygen levels and variability we are describing may be around the corner. Looking for oxygen beyond Earth is a very exciting frontier.”

Lyons and his team emphasize that they have not solved a chicken-or-egg problem. A comparative wealth of available oxygen did not automatically cause animal life to become wildly successful on Earth overnight. Instead, the team emphasizes that the delay in the rise of animals was almost certainly tied to a long period of very low oxygen. 800 million years ago when that low-oxygen barrier began to lift, the proliferation of animal life started in earnest. It is quite possible that extra oxygen in the air gave animals the breathing room they needed to make Earth a living paradise.