Havens for early life (Image: plainpicture/Reto Puppetti)

FOR the first half of Earth’s existence, there was no oxygen to be had. The air wasn’t breathable and life in the oceans was little more than primitive sludge. Even in this hostile world, though, small “oases” of oxygen-rich water persisted, fuelled by bacteria. Now it seems we have found the first hard evidence of one of these oxygen oases, preserved in ancient rocks.

If a person were to step out into this ancient world, they would die of asphyxiation within minutes. It wasn’t until around 2.4 billion years ago that oxygen flooded the oceans and gave rise to the air and seas we recognise.

“But many researchers have suspected that the first biological production of oxygen began long before that,” says Timothy Lyons of the University of California, Riverside. Rocks from 4 to 2.5 billion years ago often contain bands of iron-rich minerals. These formed when bacteria started pumping out oxygen, which reacted with dissolved iron in the ocean to form particles of rock that sank to the bottom.


So oxygen might have built up in isolated pockets – perhaps in shallow seas cut off from the global ocean. “The idea of oxygen oases in ancient seas has been around for a long time, but no one was able to pinpoint a specific example of such an oasis,” says Robert Riding of the University of Tennessee in Knoxville. He and his colleagues now say they have found one.

They collected rock samples from Steep Rock Lake in Ontario, Canada. Rocks there are 2.8 billion years old and contain a mixture of iron minerals and limestone, as well as the remains of thin mats of microbes called stromatolites. The area was once “a shallow shelf, partly isolated from the open sea by a stromatolite reef, and close to a land mass that could have supplied nutrients”, says Riding.

“Steep Rock is one of the oldest thick limestones on Earth, and is certainly the best preserved,” says Riding. His team’s analyses of the limestone suggest it has not changed since it was laid down.

That is key, adds Riding, because the calcium carbonate that makes up limestone can only form in water that has first been stripped of all its dissolved iron. He says the best explanation for the presence of the mineral is that bacteria pumped out oxygen, which reacted with all the iron in the water (Precambrian Research, doi.org/tsq).

The oasis only persisted for about 5 million years, though. After that, sea levels rose, overtopped the reef and swamped the area with a fresh influx of iron, causing oxygen levels to crash. “The existence of the oases was tenuous,” says Lee Kump of Penn State University in University Park.

Despite their name, the oases were dangerous places. Because oxygen is chemically reactive, when it first built up it was a deadly pollutant. Bacteria living in the oases would have been forced to evolve oxygen-defence mechanisms, or die. So the oases would have pushed early life to adapt to oxygen, before the gas went global.

It was a critical moment in the evolution of life on Earth. Once organisms had acquired the ability to survive in the presence of oxygen, they could evolve to harness its chemical energy, and become the world’s first oxygen-breathers.

Tiny creatures evolved to harness oxygen’s chemical energy – becoming the first oxygen-breathers

This article appeared in print under the headline “An oasis in a world without oxygen”