How Animals Managed to Survive Mega Ice Age 700 Million Years Ago

Hundreds of millions of years ago, the planet was an inhabitable snowglobe with oxygen-starved oceans and mean temperatures hovering 12 degrees Celsius below freezing—yet somehow life persevered.

Researchers from McGill University in California are closing in on this mystery, having found the first direct evidence of a "glacial oxygen pump," which they say extended an essential lifeline to the simple life forms that inhabited the Earth at the time.

In Proceedings of the National Academy of Sciences of the United States of America (PNAS), they offer new evidence supporting the presence of oxygenated marine environments at a time when the planet was experiencing the most extreme ice age in its 4-billion year history 700 million years ago. Previously, research had limited life that may have been dependent on oxygen to surface-level meltwater puddles. Most of the oceans, in contrast, would have been uninhabitable due to an oxygen deficit.

But despite being virtually uninhabitable, these harsh conditions may have actually been a key stepping stone in the evolution of more complex life, scientists suggest.

"The fact that the global freeze occurred before the evolution of complex animals suggests a link between Snowball Earth and animal evolution," Maxwell Lechte, a postdoctoral researcher in the Department of Earth and Planetary Sciences under the supervision of Galen Halverson at McGill University, said in a statement. "These harsh conditions could have stimulated their diversification into more complex forms"

Lechte refers to a "glacial oxygen pump" created by air bubbles trapped in glacial ice. These air bubbles are released into the water as the ice melts, he says, giving it oxygen. This meant that where the grounded ice sheets were beginning to float, there would have been a supply of oxygenated meltwater—and this supply of oxygenated meltwater may have been critical to the survival of life forms, specifically eukaryotes. That is, cells and organisms with a clearly defined nucleus.

What's more, the oxygen supply may have extended to areas miles away from the original source, the researchers say, pointing to the glaciers of 21st-century Antarctica as a modern-day parallel, and offered a stable environment for oxygen-breathing eukaryotes.

The steady stream of oxygen combined with iron-rich seawater could have offered enough energy for carbon-reliant life forms, allowing early animals to survive during an otherwise extreme climate.

"This study actually solves two mysteries about the Snowball Earth at once. It not only provides explanation for how early animals may have survived global glaciation, but also eloquently explains the return of iron deposits in the geological record after an absence of over a billion years," said Galen Halverson, Associate Professor Department of Earth & Planetary Sciences at McGill.

The researchers came to these conclusions after analyzing the chemical makeup of iron-rich rocks from glacial deposits across the world—in Australia, Namibia and California. This allowed them to calculate estimates of how much oxygen was present in the oceans during the Cryogenian Period (720 to 635 million years ago).

But while this shows that there may have been an otherwise unknown supply of oxygen, it does not reveal a source of food that would have been needed to survive snowball Earth. How the planet may have hosten a reliable food web is a question for future research.