Earliest record of ‘snowball Earth’ revealed by 720-million-year-old rocks

“These rocks record a time when Earth was covered in ice,” said Graham Shields, a geologist from University College London and co-author of the study. “All complex, multicellular life, such as animals, arose out of this deep freeze, with the first evidence in the fossil record appearing shortly after the planet thawed.”

The Ice Age’s impact on life’s evolution

The rocks examined in this study date back to the Sturtian glaciation, a key phase during the Cryogenian Period, which spans from approximately 635 to 720 million years ago. This glaciation is one of two major freezing events during this era. Prior to the Cryogenian, Earth was considerably warmer and home to mainly single-celled organisms and algae.

The transition from a predominantly warm planet to one covered in ice likely triggered the emergence of complex life forms. One prominent theory suggests that the extreme cold might have fostered the development of altruism among single-celled organisms, leading to the formation of multicellular life. 

As the ice advanced and retreated over thousands of years, the albedo effect—where ice reflects sunlight back into space—may have accelerated these shifts, creating a dynamic environment conducive to evolutionary change.

“The retreat of the ice would have been catastrophic. Life had been used to tens of millions of years of deep freeze,” Shields explained. “As soon as the world warmed up, all of life would have had to compete in an arms race to adapt. Whatever survived were the ancestors of all animals.”

The geological treasure of the Port Askaig Formation

To unravel the mysteries of this ancient ice age, the research team focused on the Port Askaig Formation, a significant rock formation spanning parts of present-day Scotland and Ireland. The formation consists of layers up to half a mile thick, with crucial exposures found on the Scottish Garvellachs islands.

One notable outcrop on the Garvellachs islands holds evidence of Earth’s transition from a warm, tropical climate to a frozen snowball during the Sturtian glaciation. This contrasts with other rocks from the same period found in North America and Africa, which do not show this climatic transition.

“The layers of rock exposed on the Garvellachs are globally unique. Underneath the rocks laid down during the unimaginable cold of the Sturtian glaciation are 70 meters (229 feet) of older carbonate rocks formed in tropical waters,” stated Elias Rugen, a UCL PhD candidate and co-author of the study. 

“These layers record a tropical marine environment with flourishing cyanobacterial life that gradually became cooler, marking the end of a billion years or so of a temperate climate on Earth.”

To date the rocks, the team analyzed zircon minerals. These minerals are durable and contain uranium, which decays at a steady rate. This decay rate allows geologists to estimate the age of the rocks. The zircon and other geochemical evidence suggest that the rocks were deposited between 662 and 720 million years ago.

The new dating of these rocks may lead to the designation of the site as a Global Boundary Stratotype Section and Point (GSSP), a formal marker in the geological record often referred to as a “golden spike.” The International Union of Geological Sciences is currently reviewing whether the Garvellachs outcropping will be recognized with this honor.

The research was published on August 15 in the Journal of the Geological Society of London.