Research suggests that the Port Askaig Formation, consisting of rock layers up to 1.1 kilometres thick, was likely deposited between 662 and 720 million years ago during the Sturtian glaciation. This event is believed to be the first of two global freezes that spurred the development of complex life.

A recent study highlights a unique section of exposed rock on the Garvellach Islands in Scotland, which captures the transition from a warm, tropical environment into the Snowball Earth state. Unlike similar formations in North America and Namibia, which lack this transition, the Garvellachs provide an unparalleled record of this critical shift.

Researchers believe these findings could represent the most complete evidence of Snowball Earth, a theory proposing that Earth’s oceans and landmasses were covered in ice from pole to equator during at least two extreme cooling events between 2.4 billion and 580 million years ago.

Professor Graham Shields, senior author from University College London (UCL) Earth Sciences, stated, “These rocks document a time when Earth was blanketed in ice. All complex, multicellular life, such as animals, emerged from this deep freeze, with the first fossil evidence appearing shortly after the planet thawed.”

Elias Rugen, the study’s first author and a PhD candidate at UCL Earth Sciences, added, “Our research provides the first definitive age constraints for these Scottish and Irish rocks, confirming their global significance.” He noted that, unlike most regions where ancient glaciers eroded the layers recording a tropical environment, Scotland remarkably preserves this transition.

The Sturtian glaciation, lasting approximately 60 million years, was one of two major freezes during the Cryogenian Period, which occurred between 635 and 720 million years ago. Before this, life was limited to single-celled organisms and algae. The subsequent thaw led to the rapid emergence of complex life, with most modern animals sharing fundamental traits with life forms that evolved over 500 million years ago.

One hypothesis suggests that the extreme cold forced single-celled organisms to cooperate, leading to the evolution of multicellular life.

The ice’s advance and retreat, driven by the albedo effect—whereby more ice reflects more sunlight, further cooling the planet—likely occurred over thousands of years.

Professor Shields explained, “The ice’s retreat would have been catastrophic. After tens of millions of years in a deep freeze, life had to adapt rapidly. The survivors became the ancestors of all animals.”

In this study, the research team analysed sandstone samples from the Port Askaig Formation and the older, 70-meter-thick Garbh Eileach Formation beneath it. The new age constraints for these rocks could support the designation of the site as a Global Boundary Stratotype Section and Point (GSSP), marking the beginning of the Cryogenian Period. Such sites, often called “golden spikes,” are significant for geologists and attract global attention.

The study, led by UCL researchers, is published in the Journal of the Geological Society of London.