Although geological and astronomical disasters have caused many setbacks for life, we know of only one that makes scientists wonder how Earth wasn't sterilized entirely. This is the “Snowball Earth” period 700 million years ago when the planet was so cold it was covered in thick ice even at the equator. Yet not only did life survive, its greatest burst of diversification occurred soon after the ice retreated. A new study provides evidence the planet was saved by variations in the Earth's orbit that still exist today.
Evidence for Snowball Earth is found in rocks worldwide indicating simultaneous glaciation. Nevertheless, the event was so extreme some geologists question if we've misread the signs.
“One of the most fundamental challenges to the Snowball Earth theory is that life seems to have survived,” said Dr Thomas Gernon of the University of Southampton in a statement. “So, either it didn’t happen, or life somehow avoided a bottleneck during the severe glaciation.” In Nature Communications Gernon and co-authors show Snowball Earth was real, but variations within it gave life its chance to survive.
Snowball Earth was no brief winter where single-celled organisms could have become frozen before thawing later. It is thought to have lasted around 50 million years. That doesn't mean there was no change at that time. Rocks in South Australia's Flinders Range laid down at the time alternate between iron-rich and silica-rich material. At the time, Australia lay close to the equator – if anywhere could have got through Snowball Earth without fully freezing it was there.
“The iron comes from hydrothermal vents on the seafloor,” said Gernon. “Normally, the atmosphere oxidizes any iron immediately, so Banded Iron Formations typically do not accumulate. But during the Snowball, with the ocean cut off from the air, iron was able to accumulate enough for them to form.”
Yet if ice prevented oxygen from reaching the oceans, the silica-rich bands need explaining.
“The highly variable rock layers appeared to show cycles that looked a lot like climate cycles associated with the advance and retreat of ice sheets,” said first author Professor Ross Mitchell of the Chinese Academy of Sciences.
Like iron-rich deposits today, the Flinders rocks provide a record of the magnetic field at the time they were laid down, which in turn indicates how tilted the Earth's axis was. Just as changes in the Earth's orbit and the angle of its tilt, known as Milankovitch cycles, drove ice ages and interglacials, prior to the Anthropocene, the authors conclude these changes caused intermissions in Snowball Earth. The rocks retain clear records of the Milankovitch cycles, showing they not only existed at the time but changed the Earth's climate enough to let bursts of air through.
Mitchell told IFLScience it remains uncertain whether patches of water survived unfrozen. Alternatively, the surge and retreat of glaciers on land may have created a “sheer” as they encountered the ocean, breaking up the ice and letting air through. For lifeforms, the latter would have been a precarious place to live, but perhaps it was all they needed. Either way, Mitchell told IFScience, “It's the dynamics that allowed life to survive.” A near-static world would not have been so kind.
Molecular clocks tell us some of the genes key to multicellular life, such as HOX, existed before Snowball Earth, but the first fossil evidence for complex life appears in abundance immediately thereafter. “You need two things,” Mitchell told IFLScience: “The genetic evolution and the ecological conditions.” The ending of Snowball Earth filled the oceans with nutrients from rocks the glaciers had worn down, allowing life to go from barely hanging on to exploding in abundance.