Changes in Earth's tilt led to the emergence of complex life

Researchers at the University of Southampton have just solved the mystery of how multicellular life appeared at a time when the Earth’s climate was the most hostile it has ever been. The experts report that complex life was able to emerge, and even thrive, in an inhospitable climate due to changes in Earth’s orbit. 

In collaboration with colleagues in the Chinese Academy of Sciences, Curtin University, University of Hong Kong, and the University of Tübingen, the researchers studied rocks from a time when most of Earth’s surface was covered in ice. This period of “Snowball Earth” lasted for more than 50 million years. 

“One of the most fundamental challenges to the Snowball Earth theory is that life seems to have survived,” said study co-author Dr. Thomas Gernon. “So, either it didn’t happen, or life somehow avoided a bottleneck during the severe glaciation.”

The researchers analyzed glacial rocks that formed about 700 million years ago in what is now the South Australian outback. These rocks showed that during Snowball Earth, ice sheets extended as far as the equator, which means that the icy shell almost completely covered the planet. 

The experts then focused on banded iron formations – sedimentary rocks consisting of alternating layers of iron-rich and silica-rich material. These rocks were deposited in the ice-covered ocean.

According to the study authors, the frozen ocean would have been entirely cut off from the atmosphere during the snowball glaciation. The lack of normal interactions between the air and the ocean would have caused a lack of normal climate variations.  

“This was called the ‘sedimentary challenge’ to the Snowball hypothesis,” explained study lead author Professor Ross Mitchell. “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.” 

This level of climate variability seemed to contradict the theory of an icy shell that engulfed the entire ocean.

“The iron comes from hydrothermal vents on the seafloor,” said Dr. 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.”

The experts measured the extent to which the rocks had become magnetized from exposure to Earth’s magnetic field. They discovered that the layered rocks contained evidence of nearly all orbital cycles.

The tilt and wobble of Earth’s spin axis change in cycles as our planet orbits the Sun. The cyclic changes alter the climate by modifying the amount of incoming solar radiation that reaches Earth’s surface.

“Even though Earth’s climate system behaved very differently during the Snowball, Earth’s orbital variations would have been blissfully unaware and just continued to do their thing,” explained Professor Mitchell.

The researchers concluded that changes in Earth’s orbit allowed fluctuations in the ice sheets, and ice-free regions periodically developed on Snowball Earth.

“This finding resolves one of the major contentions with the snowball Earth hypothesis: the long-standing observation of significant sedimentary variability during the snowball Earth glaciations appeared at odds with such an extreme reduction of the hydrological cycle,” said Professor Mitchell.

The results can help to explain the enigmatic presence of sedimentary rocks of this age that show evidence for flowing water at Earth’s surface when this water should have been locked up in ice sheets. “This observation is important, because complex multicellular life is now known to have originated during this period of climate crisis, but previously we could not explain why,” said Dr. Gernon.

“Our study points to the existence of ice-free ‘oases’ in the snowball ocean that provided a sanctuary for animal life to survive arguably the most extreme climate event in Earth history.”

The study is published in the journal Nature Communications.

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By Chrissy Sexton, Earth.com Staff Writer