Life survived when Earth was covered in ice - but how?

When Earth was locked in ice, where did life hide out? New research suggests that shallow, sunlit meltwater ponds – small pools on top of ancient ice sheets – may have given early complex life forms a place to survive.

These above-ice pockets could have acted as safe zones during one of the coldest, harshest chapters in our planet’s history.

The study was conducted by an international team of scientists led by the Massachusetts Institute of Technology (MIT). The research was focused on a time known as “Snowball Earth,” when much of the planet was encased in ice between 635 and 720 million years ago.

Meltwater ponds sheltered life

During Snowball Earth, temperatures dropped to minus 50 degrees Celsius. Ice sheets likely covered land and sea – possibly even near the equator. Still, life managed to survive.

The question of how life persisted has challenged scientists for years. Some have suggested that organisms could have survived in places like hydrothermal vents deep in the ocean, under the ice, or even in small areas of open water.

But the MIT-led team wanted to explore a different idea: what if meltwater ponds on the surface of the ice were key?

Fatima Husain, lead author of the study, is a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

“We’ve shown that meltwater ponds are valid candidates for where early eukaryotes could have sheltered during these planet-wide glaciation events,” said Husain. “This shows us that diversity is present and possible in these sorts of settings. It’s really a story of life’s resilience.”

Clues found in Antarctica

To test this theory, the researchers turned to modern Antarctica. Today, you can find shallow meltwater ponds scattered along the McMurdo Ice Shelf.

The ponds form when sunlight warms dust-laden patches of ice, melting the surface just enough to create small pools. These conditions are likely similar to what existed near the equator during Snowball Earth.

In 2018, MIT’s Roger Summons and colleagues collected samples from several of these ponds, first described over a century ago as “dirty ice.”

The team found thick microbial mats lining the pond bottoms – sticky layers built up over years, made mostly of photosynthetic cyanobacteria. But that wasn’t all they found.

The researchers also uncovered signs of more complex life. Not just bacteria, but eukaryotes – organisms with a cell nucleus – were present in every pond they studied.

Hidden life in meltwater ponds

To confirm the presence of eukaryotes, the scientists used two techniques. First, they looked for sterols, lipid molecules that only eukaryotes produce. Then, they analyzed ribosomal RNA, which acts as a genetic fingerprint.

These tools helped them identify not just single types of organisms, but entire communities – algae, protists, microscopic animals – thriving in each pond.

“No two ponds were alike,” said Husain. “There are repeating casts of characters, but they’re present in different abundances. And we found diverse assemblages of eukaryotes from all the major groups in all the ponds studied. These eukaryotes are the descendants of the eukaryotes that survived the Snowball Earth.”

One surprising finding: the salinity of a pond had a major impact on what lived there. Saltier ponds hosted similar communities, while fresher ponds had more variation.

“This really highlights that meltwater ponds during Snowball Earth could have served as above-ice oases that nurtured the eukaryotic life that enabled the diversification and proliferation of complex life – including us – later on,” said Husain.

Meltwater ponds hold clues to ancient life

There’s still debate about what Snowball Earth was really like – whether it was a completely frozen ball or more of a “slushball” with thin patches of open ocean. But the evidence from Antarctica shows that today’s meltwater ponds can serve as useful analogs for ancient environments.

“We’re interested in understanding the foundations of complex life on Earth. We see evidence for eukaryotes before and after the Cryogenian in the fossil record, but we largely lack direct evidence of where they may have lived during,” explained Husain.

“The great part of this mystery is, we know life survived. We’re just trying to understand how and where.” And meltwater ponds, it turns out, might be a big part of that answer.

“There are many hypotheses for where life could have survived and sheltered during the Cryogenian, but we don’t have excellent analogs for all of them,” noted Husain.

“Above-ice meltwater ponds occur on Earth today and are accessible, giving us the opportunity to really focus in on the eukaryotes which live in these environments.”

By studying one of the coldest places on Earth today, scientists are piecing together how life managed to hang on through one of the planet’s darkest and coldest times.

The full study was published in the journal Nature Communications.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–