Early Complex Life May Have Sheltered in Meltwater Ponds during Snowball Earth Episodes: Study

During the periods known as Snowball Earth, between 720 and 635 million years old, the first eukaryotes – complex cell life forms which ultimately evolved towards the diversity of multicellular life that we see today – could have expected things in the ponds of cast iron, according to new MIT research.

The impression of an artist of a “snowball land”. Image credit: NASA.

Snowball Earth is the familiar term for periods of time in the history of the earth during which the planet has frozen.

It is often used as a reference to the two consecutive glaciation events of several million years which took place during the cryogenian period, which geologists call time between 635 and 720 million years.

That the earth was more a hardened snowball or a softer “loop” is still on a debate.

But scientists are certain of one thing: most of the planet has been plunged into a deep frost, with average global temperatures in less than 50 degrees Celsius.

The question was: how and where has life survived?

“We are interested in understanding the foundations of complex life on earth,” said Fatima Husain, a graduate student at MIT.

“We see evidence of the eukaryotes before and after the cryogenian in the fossil file, but we largely lack direct evidence of the place where they could have lived during.”

“The main part of this mystery is that we know that life has survived. We just try to understand how and where. “

There are a number of ideas where organisms could have been sheltered during the land of snowball, including in certain open ocean plots (if such environments existed), in and around deep hydrothermal vents and under glacial caps.

Considering the cast iron water ponds, Husain and his colleagues have continued the hypothesis that the cast iron of surface ice can also be able to support early eukaryotal life at the time.

“There are many hypotheses for the place where life could have survived and sheltered during the cryogenian, but we have no excellent analogues for all,” said Husain.

“The above cast iron ponds occur on earth today and are accessible, giving us the possibility of really focusing on the eukaryotes that live in these environments.”

For their study, the researchers analyzed the samples taken from ponds of cast iron in Antarctica.

In 2018, scientists went to a region of the McMurdo ice platform in Eastern Antarctica, known to accommodate small ponds of melted ice, each a few feet deep and a few meters wide.

There, water freezes to the seabed, in the process trapping dark colored sediments and marine organisms.

The loss of ice from the wind of the surface creates a sort of rolling carpet which brings these debris trapped on the surface over time, where it absorbs the heat of the sun, causing the cast iron, while the surrounded debris reflect the inbound sun, which results from the formation of shallow melting water basins.

The bottom of each pond is lined with microbe carpets that have been built over the years to form layers of sticky cell communities.

“These carpets can be a few centimeters thick, colorful, and they can be very clearly superimposed,” said Husain.

These microbial carpets are made up of procaryotic cell photosynthetic organisms, procaryotes which do not have a cellular nucleus or other organelles.

Although these ancient microbes are known to survive in some of the most difficult environments of the earth, including the ponds of cast iron, researchers wanted to know if eukaryotes – complex organisms that have evolved a cell nucleus and other membrane -related ordered ordered.

Answering this question would take more than a microscope, because the determining characteristics of the microscopic eukaryotes present among the microbial carpets are too subtle to distinguish the eyes.

To characterize the eukaryotes, the authors have analyzed the carpets for specific lipids which they do called sterols, as well as genetic components called ribosomal ribonucleic acid (RNA), which can both be used to identify organisms with different degrees of specificity.

These two independent analysis sets have provided additional fingerprints for certain eukaryotic groups.

As part of their lipid research, researchers have found many sterols and RNA genes closely associated with specific types of algae, protists and microscopic animals among microbial carpets.

They were able to assess the types and the relative abundance of lipids and RNA genes from the pond at the pond, and found that the ponds hosted a surprising diversity of eukaryotal life.

“There are not two identical ponds. There are repetitive casts of characters, but they are present in different abundances,” said Husain.

“And we found various assemblies of eukaryotes from all the large groups of all the ponds studied.”

“These eukaryotes are the descendants of the eukaryotes who survived the land of snowball at the land.”

“This really emphasizes that the ponds of cast iron water during the land of the snowball could have served as oases above the ice which fed the eukaryote life which allowed the diversification and the proliferation of complex life – including us – later.”

The study was published in the journal Nature communications.

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F. Husain and al. 2025. Biosignatures of eukaryote life diversified from an analog environment of the Snowball Earth in Antarctica. Common nat 16, 5315; DOI: 10.1038 / S41467-025-60713-5