New findings from MIT indicate that early eukaryotes (complex life forms that eventually evolved into the diverse multicellular organisms we see today) may have thrived in meltwater ponds between 72 and 635 million years ago during a period referred to as Snowman Earth.
Impressions of the artist “Snowman Earth.” Image credit: NASA.
Snowman Earth is a colloquial term for a period in Earth’s history characterized by extensive ice coverage across the planet.
This term often refers to two consecutive glacial events that occurred during the Cleogen era, a timeframe geologists define as lasting from 635 million to 72,000 years ago.
The debate remains whether the Earth was akin to a solid snowball or a softer “slash ball.”
What is certain is that much of the planet experienced deep freeze conditions, with an average temperature of about 50 degrees Celsius.
The pressing question is how and where life managed to survive during this time.
“We aim to comprehend the essentials of complex life on Earth,” stated Fatima Hussain, a graduate student at MIT.
“We examine eukaryotic evidence before and after the Crazians in the Fossil Record, yet there’s limited direct evidence regarding their habitats.”
“The main mystery lies in how life persisted. We are working to uncover the specifics of how and where.”
Numerous theories suggest potential refuges for life during Snowman Earth, such as isolated areas of open ocean (if they existed), around deep-sea hydrothermal vents, and underneath ice sheets.
By examining meltwater ponds, Hussain and her team explored the idea that surface meltwater could have supported eukaryotic life during the planet’s early years.
“There are various hypotheses regarding potential survival habitats for life during the Crazians, but we lack comprehensive analogs,” Hussain remarked.
“Meltwater ponds are currently found on Earth, easily accessible, and provide a unique opportunity to focus on the eukaryotes inhabiting these environments.”
For their study, the researchers analyzed samples from meltwater ponds in Antarctica.
In 2018, scientists visited the McMurdo Ice Shelf region of East Antarctica, which is known for its small meltwater ponds.
In this area, water freezes all the way to the seabed, encompassing dark sediments and marine life.
The loss of wind-driven ice from the surface creates a conveyor belt effect, gradually bringing trapped debris to the surface, which absorbs solar warmth, melting surrounding ice and leading to the creation of shallow meltwater ponds.
Each pond is adorned with mats of microorganisms that have accumulated over time, forming sticky communities.
“These mats can be several centimeters thick and are vibrant, clearly demonstrating distinct layers,” Hussain explained.
These microbial mats consist of single-celled, photosynthetic organisms, such as cyanobacteria, which are prokaryotic and lack nuclei or other organelles.
While these ancient microorganisms are known to withstand extreme environments like meltwater ponds, researchers sought to determine if complex eukaryotic organisms—characterized by cell nuclei and membrane-bound organelles—could also survive in such harsh conditions.
To address this question, the researchers required more than just a microscope, as the defining traits of microscopic eukaryotes within microbial mats are often too subtle to discern visually.
The study involved analyzing specific lipids called sterols and a genetic component known as ribosomal ribonucleic acid (rRNA). Both serve as identifiers for various organisms.
This dual analytical approach provided complementary fingerprints for distinct eukaryotic groups.
In their lipid analysis, the researchers uncovered numerous sterols and rRNA genes in microbial mats that align closely with certain types of algae, protists, and microscopic animals.
They were able to assess the diversity and relative abundance of lipid and rRNA genes across different ponds, suggesting that these ponds are home to a remarkable variety of eukaryotes.
“The two ponds exhibit differences. There’s a recurrent cast of organisms, but they manifest uniquely in different environments,” Hussain noted.
“We identified a diverse array of eukaryotic organisms spanning all major groups in every pond we studied.”
“These eukaryotes are descendants of those that managed to survive Snowman Earth.”
“This underscores how meltwater ponds during the Snowman period globally could have nurtured eukaryotic life, enabling the diversification and emergence of complex organisms, including ourselves, in later epochs.”
Study published in the journal Nature Communications.
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F. Hussain et al. 2025. Diverse eukaryotic biosignatures from the Earth-analogous environment of Antarctic Snowman. Nat Commun 16, 5315; doi:10.1038/s41467-025-60713-5
