During periods known as Snowball Earth, between 720 and 635 million years ago, early eukaryotes — complex cellular lifeforms that eventually evolved into the diverse multicellular life we see today — could have waited things out in meltwater ponds, according to new research from MIT.
An artist’s impression of a ‘Snowball Earth.’ Image credit: NASA.
Snowball Earth is the colloquial term for periods of time in Earth history during which the planet iced over.
It is often used as a reference to the two consecutive, multi-million-year glaciation events which took place during the Cryogenian period, which geologists refer to as the time between 635 and 720 million years ago.
Whether the Earth was more of a hardened snowball or a softer ‘slushball’ is still up for debate.
But scientists are certain of one thing: most of the planet was plunged into a deep freeze, with average global temperatures of minus 50 degrees Celsius.
The question has been: How and where did life survive?
“We’re interested in understanding the foundations of complex life on Earth,” said Fatima Husain, a graduate student at MIT.
“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.”
“The great part of this mystery is, we know life survived. We’re just trying to understand how and where.”
There are a number of ideas for where organisms could have sheltered during Snowball Earth, including in certain patches of the open ocean (if such environments existed), in and around deep-sea hydrothermal vents, and under ice sheets.
In considering meltwater ponds, Husain and her colleagues pursued the hypothesis that surface ice meltwaters may also have been capable of supporting early eukaryotic life at the time.
“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,” Husain said.
“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.”
For their study, the researchers analyzed samples taken from meltwater ponds in Antarctica.
In 2018, scientists traveled to a region of the McMurdo Ice Shelf in East Antarctica, known to host small ponds of melted ice, each just a few feet deep and a few meters wide.
There, water freezes all the way to the seafloor, in the process trapping dark-colored sediments and marine organisms.
Wind-driven loss of ice from the surface creates a sort of conveyer belt that brings this trapped debris to the surface over time, where it absorbs the Sun’s warmth, causing ice to melt, while surrounding debris-free ice reflects incoming sunlight, resulting in the formation of shallow meltwater ponds.
The bottom of each pond is lined with mats of microbes that have built up over years to form layers of sticky cellular communities.
“These mats can be a few centimeters thick, colorful, and they can be very clearly layered,” Husain said.
These microbial mats are made up of cyanobacteria, prokaryotic, single-celled photosynthetic organisms that lack a cell nucleus or other organelles.
While these ancient microbes are known to survive within some of the harshest environments on Earth including meltwater ponds, the researchers wanted to know whether eukaryotes — complex organisms that evolved a cell nucleus and other membrane bound organelles — could also weather similarly challenging circumstances.
Answering this question would take more than a microscope, as the defining characteristics of the microscopic eukaryotes present among the microbial mats are too subtle to distinguish by eye.
To characterize the eukaryotes, the authors analyzed the mats for specific lipids they make called sterols, as well as genetic components called ribosomal ribonucleic acid (rRNA), both of which can be used to identify organisms with varying degrees of specificity.
These two independent sets of analyses provided complementary fingerprints for certain eukaryotic groups.
As part of their lipid research, the researchers found many sterols and rRNA genes closely associated with specific types of algae, protists, and microscopic animals among the microbial mats.
They were able to assess the types and relative abundance of lipids and rRNA genes from pond to pond, and found the ponds hosted a surprising diversity of eukaryotic life.
“No two ponds were alike. There are repeating casts of characters, but they’re present in different abundances,” Husain said.
“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.”
“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.”
The study was published in the journal Nature Communications.
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F. Husain et al. 2025. Biosignatures of diverse eukaryotic life from a Snowball Earth analogue environment in Antarctica. Nat Commun 16, 5315; doi: 10.1038/s41467-025-60713-5
