While the Earth endured icy millions of years, early life may have survived near the equator, according to a new study of microorganisms in Antarctic meltwater ponds.
The Antarctic is once again proving to be a window into the past: between around 720 and 635 million years ago, when Earth underwent two global glaciation phases lasting several million years, diverse and complex microbial communities may have survived these extreme “Snowball Earth” periods in shallow meltwater ponds — similar to those found in Antarctica today.
Evidence for this comes from a series of meltwater ponds on the McMurdo Ice Shelf in East Antarctica, which an international research team — led by the Massachusetts Institute of Technology (MIT) and involving the University of Waikato and other institutions — studied for their microbial communities. The findings were published on June 19 in Nature Communications.
Thanks to fossil evidence, it is well documented that eukaryotic life already existed before these global glaciation phases — complex single-celled organisms that later gave rise to multicellular life and, ultimately, the forms of life we know today. During the so-called Cryogenian period, average global temperatures likely dropped to as low as –50 °C. Whether the planet was entirely covered by a thick sheet of ice — a true “Snowball Earth” — or rather by a slushy, partially frozen surface remains unresolved. However, scientists agree that most of Earth’s surface was indeed frozen.
Until now, researchers have assumed that early life forms may have survived in polynyas near the equator — areas of open ocean that remained ice-free — if they existed, or at hydrothermal vents in the deep sea, or in subglacial lakes beneath kilometres of ice. The authors of the new study now introduce another possible refuge: meltwater ponds.
In view of the diverse communities detected in the ponds on the McMurdo Ice Shelf — consisting of photosynthetically active cyanobacteria as well as eukaryotic microorganisms such as microalgae, protists, and tiny animals — the researchers suspect that similar communities may have existed during the global glaciations in near-surface meltwater ponds near the equator, where temperatures might have hovered around the freezing point.
“It has been a privilege to study the types of above-ice meltwater ponds that would have existed during the Cryogenian glaciations. Long-term studies of these ponds had demonstrated that the ponds host interesting bacterial diversity, and some of the controls on that diversity have started to emerge,” writes Fatima Husain, a PhD student at MIT and first author of the study, in an email to polarjournal.net.
The research team used lipids and genetic markers to analyze the microbial communities in a total of 15 meltwater ponds located in the so-called “dirty ice” area — a name given by the Scott Expedition in 1903 due to dark sediment deposits that make the ice appear dirty. These darker surfaces absorb more sunlight, causing localized melting of the ice and the formation of small, seasonal pools just a few meters deep.
The bottoms of the ponds are covered with centimeter-thick mats of microorganisms, with no two microbial communities exactly alike. Each pond hosts a distinct community that is perfectly adapted to its specific pH, salinity, and temperature. Salinity, in particular, proved to be a decisive factor: ponds with similar salinity levels harbored strikingly similar communities — regardless of their geographic location. The researchers conclude that environmental conditions play a greater role in shaping community composition than spatial proximity.
This diversity is remarkable not only for its complexity, but also for its resemblance to what may have occurred hundreds of millions of years ago.
The fact that these shallow ponds are able to sustain diverse, functionally interconnected communities makes them particularly interesting from a scientific perspective. They could serve as modern analogs to surface meltwater ponds that may have existed on the ice sheets of “Snowball Earth” — possibly near the equator, where dust deposits warmed the ice surface and triggered localized melting. These melt ponds could have provided sufficiently stable conditions for complex microorganisms over extended periods of time, acting as near-surface refuges.
This would be of great significance for the evolution of life on our planet, as eukaryotic organisms were in a critical stage of development during the “Snowball Earth” periods. As Fatima Husain explains, the team set out to examine the eukaryotic communities not only to complement previous investigations of bacteria in these environments, but also to characterize in detail the types of eukaryotes supported by the ponds. “It was encouraging to recover such eukaryotic diversity from these ponds in light of their analogous nature to above-ice meltwater ponds on Cryogenian ice sheets,” she says.
For New Zealand polar researcher Dr. Ian Hawes from the University of Waikato — who has been studying Antarctic meltwater ponds for decades and contributed to the current research project — Antarctica holds particular significance in this context. Because of its extreme conditions, the continent’s inland ecosystems remain among the few habitats on Earth where simple prokaryotes and eukaryotes dominate — in other words, the very types of life that prevailed during the first two billion years of Earth’s history. “At the snowball earth phase, around 700 million years ago, complex life was on the rise and the simple eukaryotic lineages that we see today in Antarctic ponds were emerging,” says Hawes. The central question, he explains, is how these early organisms managed to survive for tens of millions of years under such extreme conditions — and later spread rapidly as the planet warmed again. Today’s Antarctica, he adds, serves as a kind of natural cold laboratory, allowing researchers to demonstrate that short-lived meltwater ponds could have been a potential refuge for these life forms.
The discovery that such communities also thrive in today’s extreme yet structurally comparable habitats suggests that they may likewise have endured earlier global glaciations. With this study, science gains a plausible new theory for how complex life could have survived even the most extreme chapters of Earth’s history.