When the Earth was enveloped in ice, where did life find refuge? MIT researchers suggest that one possible sanctuary might have been pools of melted ice scattered across the planet’s frigid exterior.
In a study released today in Nature Communications, the scientists indicate that 635 million to 720 million years ago, during epochs known as “Snowball Earth,” when much of the planet was encased in ice, some of our ancient cellular predecessors may have survived in meltwater lagoons.
The researchers discovered that eukaryotes — intricate cellular organisms that ultimately evolved into the diverse multicellular beings we see today — could have endured the global freeze by dwelling in shallow bodies of water. These modest, aquatic havens may have existed atop relatively thin ice sheets found in equatorial zones. In these regions, the ice surface could accumulate dark-hued dust and debris from beneath, enhancing its capability to transform into pools. At temperatures hovering around 0 degrees Celsius, the resultant meltwater ponds could have provided livable environments for certain types of early complex life.
The team drew its deductions based on the examination of contemporary meltwater ponds. Presently in Antarctica, small bodies of melted ice can be observed along the edges of ice sheets. The conditions along these polar ice sheets are akin to what likely prevailed along ice sheets near the equator during Snowball Earth.
The researchers assessed samples from various meltwater ponds situated on the McMurdo Ice Shelf, in an area first described by members of Robert Falcon Scott’s 1903 expedition as “dirty ice.” The MIT scientists uncovered distinct signs of eukaryotic life in every pond. The assemblages of eukaryotes varied among ponds, presenting an unexpected diversity of life within the setting. The team also discovered that salinity significantly influences the types of life a pond can accommodate: Ponds with higher salinity had more similar eukaryotic communities, in contrast to those with fresher waters.
“We’ve demonstrated that meltwater ponds are viable candidates for where early eukaryotes could have sought refuge during these global glaciation periods,” remarks lead author Fatima Husain, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “This illustrates that diversity is indeed present and attainable in these environments. It’s fundamentally a testament to life’s resilience.”
The study’s MIT co-authors include Schlumberger Professor of Geobiology Roger Summons and former postdoctoral researcher Thomas Evans, along with Jasmin Millar from Cardiff University, Anne Jungblut at the Natural History Museum in London, and Ian Hawes of the University of Waikato in New Zealand.
Polar plunge
“Snowball Earth” is the informal term for periods in Earth’s history when the planet became encased in ice. It often refers to the two consecutive, multi-million-year glaciation events that took place during the Cryogenian Period, which geologists date between 635 and 720 million years ago. Whether the Earth resembled a solidified snowball or a softer “slushball” remains subject to discussion. However, scientists agree on one point: Most of the planet underwent a profound freeze, with average global temperatures around minus 50 degrees Celsius. The crucial question has been: How and where did life endure?
“We’re keen on uncovering the roots of complex life on Earth. We observe evidence for eukaryotes before and after the Cryogenian in the fossil record, yet we largely lack direct evidence regarding where they may have existed during that time,” Husain explains. “The intriguing part of this mystery is that we know life persisted. We are simply trying to decipher how and where.”
Numerous theories exist regarding where organisms could have found refuge during Snowball Earth, such as certain areas of the open ocean (if such environments were present), near deep-sea hydrothermal vents, and beneath ice sheets. In evaluating meltwater ponds, Husain and her colleagues explored the hypothesis that surface meltwaters may have also supported early eukaryotic life during that period.
“Many hypotheses exist regarding where life could have survived and sheltered during the Cryogenian, yet we lack excellent parallels for all of them,” Husain observes. “Above-ice meltwater ponds exist on Earth today and are accessible, allowing us to focus on the eukaryotes inhabiting these environments.”
Small pond, big life
For their recent study, the scientists scrutinized samples collected from meltwater ponds in Antarctica. In 2018, Summons and colleagues from New Zealand ventured to a section of the McMurdo Ice Shelf in East Antarctica, known to contain small ponds of melted ice, each only a few feet deep and few meters wide. In this area, water freezes all the way to the seafloor, trapping dark sediments and marine organisms in the process. Wind-driven loss of ice from the surface generates a type of conveyor belt that gradually brings this trapped debris to the surface, where it absorbs solar warmth, causing ice to melt, while surrounding clear ice reflects incoming sunlight, resulting in the formation of shallow meltwater ponds.
The bottom of every pond is coated with mats of microbes that have accumulated over the years to form layers of sticky cellular communities.
“These mats can be several centimeters thick, colorful, and distinctly layered,” Husain states.
These microbial mats consist of cyanobacteria, prokaryotic, single-celled photosynthetic organisms that lack a cell nucleus or distinct organelles. While these ancient microbes are known to endure some of the harshest environments on Earth, including meltwater ponds, the researchers wanted to determine whether eukaryotes — complex organisms that evolved a cell nucleus and other membrane-bound organelles — could also withstand similar adverse conditions. Addressing this question would require more than just a microscope, as the defining traits of the microscopic eukaryotes among the microbial mats are too subtle for visual identification.
To characterize the eukaryotes, the team examined the mats for specific lipids they produce called sterols, as well as genetic components known as ribosomal ribonucleic acid (rRNA), both of which can aid in identifying organisms with varying degrees of specificity. These two independent analyses provided complementary signatures for particular eukaryotic groups. In the lipid research, the team discovered a variety of sterols and rRNA genes closely linked with specific types of algae, protists, and microscopic animals within the microbial mats. The researchers assessed the types and relative abundance of lipids and rRNA genes from pond to pond, revealing a surprising diversity of eukaryotic life across the ponds studied.
“No two ponds were identical,” Husain asserts. “There are recurring characters, but they’re present in varying proportions. We found diverse assemblages of eukaryotes from all major groups across all the ponds analyzed. These eukaryotes descended from those that survived the Snowball Earth. This truly emphasizes that meltwater ponds during Snowball Earth could have acted as above-ice oases that nurtured the eukaryotic life crucial for the diversification and proliferation of complex life — including us — later on.”
This research received support, in part, from the NASA Exobiology Program, the Simons Collaboration on the Origins of Life, and a MISTI grant from MIT-New Zealand.