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Hundreds of millions of years ago, long before the first plants and animals evolved, the planet was almost entirely covered in ice.

During this period known as Snowball Earth, temperatures across the planet repeatedly plummeted to well below freezing. But the cellular life that had already evolved managed to endure.

New research suggests that our ancient microscopic ancestors may have survived this icy period by sheltering in pools of water that formed on top of the relatively shallow ice sheets near the Earth’s equator.

To test these theories, scientists have been exploring meltwater ponds on the McMurdo Ice Shelf in Antarctica. They believe that the conditions here are likely similar to those that occurred in the equatorial regions during the Snowball Earth event.

Dr Anne Jungblut, a microbial researcher at the Natural History Museum, was involved in this latest research.

“We analysed samples from a variety of these ponds and found that they can support diverse communities of microorganisms,” says Anne.

“Each pond had clear traces of eukaryotic life, which are complex organisms whose cellular ancestors eventually gave rise to the huge diversity of life, including animals and plants, that we see today.”

“We can see from fossils that eukaryotes were around before and after Snowball Earth, so we know they made it through this period of intense freezing, and meltwater ponds might be how they did it!”

The study, which has been published in Nature Communications, was led by researchers at the Massachusetts Institute of Technology with co-authors from Cardiff University and University of Waikato in New Zealand.

How did life survive during Snowball Earth?

Snowball Earth is often used to refer to two consecutive glaciation events that took place between 635 and 720 million years ago, during a time known as the Cryogenian Period.

During these events the global average temperature plummeted to below -50°C, but conditions at the equator may have been somewhat more variable.

The slightly warmer temperatures around the middle of the Earth melted the top layers of ice to form meltwater ponds that hovered around 0°C. This stable, warmer temperature could have served as a habitable refuge for some forms of complex life.

The diverse communities of microorganisms that lived in these ponds would have created their own ecosystems that allowed life to survive. It is these communities that Anne and her colleagues have been studying in Antarctica.

“In Antarctic meltwater ponds, the bottom is often covered with microbial mats,” says Anne. “These mats contain colonies of microorganisms, including bacteria and eukaryotes such as amoeba, fungi and ciliates.”

Microbial mats form from the build-up of multiple layers of bacteria, such as cyanobacteria. Cyanobacteria evolved before the Snowball Earth event, so these mats may have been present in the meltwater ponds during this time.

“These mats are super exciting to study because they are entire ecosystems of microscopic life,” explains Anne. “They are almost like forests where the cyanobacteria are the trees that provide shelter and resources for other microorganisms. Some eukaryotes graze on the bacteria, while others are predatory. We can see so many interactions going on that can tell us a lot about how life interacted during early Earth.”

How can this help with the search for life on other planets?

The study of microorganisms in extreme environments not only provides insight into early Earth but can aid in the search for life on other icy worlds in the solar system.

This is because the way in which the scientists detect the presence of life in Antarctic ponds. Rather than looking for the microorganisms themselves, they can search for biosignatures. These biosignatures include molecules like DNA and lipids. The latter are a group of organic compounds that make up the cell walls and are useful for energy storage in living organisms.

One type of these lipids that occurs in all eukaryotes are called Sterols. The research team were able to use these to detect the presence of complex eukaryotic life in these ponds.

By using the same method of detecting and interpreting biosignatures, scientists think this could help in the search for life on other objects in the solar system.

“The more we understand about these biosignatures, the more we can learn about how they differ between organisms and how they might be affected by their environment,” says Anne. “This work can help us understand the signatures to look for during the search for life elsewhere in the solar system.”

For instance, Saturn’s moon Enceladus is a small world with liquid water beneath an icy crust that scientists believe could potentially support life.

Enceladus also has geyser-like jets that spew water vapour and ice particles into space. Future missions could include an orbiter that will pass through these geysers and capture liquid which could then be analysed for biosignatures of life.

You can learn more about the search for life on the icy moons of Jupiter and Saturn in our latest exhibition, Space: Could Life Exist Beyond Earth?