This article was originally featured on Hakai Magazine, an online publication on science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.
Planet Earth used to be something like a cross between a deep freeze and a car wrecker. During much of the planet’s history, everything from pole to pole was sandwiched under a blanket of ice a kilometer or more thick. Scientists call this snowball Earth.
Some early animals were able to withstand this icy epoch from about 720 to 580 million years ago, but they had their work cut out for them. Despite their valiant successes, the repeated expansion and contraction of giant ice sheets pulverized the hardy extremophiles’ remains, leaving almost no trace of them in the fossil record and scientists with little or no idea how they managed to survive.
“It’s basically like having a giant bulldozer,” says Huw Griffiths of the British Antarctic Survey. “The next glacial expansion would have just erased all that and turned it to mush, basically.”
Despite the lack of direct evidence thanks to all that glaciation, Griffiths argues that it’s reasonable to suggest that a diverse range of wildlife inhabited the snowball land. He suggests that this flourishing would have predated the so-called Cambrian explosion, a period around 540 million years ago when a vast and unprecedented diversity of animal life appeared on Earth. “It’s not a huge leap of the imagination that there were much smaller, simpler things that existed before that,” says Griffiths.
The full picture of wildlife during this time is lost, but Griffiths and his colleagues take a stab in a recent paper to try to find out what maybe has looked.
The team considered three different frozen periods. The first was the Sturtian snowball Earth, which began about 720 million years ago. It lasted up to 60 million years. This is an incredibly long time – it is almost as long as the period between the end of the dinosaur era and today. Then came the Marinoan Snowball Earth, which started 650 million years ago and lasted only 15 million years. It was eventually followed by the Gaskiers Ice Age around 580 million years ago. This third ice age was even shorter and is often called a slushball rather than a snowball Earth because the ice cover was probably not as extensive.
Although the ice crushed most of the fossils from these periods, scientists have found a handful of remains. These rare fossils depict the strange animals that existed around the Gaskier Ice Age. Among these ancient slushball earth dwellers were the frondomorphs – organisms that looked a bit like fern leaves. Frondomorphs lived attached to the sea floor under the ice, possibly absorbing nutrients from the water as it flowed around them.
Short of direct evidence, Griffiths and his colleagues argue instead that the survival strategies of animals during the Great Freeze of the past are likely mirrored by life inhabiting the most similar environment on Earth today—Antarctica.
Some modern Antarctic inhabitants such as anemones live upside down attached to the underside of the sea ice. One of the favorite feeding strategies of krill is to graze on microorganisms on this upturned plane. Perhaps early animals sought and found shelter in such places as well, Griffiths and his colleagues suggest.
It is also possible that the waxing and waning of sea ice introduced algae or other micro-organisms that live on the ice into the seawater so that they could flourish, which may have provided food for other early animals.
One of the challenges that the inhabitants of a snowball earth faced was a possible lack of oxygen, both because the oxygen level in the air was low and because there was limited mixing from the atmosphere into the water. But oxygenated meltwater high in the water column may have supported animals that depended on it. Some inhabitants of the Antarctic seafloor today, such as certain species of feather stars, solve this problem by relying on water currents to bring a steady flow of oxygen and nutrients from the small areas of open water at the surface to deep beneath the ice shelves . There’s no reason to believe this didn’t happen during Gaskier’s slushball Earth period as well.
“We’re really talking about very basic life forms … but at the time that’s all you needed to be the king of the beasts,” says Griffiths.
Alongside frondomorphs, the seabed may also have been inhabited by sponges. Some fossil evidence of sponges dates back to well before the Sturtian Snowball Earth, although there is some debate about this, says Griffiths.
Ashleigh Hood, a sedimentologist at the University of Melbourne in Australia who was not involved in the research, jokes that “everyone, including us, has their oldest sponge that they found in the record, and nobody else believes them.”
Some modern sponges live symbiotically with bacteria, which can help them gain access to nutrients when other food is in short supply. “It’s probably based on a survival strategy they had very early in history,” Hood suggests.
Andrew Stewart, assistant curator at the Museum of New Zealand Te Papa Tongarewa who was also not involved in the paper, has studied countless species from harsh Antarctic environments. Many of these organisms thrive in incredibly dark, cold or chemically toxic places. For Stewart, Antarctic extremophiles are a reminder of how resilient life on Earth really is – and perhaps always has been.
“It’s just the most amazing place,” he says. “You go, no, nonsense, nothing can survive there! Well, actually it can.”