Icy Origins: How Cold Water Could Have Sparked Life

Geologists refer to the period when Earth was covered in ice as “Snowball Earth.” During this time, thick layers of ice and glaciers extended all the way to equator. Although this ice age might seem harsh for life, it’s surprising that the first multicellular animals appeared during a milder period between ice ages. This discovery raises intriguing questions about how cold water could have sparked the development of life.

Researchers are wondering how life could exist in such a cold period. Paleobiologist Carl Simpson and his colleagues hypothesize that the secret may be in the way that cold saltwater impacts microscopic life. Seawater gets more viscous as it cools, similar to swimming through honey rather than water. Small creatures find it more difficult to migrate and obtain nourishment as a result. In order to overcome these obstacles they may have evolved to form larger groups & collaborate more successfully which resulted in the emergence of multicellular life.

Simpson and his team studied green algae in the lab to test their theory. They found that the algae formed larger, coordinated groups to move and feed better when placed in thicker gel. These groups stayed together for 100 generations after the experiment ended.

This research gives new insights into how multicellular life might have begun. While much research looks at environmental changes, Simpson’s study highlights how individual organisms adapt and evolve.

Cohen praised the experiment, noting it’s great to see research exploring how early organisms experienced their environment. However, the study is still pending peer review and is only a preprint.

The research suggests that Snowball Earth may have led to the evolution of complex life because of the effects of cold water.

A Frozen Paradox

Snowball Earth was a hot topic in the late 1990s. Geochemist Joseph Kirschvink first proposed evidence for global ice coverage, and Paul Hoffman’s 1998 paper supported this with sedimentary data showing glaciers in warm regions around 700 million years ago.

Simpson found this timing puzzling. He noted that while Snowball Earth seemed to contradict the evidence of evolving complexity, fossils from before this period are tiny, whereas those from after are large and complex.

Although dating the rise of multicellular animals is tricky, molecular clocks suggest their common ancestor appeared during the Sturtian Snowball Earth, about 717 to 660 million years ago. Larger, complex animals appeared in the fossil record after Earth’s ice melted around 635 million years ago.

The puzzle of how life evolved during Snowball Earth, a time when the planet was covered in ice, fascinated Simpson throughout his career. In 2018, as an assistant professor, he realized that colder seawater becomes thicker and more viscous. This meant that during Snowball Earth, the ocean was much thicker than usual.

Simpson thought about what it would be like for tiny organisms living in such thick water. Cold and viscous water makes it hard for single-celled organisms to get enough food because diffusion slows down, meaning nutrients move less efficiently. Even those that can move, like cells with tails, find it harder to find food in thicker water.

Larger organisms or cell groupings, on the other hand, are better adapted to survive in thick water. They can push through it easier because of their combined mass. Simpson (2021) postulated that during Snowball Earth, presence of thicker water may have forced certain species to evolve into multicellular forms in order to survive. To test this theory, he and his colleagues developed mathematical models. They discovered that whereas self-propelling cells that could cling together formed larger groupings, single cells that fed by diffusion tended to decrease in thicker fluids. It suggests that larger and more sophisticated organisms may have been encouraged to form in the thick water.

The results were interesting, but they were based only on computer models. Simpson wondered, “What if we tested this on real organisms?”

His colleague, geologist Boswell Wing, had a colony of Chlamydomonas reinhardtii algae in his lab. These algae can move using their flagella and are usually single-celled. However, under stress, they can become multicellular. Simpson wondered if higher viscosity, like in Snowball Earth’s thick oceans, could trigger this change.

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Life in Thick Water

Biologists can’t go back in time to test Snowball Earth’s conditions, but they can recreate parts of it in the lab. Halling and Simpson made a custom petri dish with a bull’s-eye pattern of agar gel. The center had normal viscosity for algae, and each ring around it got thicker, with the outer ring being four times more viscous. They placed algae in the middle, turned on a camera, and observed them for 30 days, allowing about 70 generations of algae to live and adapt.

Halling and Simpson thought that as algae reproduced in normal conditions, those that could handle thicker, more viscous water might spread outward to the outer rings. They wondered if algae that reached the highest viscosity ring would adapt in some way, such as increasing their swimming speed.

After 30 days, the algae in the center remained single-celled, but in the thicker, outer rings, they formed larger clusters. Some of the largest clusters had hundreds of cells, while smaller clusters had four to sixteen cells with their flagella arranged on the outside. These clusters moved by coordinating their flagella with cells at the back staying still and those at the front wriggling.

Simpson found that these clusters swam at the same speed as single cells. By working together, they maintained their mobility. When scientists moved these clusters from thick to thin water, cells continued to stick together for about 100 more generations. This suggests that the changes made to survive in thick water were long-lasting and possibly a step toward evolution.

Modern algae aren’t early animals, but Simpson finds it striking that physical pressures made single-celled organisms form lasting groups. He thinks that understanding how small organisms cope with thick fluids could reveal more about the origins of complex life.

As large creatures, we don’t notice how fluid thickness affects us. But for tiny organisms, viscosity is a major challenge. Simpson believes this might have played a big role in the evolution of complex life.

Other researchers, like Nick Butterfield, find Simpson’s ideas novel but somewhat unconventional. Most theories focus on oxygen levels affecting evolution, not fluid thickness. Jochen Brocks, however, thinks Simpson’s hypothesis has some gaps and questions the connection between Snowball Earth and the origins of animals.

Despite the debate, Brocks appreciates Simpson’s experiment. It’s valuable to explore how organisms might adapt to thick fluids and develop collective behaviors, regardless of whether it directly relates to Snowball Earth or not.

Brocks is interested in testing Simpson’s ideas with choanoflagellates, tiny creatures closely related to animals. Unlike algae, choanoflagellates hunt for food and would be more affected by thick fluids. If they evolved multicellular forms in high viscosity, it could support Simpson’s theory about how life adapts to its environment.

Simpson is already studying choanoflagellates to understand their behavior. These creatures are fascinating and versatile: they swim fast or slow, stick to surfaces, form colonies, and even transform into amoeba-like forms when squeezed. Simpson believes they have many ways to adapt to new challenges.