Highlights
When seawater gets cold, it gets viscous. This fact could explain how single-celled ocean creatures became multicellular when the planet was frozen during “Snowball Earth,” according to experiments.
A series of papers from the lab of Carl Simpson proposes an answer linked to a fundamental physical fact: As seawater gets colder, it gets more viscous, and therefore more difficult for very small organisms to navigate. Imagine swimming through honey rather than water. If microscopic organisms struggled to get enough food to survive under these conditions, as Simpson’s modeling work has implied, they would be placed under pressure to change — perhaps by developing ways to hang on to each other, form larger groups, and move through the water with greater force. Maybe some of these changes contributed to the beginning of multicellular animal life.
The experiment comes with a few caveats, and the paper has yet to be peer-reviewed; Simpson posted a preprint on biorxiv.org earlier this year. But it suggests that if Snowball Earth did act as a trigger for the evolution of complex life, it might be due to the physics of cold water.
It is difficult to precisely date when animals arose, but an estimate from molecular clocks — which use mutation rates to estimate the passage of time — suggests that the last common ancestor of multicellular animals emerged during the era known as the Sturtian Snowball Earth, sometime between 717 million and 660 million years ago. Large, unmistakably multicellular animals appear in the fossil record tens of millions of years after the Earth melted following another, shorter Snowball Earth period around 635 million years ago.
The paradox — a planet seemingly hostile to life giving evolution a major push — continued to perplex Simpson throughout his schooling and into his professional life. In 2018, as an assistant professor, he had an insight: As seawater gets colder, it grows thicker. It’s basic physics — the density and viscosity of water molecules rises as the temperature drops. Under the conditions of Snowball Earth, the ocean would have been twice or even four times as viscous as it was before the planet froze over.
As large creatures, we don’t think much about the thickness of the fluids around us. It’s not a part of our daily lived experience, and we are so big that viscosity doesn’t impinge on us very much. The ability to move easily — relatively speaking — is something we take for granted. From the time Simpson first realized that such limits on movement could be a monumental obstacle to microscopic life, he hasn’t been able to stop thinking about it. Viscosity may have mattered quite a lot in the origins of complex life, whenever that was.
“Putting this into our repertoire of thinking about why these things evolved — that is the value of the entire thing,” he said. “It doesn’t matter if it was Snowball Earth. It doesn’t matter if it happened before or after. Just the idea that it can happen, and happen quickly.”