The Physics of Cold Water May Have Jump-Started Complex Life
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.
Once upon a time, long ago, the world was encased in ice. That’s the tale told by sedimentary rock in the tropics, many geologists believe. Hundreds of millions of years ago, glaciers and sea ice covered the globe. The most extreme scenarios suggest a layer of ice several meters thick even at the equator.
This event has been called Snowball Earth, and you’d think it would be a terrible time to be alive—and maybe, for some organisms, it was. However, in a warmer period between glaciations, the first evidence of multicellular animals appears, according to some interpretations of the geological record. Life had taken a leap. How could the seeming desolation of a Snowball Earth line up with this burst of biological innovation?
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.
To test the idea, Simpson, a paleobiologist at the University of Colorado, Boulder, and his team conducted an experiment designed to see what a modern single-celled organism does when confronted with higher viscosity. Over the course of a month, he and his graduate student Andrea Halling watched how a type of green algae—members of a lab-friendly species that swims with a tail-like flagellum—formed larger, more coordinated groups as they encountered thicker gel. The algae collectively motored through the fluid to keep up their feeding pace. And, intriguingly, the groups of cells remained stuck together for 100 generations after the experiment ended.
The research offers a novel take on the emergence of multicellular life, said Phoebe Cohen, a paleontologist at Williams College who has spoken with Simpson about his idea over the years but was otherwise uninvolved with the work. The field is overflowing with papers about triggers for the evolution of animal multicellularity that draw on geochemical measurements, she said, but few consider the biology of individual organisms.
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