Anika Begay
After 30 days, the algae in the center were still single-celled. But when the scientists held increasingly thick rings of algae under the microscope, they found larger clusters of cells. The largest were groups of hundreds. But what Simpson was most interested in were the mobile clusters of four to 16 cells, arranged so that their flagella were all on the outside. These clusters moved by coordinating the motion of their flagella, with those at the back of the cluster staying still and those at the front wriggling.
Comparing the speed of these clusters to the individual cells in the center, something interesting emerged. “They’re all swimming at the same speed,” Simpson said. By working together as a collective, the algae were able to preserve their mobility. “I was really pleased,” he said. “With the rough mathematical framework, there were some predictions I could make. To actually see it empirically means there’s something to this idea.”
Interestingly, when the scientists took these little clusters out of the high-viscosity gel and put them back into the low-viscosity one, the cells stuck together. They stayed that way, in fact, for as long as the scientists continued to observe them, about another 100 generations. Clearly, whatever changes they had undergone to survive the high viscosity were difficult to reverse, Simpson said, perhaps a step toward evolution rather than a short-term change.
ILLUSTRATION
Caption: In a viscous gel like ancient oceans, algal cells began to work together. They huddled together and coordinated the movements of their tail-like flagella to swim faster. When they were returned to normal viscosity, they stayed together.
Credit: Andrea Halling
Modern algae are not primitive animals. But the fact that these physical pressures forced a single-celled creature into an alternative lifestyle that was difficult to reverse seems pretty powerful, Simpson said. He suspects that if scientists explored the idea that when organisms are very small, viscosity dominates their existence, we might learn something about the conditions that might have led to the explosion of large life forms.
A cell’s perspective
As large creatures, we don’t think much about the thickness of the fluids around us. It’s not part of our everyday experience, and we’re so big that viscosity doesn’t affect us much. The ability to move easily, relatively speaking, is something we take for granted. Ever since Simpson first realized that such limits to movement could pose a monumental obstacle to microscopic life, he hasn’t been able to stop thinking about it. Viscosity may have played some role in the origins of complex life, whatever that was.
“[This perspective] “It allows us to think about the deep history of this transition,” Simpson said, “and what was going on in Earth history when all the obligately complicated multicellular groups evolved, which are relatively close together, we think.”
Other researchers find Simpson’s ideas novel. Before Simpson, no one seemed to have given much thought to the physical experience of organisms in the ocean during a snowball Earth, said Nick Butterfield of the University of Cambridge, who studies the evolution of early life. But he noted cheerfully that “Carl’s idea is marginal.” That’s because the vast majority of theories about how snowball Earth influenced the evolution of multicellular animals, plants, and algae focus on how oxygen levels, inferred from isotope levels in rocks, might have tipped the scales one way or the other, he said.
