After 30 days, the algae inside were still single-celled. But when the scientists put the algae from thicker and thicker rings under a microscope, they found larger clusters of cells. The largest were clusters of hundreds. But what interested Simpson most were motile clusters of four to 16 cells, arranged so that their flagella were on the outside. These clusters moved by coordinating the movements of their flagella, the clusters at the back held still, the clusters at the front wriggling.
Comparing the speeds of these clumps with the individual cells inside revealed something intriguing. “They’re all swimming at the same speed,” Simpson said. By working together as a collective, the algae could maintain their mobility. “I was really pleased,” he said. “With a simple mathematical framework, I could make some predictions. Actually seeing it empirically means there’s something to the idea.”
Interestingly, when the researchers took these compact clusters out of the high-viscosity gel and put them back into the low-viscosity gel, the cells stuck together. They stayed that way, in fact, for about 100 generations as long as the researchers observed them. It’s clear that whatever changes they underwent to survive in the high-viscosity gel were tough to reverse, Simpson said—perhaps a move toward evolution rather than a short-lived change.
ILLUSTRATION
Caption: In a gel as viscous as the old oceans, algae cells began to cooperate. They clung together and coordinated the movements of their tail-like flagella to swim faster. When placed back into their normal viscosity, they stayed together.
Source: Andrea Halling
Up-to-date algae are not early animals. But the fact that these physical pressures forced a single-celled creature into an alternative way of life that was strenuous to reverse seems pretty powerful, Simpson said. He suspects that if scientists explore the idea that when organisms are very compact, viscosity dominates their existence, we might learn something about the conditions that could have led to an explosion of enormous life forms.
Cell Perspective
As enormous creatures, we don’t think much about the density of the fluids around us. It’s not part of our everyday experience, and we’re so gigantic that viscosity doesn’t affect us much. The ability to move around easily—relatively speaking—is something we take for granted. Ever since Simpson first realized that such restrictions on movement could be a monumental obstacle to microscopic life, he couldn’t stop thinking about it. Viscosity may have played a gigantic role in the beginnings of sophisticated life, whenever that was.
“[This perspective] allows us to think about the deep-rooted history of this transformation,” Simpson said, “and what was happening in Earth’s history when all of these obligately complex multicellular groups evolved, which we think happened relatively close together.”
Other researchers consider Simpson’s ideas quite novel. Before Simpson, no one seemed to have thought much about the physical experience of organisms in the ocean during Snowball Earth, he said. Nick Butterfield from the University of Cambridge, who studies the evolution of early life. But he was pleased to note that “Carl’s idea is marginal.” That’s because the expansive majority of theories about how the 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 another, he said.