After 30 days, the algae in the middle were still single-celled. However, as the scientists placed algae in increasingly thick rings under the microscope, they found larger clumps of cells. The largest were bundles of hundreds of cells. But what interested Simpson most were the mobile groups of four to 16 cells, arranged so that their flagella were all facing outward. These groups moved by coordinating the movement of their flagella: those at the back of the group stayed still, and those at the front moved.
Comparing the speed of these groups to that of individual cells in the medium revealed something interesting. “They all swim at the same speed,” Simpson said. By working together as a collective, the algae were able to preserve their mobility. “I was very pleased,” he said. “With the basic mathematical framework, I was able to make some predictions. Seeing it empirically means there is some truth to this idea.”
Interestingly, when the scientists took these little clumps out of the high-viscosity gel and put them back into a low-viscosity gel, the cells stuck together. In fact, they stayed that way for as long as the scientists continued to observe them — about 100 more generations. It’s clear that the changes they underwent to survive in a high-viscosity gel were difficult to reverse, Simpson said — perhaps a step toward evolution rather than a short-term change.
ILLUSTRATION
Caption: In a gel as viscous as ancient oceans, algae cells began to work together. They grouped 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
Today’s algae are not primitive animals, but the fact that these physical pressures forced a single-celled creature into an alternative way of life that was difficult to reverse is very striking, Simpson said. He suspects that if scientists explore 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 motion could be a monumental obstacle to microscopic life, he hasn’t been able to stop thinking about it. Viscosity may have been very important in the origins of complex life, whenever that was.
“(This perspective) allows us to think about the deep history of this transition,” Simpson said, “and what was happening in Earth history when all the complicated, obligate multicellular groups evolved, which are relatively close to each other, we think.”
Other researchers consider Simpson’s ideas to be quite novel. Before Simpson, no one seemed to have thought much about the physical experience of organisms being in the ocean during Snowball Earth, he said. Nick Butterfield Carl Carlson of Cambridge University, who studies the evolution of early life, cheerfully noted, however, that “Carl’s idea is marginal.” That’s because the vast majority of theories about Snowball Earth’s influence on the evolution of multicellular animals, plants and algae focus on how oxygen levels, inferred from isotope levels in rocks, might have tipped the balance one way or another, he said.
