After 30 days, the algae in the middle was still single-celled. But as the scientists examined algae from thicker and thicker rings under a microscope, they found larger clumps of cells; the largest contained hundreds of cells. But what most interested Simpson were the mobile clumps of four to 16 cells arranged with their flagella all on the outside. These clumps moved around by coordinating the movements of their flagella, while those at the back of the clump were stationary and those at the front were wiggling.
Comparing the speed of these clusters to that of a single cell in the middle revealed something interesting: “They’re all swimming at the same speed,” Simpson says. By working together as a collective, the algae can maintain their motility. “That was really exciting,” he says. “In a rough mathematical framework, we were able to make some predictions. To actually see it empirically means there’s something to this idea.”
Interestingly, when the scientists removed these tiny globs from the high-viscosity gel and put them back into the low-viscosity gel, the cells stuck together. In fact, the cells continued to stick together for as long as the scientists continued to observe — about 100 generations. The changes the cells underwent to survive in the high viscosity were clearly difficult to reverse, Simpson said. Perhaps this is not a short-term change, but an evolutionary move.
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Caption: In a gel as viscous as the ancient ocean, the algae cells began to cooperate. They clumped together, coordinated the movement of their tail-like flagella, and swam faster. When the viscosity was returned to normal, the cells stayed together.
Credit: Andrea Harring
Modern algae weren’t early animals, but the fact that these physical pressures forced single-celled organisms into a different way of life that was hard to reverse is pretty striking, Simpson says. Simpson thinks that if scientists study the idea that viscosity determines the existence of organisms when they’re very small, it might tell them something about the conditions that may have led to an explosion of larger organisms.
A cellular perspective
As large organisms, we don’t really think about the thickness of the liquid around us. It’s not part of our daily lives, and we’re so big that viscosity doesn’t affect us much. Being able to move around with relative ease is something we take for granted. It’s something that Simpson couldn’t stop thinking about when he first realized that restricted movement could be a big obstacle for tiny life. We don’t know when complex life originated, but viscosity may have played a big role.
“(This perspective) allows us to think about the long history of this transition,” Simpson said, “and we think about what was happening in Earth’s history when all the absolutely complex multicellular groups evolved, which happened relatively recently.”
Other researchers find Simpson’s ideas highly novel. Before Simpson, no one seems to have thought deeply about the physical experiences of organisms in the oceans during Snowball Earth, says Nick Butterfield, who studies the evolution of early life at the University of Cambridge. But he’s quick to point out that “Karl’s ideas are heretical.” That’s because most theories about Snowball Earth’s influence on the evolution of multicellular animals, plants, and algae focus on the possibility that oxygen levels, as inferred from isotope levels in rocks, might have somehow tipped the scales, he says.
