Modern-day algae are not early animals. But the fact that these physical pressures forced a unicellular creature into an alternate way of life that was hard to reverse feels quite powerful, Simpson said. He suspects that if scientists explore the idea that when organisms are very small, viscosity dominates their existence, we could learn something about conditions that might have led to the explosion of large forms of life.

A Cell’s Perspective

As large creatures, we don’t think much about the thickness of the fluids around us. It’s not a part of our daily lived experience, and we are so big that viscosity doesn’t impinge on us very much. The ability to move easily — relatively speaking — is something we take for granted. From the time Simpson first realized that such limits on movement could be a monumental obstacle to microscopic life, he hasn’t been able to stop thinking about it. Viscosity may have mattered quite a lot in the origins of complex life, whenever that was.

“[This perspective] allows us to think about the deep-time history of this transition,” Simpson said, “and what was going on in Earth’s history when all the obligately complicated multicellular groups evolved, which is relatively close to each other, we think.”

Other researchers find Simpson’s ideas quite novel. Before Simpson, no one seems to have thought very much about organisms’ physical experience of being in the ocean during Snowball Earth, said Nick Butterfield of the University of Cambridge, who studies the evolution of early life. He cheerfully noted, however, that “Carl’s idea is fringe.” 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 levels of oxygen, inferred from isotope levels in rocks, could have tipped the scales in one way or another, he said.

That novelty is a strength, said the geobiologist Jochen Brocks of the Australian National University. However, in his assessment, Simpson’s hypothesis makes a few logical leaps that don’t hold up. It’s not clear that the earliest animals would have been swimming freely in water, Brocks said. Some of the first fossils that can be confidently called “animals” were anchored on the ocean floor.

Perhaps more importantly, the timeline of animal origins is very uncertain. Some estimates suggest that the Snowball Earth period might line up with the last common ancestor of animals. But these are based on molecular inferences from DNA that are hard to confirm, Brocks said. In his opinion, it’s difficult to say how much importance to assign to this era. Butterfield also remarked on this uncertainty: “There’s no evidence of anything getting large until quite a bit after [Snowball Earth].”

That said, Brocks found Simpson’s experiment quite clever and beautiful. The fact that organisms might respond to high viscosity by developing collective behavior deserves to be better understood, he said — whether Snowball Earth led to the evolution of complex animal life or not.

“Putting this into our repertoire of thinking about why these things evolved — that is the value of the entire thing,” he said. “It doesn’t matter if it was Snowball Earth. It doesn’t matter if it happened before or after. Just the idea that it can happen, and happen quickly.”

Brocks is curious about what would happen if a similar experiment were performed with choanoflagellates, little creatures that are more closely related to animals than algae are. They rely entirely on hunting to get food — they can’t photosynthesize — so they would be especially vulnerable to slowdowns caused by high viscosity. If they started to take on multicellular forms under those conditions, that would suggest that Simpson’s results represent a more general truth about how life responds to its environment. “It would be absolutely ultra-exciting,” he said.

Simpson is, in fact, currently working with choanoflagellates. Right now, he is trying to understand how they live.

“They’re really beautiful and complicated creatures,” he said. They can take on many different forms: There are fast swimmers with long flagella, slow swimmers that meander, ones that stick to a surface to grow. “They can grow these little tendrils off the tip and walk around like on stilts; they have sex, and they fuse, and they form chain colonies and rosette colonies … and if you squeeze them, apparently they’ll lose their flagella and turn into an amoeba,” he said. When it comes to responding to the challenges of a radical new environment, he reflected, “they’ve got a lot to work with.”