From single cells to complex life: Was cooperation the first step?

Single-celled organisms that group together can generate stronger water currents to pull in food, a benefit that may have encouraged early life forms to assemble into colonies, according to a new study. This could have set the stage for more complex, multicellular life.

Ultimately, the research suggests that the evolution of multicellular organisms may have been shaped by the fluid dynamics of cooperative feeding.

“So much work on the origins of multicellular life focuses on chemistry,” said Shashank Shekhar, lead author of the study and assistant professor of physics at Emory University. “We wanted to investigate the role of physical forces in the process.” 

Shekhar was inspired by observing the feeding behavior of stentors – large, trumpet-shaped, single-celled organisms. He began exploring how their movements through liquid might provide clues about the transition from single cells to complex life. 

“The project started with beautiful images of the fluid flows,” Shekhar said. “Only later did we realize the evolutionary significance of this behavior.”

The physics of feeding

Stentors are visible to the naked eye and are commonly found floating near the surface of freshwater ponds. These organisms can grow up to two millimeters in length – roughly the size of a pencil tip. 

They attach to leaves, twigs, or other floating matter using a sticky secretion from one end of their body, while the wider, open end is surrounded by cilia – tiny hair-like structures that beat rhythmically to create water currents and pull in food particles.

Using microscope video, Shekhar captured how a single stentor feeds by creating twin vortices at its mouth, which effectively draw in food like bacteria and algae. 

To make the fluid motion visible, he added plastic beads small enough to trace the currents. This experiment revealed the efficiency of the cilia-generated flow in solitary feeding.

However, Shekhar noticed that stentors sometimes clustered together, forming pairs or even colonies in a semi-spherical shape. When he recorded the feeding dynamics of these groups, he discovered something intriguing: 

“I call that movement ‘I love you, I love you not,’” Shekhar said, referring to how pairs of stentors would sway toward and away from each other. When their heads drew closer together, their fluid flows merged into a single, stronger vortex that attracted food from a larger area.

Single-celled organisms working together 

The feeding advantage increased further when stentors joined larger groups. Colonies were able to generate currents significantly more powerful than individual organisms or even pairs, drawing in more food from farther away.

This finding raised a question: if cooperation improved feeding, why did some stentors occasionally leave the group?

The team proposed that not all stentors gained equally from forming colonies. “The colonies are dynamic as the stentors keep changing partners,” Shekhar explained. “The stronger ones are being taken advantage of, in a sense. They change partners often so that everyone benefits similarly.” 

This suggested that stronger individuals might temporarily withdraw from the group to avoid being exploited by weaker ones, while the weaker stentors had more to gain by staying attached.

Cooperation in organisms without a brain

To test these ideas, the researchers turned to mathematical modeling. Co-authors Eva Kanso of the University of Southern California and Haniliang Guo of Ohio Wesleyan University helped construct simulations that reproduced the dynamics seen in the lab. 

The models confirmed that in pairings, one organism always had a greater advantage. Meanwhile, larger groups, especially those in which members periodically changed positions, increased the overall food intake for each member on average.

These findings suggest that the benefits of forming temporary colonies – especially the ability to draw in more food – could have played a role in the early transition from single cells to complex life. 

“It’s amazing that a single-celled organism, with no brain or neurons, developed behaviors for opportunism and cooperation,” Shekhar said. “Perhaps these kinds of behaviors were hard-wired into organisms much earlier in evolution than we previously realized.”

The evolution of multicellular organisms

The research emerged from a decade-long journey that began in 2014, when Shekhar joined a summer program at the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts. There, he was able to collaborate with scientists from various universities. 

“Renowned scientists come there every summer from around the world for organic collaborations,” Shekhar said. “You have the time and resources to explore extreme questions that capture your interest.”

Marshall’s research on stentors and their ability to regenerate has sparked even greater curiosity about these unique cells. Their remarkable properties and visible behaviors made them ideal subjects for studying physical phenomena that could drive evolutionary change.

“You can chop up a stentor and each tiny piece will become a complete organism within 12 hours,” said Shekhar. 

From single cells to complex life

Though Shekhar is known for his work on actin, a protein essential to cell mobility, this project marked a new direction for his lab. 

“The stentor work was a passion project,” he said. “It’s wonderful to work at your own pace, over many years, on a question that fascinates you and wind up with such beautiful and significant results.”

The study offers a fresh perspective on how simple, physical interactions between organisms could have contributed to the emergence of complex life. 

While chemistry has long dominated theories about the origins of multicellularity, these results highlight the potential role of fluid mechanics and cooperation in shaping the earliest stages of biological organization.

The study is published in the journal Nature Physics.

Image Credit: Shashank Shekhar

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