A new clue to how multicellular life may have evolved
But what catalyzed the leap from single-celled organisms to complex multicellular life?
A new study published in Nature Physics suggests that physical forces, particularly fluid dynamics associated with cooperative feeding, may have played a key role.
"So much work on the origins of multicellular life focuses on chemistry," explained Shashank Shekhar, lead author and assistant professor of physics at Emory University. "We wanted to investigate the role of physical forces in the process."
Shekhar drew inspiration while observing stentors, trumpet-shaped single-celled ciliates that filter feed near pond surfaces. Using time-lapse microscopy, he captured how individual and grouped stentors influence fluid motion to gather food.
"The project started with beautiful images of the fluid flows," Shekhar said. "Only later did we realize the evolutionary significance of this behavior."
The research team found that when stentors cluster, they collectively produce stronger currents, allowing them to pull in food particles from farther away. This cooperative behavior offers a potential window into how early life forms began to function in multicellular arrangements.
The study positions stentors as a model for understanding the shift from unicellular to multicellular life, highlighting how physical interactions could foster evolutionary advantages.
Shekhar collaborated with John Costello, a marine biologist at Providence College, and Eva Kanso, a mathematician at the University of Southern California. Their work originated during a 2014 research program at the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts.
"Renowned scientists come there every summer from around the world for organic collaborations," Shekhar said of the MBL. "You have the time and resources to explore extreme questions that capture your interest."
Key contributors included Sean Colin, a marine biologist at Roger Williams University, and Wallace Marshall, a cell biologist at the University of California, San Francisco, who studies regeneration using stentors.
Stentors, named after a loud-voiced Greek mythological figure, are among the largest known single-celled organisms. Their horn-like shape includes a narrow base with a sticky holdfast that anchors them to surfaces, and a wide mouth edged with cilia that generate currents to draw in bacteria and algae.
These ciliates can temporarily group into dome-like colonies using a sticky secretion, creating half-hemisphere formations that intensify feeding flows. Remarkably, each stentor is about 1 to 2 millimeters long, making them visible to the naked eye and ideal for detailed microscopic imaging.
In his lab, Shekhar introduced plastic microspheres into a fluid dish to visualize the flow patterns around Stentor coeruleus. The experiment revealed twin vortices created by the cilia.
He then examined how paired stentors interacted. As they oscillated toward and away from each other, their combined vortex strengthened, drawing in more food. Shekhar jokingly dubbed this behavior "I love you, I love you not."
Curiously, stentors periodically disengaged from these pairings. Larger colonies also exhibited dynamic head movements, enhancing fluid flow even further. The researchers proposed that weaker individuals benefited more from group feeding, while stronger ones rotated through partnerships to balance the advantages.
Mathematical modeling by Kanso and co-author Haniliang Guo of Ohio Wesleyan University supported these observations. The analysis confirmed that asymmetric benefits in pairs were offset in larger colonies, where dynamic group formations optimized feeding efficiency.
The findings offer a new perspective on how collective behavior and physical forces may have influenced the earliest steps toward multicellular 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."
This project marks a new research trajectory for Shekhar, whose lab traditionally focuses on actin proteins crucial to cell movement.
"The stentor work was a passion project," Shekhar 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."
Research Report:Cooperative hydrodynamics accompany multicellular-like colonial organization in the unicellular ciliate Stentor
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