Humans often believe that being multicellular provides distinct advantages, especially considering that around 80% of life on Earth consists of single-celled organisms, many of which thrive in environments inhospitable to larger creatures. The mystery of how and why multicellular life emerged has long baffled biologists.

The earliest known multicellular organisms appeared approximately 2.5 billion years ago when marine cyanobacteria began forming filamentous colonies. However, the mechanisms behind this dramatic transition and the benefits it conferred upon the individual cells remain unclear.

Recent research from the Marine Biological Laboratory (MBL) provides a groundbreaking perspective, suggesting that physical forces, rather than solely chemistry, have played a significant role in the evolution of multicellular life. The study is detailed in the prestigious journal Nature Physics and focuses on the cooperative feeding dynamics of the Stentor, a large unicellular organism.

John Costello, a senior author on the study from Providence College, started by posing a critical question: Why did these independent cells start clustering into colonies? The team aimed to explore this phenomenon through the lens of physics instead of relying on traditional biochemical explanations.

The Power of Cooperation

Stentor is a trumpet-shaped single-celled organism that can reach lengths of up to 2 mm. In its natural pond habitat, it secures itself by attaching its holdfast to leaves or twigs, while its trumpet-shaped end moves freely, creating water vortexes to capture food with its cilia-lined mouth.

Experimental observations revealed that when Stentors are introduced into a controlled lab environment, they quickly form loosely connected colonies, where their holdfasts touch but do not permanently attach. Researchers discovered that these neighboring Stentors can significantly enhance their collective food intake, doubling the flow rate of water that reaches their mouths when they coordinate their feeding.

An Unexpected Twist: Unequal Benefits in Cooperation

Intriguingly, the study found that not all Stentors benefit equally from this collaboration. The weaker individual in a pair tends to gain more from the dynamic partnership compared to its stronger counterpart. A fascinating behavior emerged during their observations, which lead author Shashank Shekhar describes as "she loves me, she loves me not." When two Stentors sway together to optimize their feeding, they regularly veer apart and then return to one another—a phenomenon that prompted deeper investigation.

Through mathematical modeling of fluid dynamics within the colony, co-authors Hanliang Guo and Eva Kanso identified a pattern of "promiscuity." The Stentors continually switch between neighbors, resulting in all individuals within the colony experiencing enhanced feeding flows. This strategy allows each Stentor to maximize its energetic returns.

Ephemeral Collaborations: Why Stentors Go Solo

Despite their collective advantages, Stentor colonies are temporary, easily dispersed by minor disturbances. This raises a compelling question: If cooperation is beneficial, why do they ultimately separate? The researchers note that Stentors tend to cluster more when food is plentiful, indicating that resource availability drives their cooperative behavior. Conversely, when resources dwindle, they revert to solitary foraging.

The intriguing parallel to human behavior highlights how social cooperation often hinges on resource availability, suggesting deeper biological narratives about survival strategies across species.

A Model for Early Multicellularity

Stentor colonies differ notably from other models of early multicellular life, such as the genetically uniform colonies of Volvox carteri, where improper cell division leads to the formation of a stable, differentiated organism. The Stentor's colonies consist of genetically distinct individuals, suggesting that their behavior may represent an earlier evolutionary stage toward true multicellularity.

Shekhar emphasizes the novelty of this model: "What we observed is a foundational step, where happy single cells decided to come together for mutual benefit, only to revert to single living when circumstances changed. True multicellularity, it seems, was still a distant goal."

This research not only expands our understanding of evolutionary biology but also reshapes how we perceive the transition from solo survival to intricate social collaboration, setting the stage for future studies on the evolution of multicellularity.