By Mari Hansen 
A cluster of choanoflagellates appears almost hesitant when viewed under a microscope for the first time; tiny collars of hairlike filaments beat in unison to form a delicate sphere that appears ready to dissolve at any time. It’s difficult to ignore how tentative the structure seems, as though the cells are experimenting with an old notion: what if we stick together rather than drifting apart?
Life on Earth was confined to isolated cells for the majority of its history. Free-living microorganisms inhabited the oceans, feeding, dividing, and floating in currents that never needed cooperation. Somewhere along the line, some cells started adhering to each other. Eventually, forests, insects, whales, and humans emerged as a result of that silent choice that changed the course of life. It used to seem like there was a huge difference between single-celled independence and multicellular cooperation. Scientists are beginning to believe that rather than a single, spectacular leap, it was bridged through incremental, repeatable steps.
| Key Information | Details |
|---|---|
| Scientific Concept | Multicellularity (transition from single-celled to multicellular life) |
| Key Organism Studied | Choanoflagellates (e.g., Choanoeca flexa) |
| Research Focus | Environmental triggers and genetic mechanisms enabling cells to aggregate and cooperate |
| Research Institutions | Pasteur Institute (France), University of California Berkeley, Georgia Tech |
| Evolutionary Timeline | Multicellularity emerged independently multiple times over ~1 billion years |
| Mechanisms | Clonal multicellularity, aggregative multicellularity, cellular differentiation |
| Importance | Enabled complex organisms, division of labor, and biological specialization |
| Reference | https://www.science.org |
This story is being given more depth by recent research on aquatic microbes. A surprising discovery was made by researchers studying Choanoeca flexa, a choanoflagellate that inhabits transient coastal puddles: the organism alternates between solitary and multicellular forms based on environmental factors. The cells divide and stay attached, creating clonal clusters, when water fills the puddles and the salinity stays low. Unrelated cells start adhering as evaporation concentrates the salt, forming mixed assemblies. The cluster disperses once more as the situation deteriorates. Seeing this cycle is more like watching a negotiation with the environment than seeing a fixed organism.
It seems as though this adaptability calls into question the neat classifications that biology previously depended on. Researchers have long made a distinction between aggregative multicellularity, in which genetically different cells congregate, and clonal multicellularity, in which daughter cells remain together. The choanoflagellate that lives in puddles seems to do both, adapting its tactics to the changing environment. Similar experiments may have taken place in early life, testing cooperation as a survival strategy rather than as a permanent state.
The actual environment is important. Imagine sun-heated, evaporation-salted shallow pools forming in coastal rock. concentrate of nutrients. Oxygen concentrations vary. There are predators out there. Clustering has benefits under these pressures: shared metabolism reduces environmental stress, coordinated movement enhances feeding, and larger size deters predators. Aggregation, however, also increases the risk of conflict because freeloading cells may take advantage of the group. Even now, there is a delicate balance between competition and cooperation.
Another hint comes from genetics. Close relatives of animals, choanoflagellates have genes that were previously believed to be exclusive to complex organisms, such as those involved in communication and cell adhesion. These single-celled organisms already contain hundreds of gene families associated with multicellular functions, according to research conducted by Nicole King’s lab at UC Berkeley. The implication is subtly radical: multicellularity’s toolkit predated multicellular life. It’s possible that evolution reused pre-existing equipment rather than creating entirely new parts.
This notion is supported by laboratory experiments. Within a few hundred generations, biologists have managed to get yeast, which is typically single-celled, to form clusters resembling snowflakes. The selection pressure was straightforward: larger clumps were preferentially retained and settled more quickly. Some cells even developed the ability to self-sacrificing over time, disassembling clusters to liberate reproductive units. Researchers observe the improvisational nature of evolution as these snowflake colonies form in glass flasks, with cooperation emerging from self-interest under the correct conditions.
Uncertainty still persists. Whether early multicellular clusters developed primarily to manage environmental stress, increase feeding efficiency, or avoid predators is still unknown. It’s likely that different lineages took different routes. Numerous independent evolutions of multicellularity indicate that the transition was a recurrent ecological problem-solving mechanism rather than an isolated miracle.
The transition appears more like a series of tentative experiments than a sudden revolution when viewed through the lens of deep time. Cells adhered to one another, split apart, specialized, and then tried again. A few assemblies didn’t work. Some persevered long enough to improve collaboration, dividing work and stifling internal strife. These brittle collectives eventually became stable organisms whose cells no longer existed for their own sake.
There is an odd familiarity in witnessing contemporary microbes alternate between cooperation and solitude. It seems as though the origins of complex life are still discernible in these microscopic negotiations, which are taking place in lab dishes, puddles, and sediments. Perhaps the first multicellular clusters did not herald a new era. They just hung on a bit longer than they had previously, and that made all the difference.