In a laboratory in Indiana, single-celled organisms did something their ancestors haven't done in a billion years: they stuck together. Ruibao Li and his team at Indiana University Bloomington discovered that Ministeria vibrans, a humble unicellular organism distantly related to animals, begins to cluster and form multicellular clumps when exposed to a specific bacterium. This seemingly small moment may illuminate one of evolution's most profound mysteries—how the first animals emerged from a world of solitary cells.
For billions of years before animals evolved, Earth was inhabited exclusively by single-celled organisms. Yet at some point, cells began adhering to one another, cooperating, and reproducing together as multicellular entities. Some of these early collectives evolved into plants and fungi; others became animals. Today, every animal body is built from trillions of cells working in concert. But how and why this cooperation began has long puzzled scientists. Li and his colleagues set out to find answers by studying M. vibrans, a unicellular organism that shares ancient ancestry with modern animals.
The mechanism behind the discovery is elegantly simple. M. vibrans survives by consuming bacteria, so Li systematically tested different bacterial diets. When he introduced one particular bacterium, something remarkable happened: the cells began sticking together. The reason, Li's team determined, was survival efficiency. The bacteria became trapped within the aggregating cell cluster, making it far more efficient for M. vibrans to collect and consume food as a multicellular unit rather than as isolated cells hunting individually. By forming groups, the cells could also protect their trapped food source from competing organisms—a cooperative advantage that evolution would reward.
There's more. Sticking together opened new evolutionary possibilities. When cells cluster, they can exchange genes through mating, generating genetic diversity that enables adaptation to new environments. This mechanism may explain how unicellular organisms began experimenting with multicellularity in Earth's distant past.
The findings grew even more striking when Li and Jennah Dharamshi examined the molecular machinery behind aggregation. The multicellular M. vibrans produced the same adhesion proteins that modern animal cells use to stick together. The organisms also generated proteins that animal cells employ for communication and coordination. "What this organism is most powered to answer is what the unicellular ancestor of animals was like," said J. P. Gerdt, associate professor of chemistry at Indiana University Bloomington, who led the research alongside Iñaki Ruiz-Trillo. "It's one of the best systems we have to go back a billion years to see what our ancestors were like at that stage."
The research, published in Nature and conducted through collaboration between Indiana University Bloomington, the Institute of Evolutionary Biology in Spain, and Uppsala University in Sweden, offers more than historical insight. Because M. vibrans is far simpler than complex animals, it serves as an ideal experimental system for understanding multicellular development. The team believes the organism could help reveal previously overlooked genes involved in developmental processes or diseases. For now, though, the discovery stands as a window into life's great transformation—a moment when cells learned that together, they could become something entirely new.