The original version on this story shows the Quanta Magazine.

Far from solo operators, most are single-celled VIRUS are in complex relationships. In the ocean, on land, and in your gut, they can fight and eat each other, exchange DNAcompete for nutrients, or feed on each other’s products. Sometimes they become closer: One cell can slip inside the other and become comfortable. If the conditions are right, it can stay and be welcomed, sparking a relationship that will last for generations—or billions of years. This phenomenon of one cell living inside another, called endosymbiosis, fueled the evolution of complex life.

Examples of endosymbiosis are everywhere. Mitochondria, the energy factories of your cells, previously free-living bacteria. Photosynthetic plants owe their sugars produced by the sun to the chloroplast, which was originally also an independent organism. Many insects obtain important nutrients from the bacteria that live inside it. And last year researchers discovered “nitroplast,” an endosymbiont that helps some algae process nitrogen.

Most of life depends on endosymbiotic relationships, but scientists struggle to understand how this happens. How does the internalized cell avoid digestion? How does it learn to reproduce within its host? What turns a random union of two independent organisms into a strong, long-lasting partnership?

Now, for the first time, researchers are looking at the opening choreography of this microscopic dance of inducing endosymbiosis in the lab. After injecting the bacteria into the fungus—a process that required creative problem solving (and a bicycle bomb)—the researchers were able to induce cooperation without killing the bacteria or the host. Their observations offer a glimpse into the conditions that make the same thing possible in the microbial wild.

The cells even adjust to each other faster than expected. “To me, this means that organisms want to actually live together, and symbiosis is the norm,” said Vasilis Kokkorisis a mycologist who studies the cell biology of symbiosis at VU University in Amsterdam and was not involved in the new study. “So that’s big, big news for me and this world.”

Early failed attempts reveal that most cellular love activities are unsuccessful. But by understanding how, why, and when organisms accept endosymbionts, researchers can better understand key evolutionary moments, and possibly develop synthetic cells engineered with superpowered endosymbionts.

The Penetration of the Cell Wall

Julia Vorholta microbiologist at the Swiss Federal Institute of Technology Zurich in Switzerland, has long puzzled over conditions of endosymbiosis. Researchers in the field believe that once a bacterium enters a host cell, the relationship between infection and harmony. If bacteria proliferate quickly, they risk depleting the host’s resources and triggering an immune response, resulting in the death of the guest, the host, or both. If it reproduces slowly, it does not establish itself in the cell. In rare cases, they think, bacteria achieve a Goldilocks reproductive rate. Then, to become a true endosymbiont, it must enter the reproductive cycle of its host to ride the next generation. Finally, the host genome must eventually mutate to accommodate the bacteria—allowing the two to evolve as a unit.

“They were addicted to each other,” Vorholt said.