Today, such a relationship is called a “symbiosis,” and it’s considered the norm rather than the exception. Corals rely on the beneficial algae in their tissues. Humans are influenced by the trillions of microbes in our guts. Plants grow thanks to the fungi on their roots. We all live in symbiosis, but few organisms do so to the same extreme degree as lichens. If humans were to spend their lives in the total absence of microbes, they’d have many health problems but would unquestionably still be people. But without its alga, a lichen-forming fungus bears no likeness to a lichen. It’s an entirely different entity. The lichen is an organism created by symbiosis. It forms only when its two partners meet.

Or does it?

Lichen-forming fungi mostly belong to a group called the ascomycetes. But in 2016, Spribille and his colleague Veera Tuovinen, of Uppsala University, found that the largest and most species-rich group of lichens harbored a second fungus, from a very different group called Cyphobasidium. (For simplicity, I’ll call the two fungi ascos and cyphos). The whole organism resembles a burrito, with asco fillings wrapped by a shell that’s rich in algae and cyphos.

For many, it was a game-changing discovery. “The findings overthrow the two-organism paradigm,” Sarah Watkinson of the University of Oxford told me at the time. “Textbook definitions of lichens may have to be revised.” But some lichenologists objected to that framing, arguing that they’d known since the late 1800s that other fungi were present within lichens. That’s true, Spribille countered, but those fungi had been described in terms that portrayed them as secondary to the main asco-alga symbiosis. To him, it seemed more that the lichens he studied have three core partners.

But that might not be the whole story, either.

Look on the bark of conifers in the Pacific Northwest, and you will quickly spot wolf lichens—tennis-ball green and highly branched, like some discarded alien nervous system. When Tuovinen looked at these under a microscope, she found a group of fungal cells that were neither ascos nor cyphos. The lichens’ DNA told a similar story: There were fungal genes that didn’t belong to either of the two expected groups. Wolf lichens, it turns out, contain yet another fungus, known as Tremella.

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This isn’t entirely new. Over the years, other lichenologists have detected Tremella in wolf lichens, but only ever in three specimens, and only in the context of abnormal swollen structures called galls. “It was thought to be a parasite,” Tuovinen says. “But we found it in completely normal wolf lichens that don’t have any kinds of bumps.” Tremella is right there in the shell of the lichen burrito, next to the cyphos. It seems to make extremely close contact with the algae, hinting at some kind of intimate relationship. And it’s everywhere. Tuovinen analyzed more than 300 specimens of wolf lichens from the U.S. and Europe, and found Tremella in almost all of them.