A lichen is what happens when a fungus hugs an algae and doesn’t let go. It’s a sweet arrangement: The fungus offers shelter, and algae feed the fungus. They’re still separate species, but tear them apart and the fungi typically can’t survive. So they’ve long been studied as a single organism.

These relationships are so successful, they’ve lasted hundreds of millions of years. But they’re not as old as once thought. Contrary to conventional wisdom that lichens were some of the earliest arrivals on land, a study published Thursday in Geobiology adds to the case that lichens most likely made their way to land some 100 million years after vascular plants, such as ferns. The study’s findings upturn our understanding of deep time, the tree of life and how lichens, plants and fungi transformed Earth’s evolving climate, eventually making it possible for other organisms to evolve and live on our planet today.

“Even though they can live in these kind of really extreme areas, and they’re often some of the first colonizers, that doesn’t mean that historically they have always been,” said Matthew Nelsen, an evolutionary biologist at Chicago’s Field Museum and an author of the study.

Lichens dominate about 7 percent of the planet’s surface. You’ll find more than 13,500 species embellishing soil, rocks, tree bark and even dead animals, even in Earth’s harshest climates.