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.
It’s long been known that lichens were among the earliest colonizers of land. And during more than a century of study, many scientists inferred that as they moved from water to land, lichens created more hospitable settings for vascular plants. It made sense: We can observe contemporary lichens moving into barren, severe environments before anything else.
Dr. Nelsen didn’t intend to refute those assumptions. He wanted to get to the bottom of another debate: What was the origin of the hugging habit of most lichen-forming fungi? Could this lichenization be traced to a single origin, or had several fungi picked it up at different times and places over the years?
Studies supporting both arguments have relied on varying methods yielding incomplete data, sometimes tracing only to the fungal side of the relationship or depending on ambiguous fossil evidence, Dr. Nelsen and others say.
To clear things up, Dr. Nelsen and his team compared the ages of the group that contains 98 percent of lichen-forming fungi (ascomycota, which include yeast, morels and penicillium) and algae to vascular plant fossils from around 425 million years ago.
They determined that lichen-forming fungi first evolved about 250 million years ago, on average — long after those plants were rooted on land. The timing varies though, suggesting that different fungi developed their algae-hugging habits independently, and didn’t inherit it from one main ancestor.
In a study published last year in Nature Communications, François Lutzoni, a lichenologist at Duke University, also found that lichens evolved with or after, but not before, vascular plants. Dr. Nelsen’s study expands on this finding with additional data and evidence for the origin of lichenization.
“To me, this new study really nails it,” said Cécile Gueidan, who researches fungi and lichen evolution at the Commonwealth Scientific and Industrial Research Organization, Australia’s national science research agency, and was not involved in either study.
Placing the development of lichenization in geological time provides a better understanding of how plants and lichens influenced our world’s evolving climate. Plants most likely weren’t competing with lichens, and lichens weren’t the big climate changers that readied the world for vascular plants.
The next step will be studying genomes to better understand how various lichen relationships developed and where each fits on the timeline.
“Are they scattered throughout time or clumped through different periods of time?” Dr. Nelsen asked. “Are there certain events or circumstances that might predispose this to happen, or is there something in the genomes?”