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Fossils Shed Light on Lichen Evolution from Sea to Land

Scientists declared they have found a 600-million-year old fungi-algae symbiontic organism in marine fossils believed to be the ancestors of the earliest land-based lichens ever found, shedding light on the sea-to-land evolution of plant life.


The findings were published in Friday's issue of Science. The report was co-authored by Xunlai Yuan, a paleontologist with the Nanjing Institute of Geology and Paleontology, Shuhai Xiao, assistant professor of geosciences at Virginia Tech, and Thomas N.Taylor, professor of ecology and evolutionary biology at the University of Kansas.


The previous earliest evidence of lichen was 400 million years old, discovered in Scotland. The latest finding proved the plant's ancestors might have appeared about 200 million years earlier, Yuan said.


Yuan, Xiao, and their collaborators have been exploring the Doushantuo Formation in south China for a decade. Taylor, a member of the National Academy of Sciences, is a paleobotanist who has reported on fossil lichens in Scotland.


Lichen is a consortium of two organisms that collaborate to survive in a harsh environments, such as exposed rock. One partner, a cyanobacterium or a photosynthetic alga, or both, are able to form food from carbon dioxide, while the other partner, a fungus, provides moisture, nutrients, and protection for the consortium.


"When and where did they first learn the tricks to form this collaboration?" Xiao asked. "The earliest lichen fossils described by Professor Taylor were from non-marine deposits about 400 million years old, when plants began to massively colonize the land. But did cyanobacteria or other algae form similar relationships with fungi in the marine environment, perhaps long before the evolution of land plants?"


At a site where abundant algae live in a shallow sub-tidal environment about 600 million years ago, Yuan and Xiao found three specimens that have evidence of two partners in a familiar relationship.


"The ability to form a symbiotic relationship between fungi and algae may have evolved long before the colonization of land by land-based lichens and green plants, which also form symbiotic relationships with various fungi," Xiao said.


The Doushantuo Formation in southwest China's Guizhou Province has yielded many marine fossils.


(Xinhua News Agency May 13, 2005)


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