Land plants descended from an alga that learned a critical trick from a friend.
Green algae evolved over a billion years ago from amoeba-like marine organisms which turned photosynthesizing bacteria in endosymbiotic buddies. Then some of these little mariners took to freshwater.
One branch of freshwater algae adopted sophisticated growth systems. This algal family – charophytes – grow complex, branching bodies as multicellular organisms. “They look like underwater plants,” said German botanist Stefan Rensing.
But land plants did not come from these charophytes, as long suspected. Instead, plants evolved from zygnematophytes, which now appear as less complex than charophytes, having undergone morphological simplification in evolutionary time. “People knew about these algae for a long time, but they didn’t think they were the closest relatives to the land plants because there’s an assumption that there’s a linear increase in complexity,” said Chinese Canadian genomicist Gane Ka-Shu Wong.
The perpetual problem of life as a land plant is desiccation. Being able to survive dry spells is essential. About a half billion years ago, a zygnematophyte picked up a genic trick from a bacterium that befriended it.
This freshwater alga made a spongy coat to soak up water. The bacterium fed on the carbohydrates that comprise the coat. In return, the bacterium produced vitamins which were nourishing for the alga; a nice mutualism.
The alga got something else from the bacterium: the genetics to survive dryness. This allowed the alga to survive on land that wasn’t always soaking wet. Whence the terrestrial verdure venture began.
Carl Zimmer, “How did plants conquer land? These humble algae hold clues,” The New York Times (14 November 2019).
Shifeng Cheng et al, “Genomes of subaerial Zygnematophyceae provide insights into land plant evolution,” Cell (14 November 2019).