Traditionally, biologists thought terrestrial plant life must have arisen from the oceans. Marine algae, adapted to the salty sea environment, seemed much better at retaining water than their freshwater counterparts, and thus at living on land, where dehydration is a constant threat. Tidepool algae seemed the most likely candidate predecessors of land plants, but recently, botanists comparing the anatomy of plants began to suspect that freshwater algae were the real ancestors.
Researchers collaborating in the international "Deep Green" project are addressing such questions by putting together a massive phylogeny, or family tree, of the green plants, based on a comparison of their genes and phenotypes. Armed with this history book, they now can trace the evolution of traits through time and across species. Their tree indicates that all modern land plants are descendants of a single group of freshwater algae called charophytes. But the tree also reveals three other, less successful conquests, Russell Chapman of Louisiana State University told the meeting. Only one of these originated in saltwater; it gave rise to the Trentepohliales, a group of 60 species of rock- and tree-hugging algae that look like orange fuzz.
Why freshwater algae diversified into the 300,000 or so land plant species that blanket the Earth, while the marine ones never took off, is unclear. But Chapman suggests that marine invaders, adapted to saltwater, may face a steep challenge from the freshwater they'd experience on land in the form of rain. Interestingly, the algal lineages that moved onto land all share a type of cell division, called phragomoplastic, that other algae lack, but Chapman says it's not yet clear how or whether this trait might have aided a shift onto land.
Rick McCourt, curator of botany at the Academy of Natural Sciences in Philadelphia, praises the work, but says additional plants added to the analysis in the future could increase the number of known invasions of land. Deep Green researchers are now engaged in "Deep Gene," an attempt to integrate the phylogenetic data with information from whole genome sequences of plants. This may allow them to trace the evolution of gene complexes involved in traits such as desiccation tolerance and cell division, says Brent Mishler of the University of California, Berkeley--and perhaps to determine whether groups like the charophytes have special features that spur diversification.