Liverworts: The Original Landed Gentry

Medieval herbalists named liverworts after the plant's liver-shaped lobes, whose extracts they believed could cure jaundice and other liver problems. Although the liverwort can't claim fame as a wonder drug these days, the plant has at least won a new measure of respect from scientists: A report in tomorrow's issue of Nature reveals that liverworts are the oldest known land plants. The findings also suggest that liverworts can be used as models for studying how plants have adapted to life on land.

The first evidence of land plants in the fossil record are plant spores dating to about 476 million years ago. But so far, researchers haven't discovered any fossils of the plants that made these spores, which resemble those of modern-day bryophytes, a group that includes liverworts and hornworts. The earliest fossils of land plants unearthed to date are 425 million years old. Lacking a smoking gun, "people have been arguing [over the identity of the first land plant] for a long time," says Thomas Ranker, an evolutionary biologist at the University of Colorado Herbarium in Boulder.

Molecular evolutionist Jeffrey Palmer of Indiana University in Bloomington turned to genetics in an effort to close this 50-million-year gap. Palmer's group examined the DNA in plant mitochondria, organelles in cells that act as tiny power plants and have their own DNA, which is a favorite target for researchers seeking to probe ancestral DNA. They homed in on three introns--DNA sequences in genes that do not code for proteins--known to be found in many plants. Systematically probing 352 species from all major groups of land plants, Palmer's team found that only the 11 liverwort species lacked the introns. All other plant groups, including hornworts and other bryophytes, had at least two of the telltale DNA swaths. The group found that algae and other sea-dwelling plants also lack the introns. Palmer says the most likely explanation for liverworts' utter lack of these three introns is that they were the original land plants, and other plants branched from them picking up the introns early on.

Palmer's conclusions are "right on," says Ranker. The findings suggest that a single species of plant adapted to land first, giving rise to all subsequent land plants. Aside from resolving that long-standing question, liverworts and their presumably ancient genetic lineage can be used by researchers to understand the evolution of plant traits seen in today's modern plants.