The Ur-Plant

Land plants, the fossil record shows, evolved some time before 476 million years ago. Unfortunately, the earliest fossils are just spores and don't reveal much about what sort of plants they came from. But where fossil hunters have failed, molecular botanists studying plant genes may have succeeded. Genetic studies suggest that liverworts--humble, stumpy plants related to mosses, which now commonly grow on damp soils--are the most ancient group of plants alive today. Liverworts share some traits with green algae, the single-celled marine plants from which land plants most likely arose. For example, alone among land plants, liverworts don't have stomata, or pores, in their leaves. The study looked at the DNA of 352 plant species ranging from liverworts to flowering plants. All the plant groups the researchers examined, except liverworts, contained at least one of three distinct introns--useless chunks of DNA located inside the coding sequence of a gene--in two different genes. That means that liverworts probably branched off the land-plant family tree before the introns were acquired and are the descendants of an ancient species ancestral to all other land plants. "We are not trying to imply that liverworts were really the very first land plants," says Jeffrey Palmer of Indiana University, who helped conduct the experiment. "Most likely there is some distinct lineage that represented the very first land plants, and it gave rise to two extant lineages. One consists of the liverworts; the other, everything else."