The emergence of land plants approximately 480 million years ago was a monumental event in the history of life. It had major impact on the Earth's environment, for example, changing atmospheric carbon dioxide and oxygen levels, lowering surface temperature, producing soil, and increasing mineral nutrient release from land crust into oceans. Through interacting coevolution with animals (consumers) and fungi (decomposers), the plants as primary producers initiated development of the entire modern terrestrial ecosystems and thus fundamentally changed the course of evolution of life.

To understand how all these events happened, plant evolutionary biologists have long been interested in reconstructing the evolutionary history, or phylogeny, of early land plants. Most of these plants fall into the category of bryophytes, which include three groups, liverworts, mosses, and hornworts (all characterized by the lack of true roots, stems, and leaves). Over the last few years, the massive infusion of molecular biology techniques (for example, automated DNA sequencing and bioinformatic tools) into systematics has significantly improved our understanding of relationships among early land plants.

At present, it is clear that (1) all land plants share a common origin, (2) they evolved from green algae that resemble today's charophytes (aquatic plants that are found on the lake shore), and (3) bryophytes preceded vascular plants during early evolution of land plants. Since bryophytes represent the transitional group between algae and vascular plants, their phylogeny becomes especially relevant in our understanding of the origin and early evolution of land plants. In particular, two questions have been explicitly pursued by plant systematists over the last few years: (1) do liverworts or hornworts represent the earliest land plants, and (2) are mosses or hornworts sister groups to vascular plants?