The tree of life is inherently fractal. Look closely at one lineage of a phylogeny and it dissolves into many separate lineages, and so on down to a very fine scale. There is now a great body of phylogenetic research that has provided numerous tools applicable at particular, usually fairly constrained, scales. These tools have left many phylogenetic questions unanswered. We think these questions will remain unanswered until it is possible to do analyses across multiple scales.

We believe that the green plant lineage represents the most suitable system for such research. It is one of the oldest and most diverse branches of the tree of life, and it contains good examples of the known phylogenetic problems. Investigations on it may draw on a tradition of interdisciplinary collaborative research, facilitated by the Green Plant Phylogeny Research Coordination Group (GPPRCG or "Deep Green").

Many interesting questions remain to be tested in the green plants, once a better resolved phylogeny is available, such as: How many times was land colonized from the water by "green algae?" Where did the key adaptive features for life on land come from? How many times has multicellularity arisen in the green plants? Did multicellularity ever reverse? How many times did alternation of generations and diploid-dominant life-cycles arise? How has tempo and mode of macroevolution changed during diversification?

Our overall objective is to resolve the primary pattern of evolutionary diversification among green plants and establish a model for doing so that will be applicable to other groups of organisms with long evolutionary histories. A solid backbone based on genomic and ultrastructural data for relatively few taxa will enable the integration of previous and ongoing studies of many more taxa into a comprehensive picture of green plant phylogeny.

We gratefully acknowledge funding (details here) by the National Science Foundation under the Assembling the Tree of Life Program.