MOLECULAR EVIDENCE FOR EARLY COLONIZATION OF LAND BY EUKARYOTES AND RELATION TO NEOPROTEROZOIC GLACIATIONS

Fossil evidence suggests that land was first colonized by eukaryotes (plants) about 480 million years ago (Ma). It has been proposed that successful colonization was facilitated through a partnership (symbiosis) between a phototroph and a fungus. The most widespread of these symbioses today are lichens, consisting of cyanobacteria or green algae and a fungus, and vesicular-arbuscular (VA) mycorrhizae, which pair a plant with a glomalean fungus. Previous attempts to estimate divergence times within fungi, using a ribosomal RNA gene, yielded estimates of 350-600 Ma for the origin of terrestrial fungi. To further resolve and date the major splits among fungi, and of green algae and land plants, we analyzed protein sequence data from >100 nuclear genes.

Divergence time estimates indicate that most major groups of fungi had evolved by one billion years ago. Because of the widespread symbiotic relationships between fungi and plants (including green algae), we also obtained divergence time estimates for a green alga (Chlorophyta) versus higher plants (Streptophyta) and a moss (Bryophyta) versus a vascular plant (Tracheophyta). The resulting time estimates provide additional support for the early colonization (Neoproterozoic) of land by eukaryotes. Calibration with the fossil records of fungi and red algae also supports an early colonization of land.

Lichens and bryophytes form a rock and soil crust flora in harsh terrestrial environments today. It is possible they formed a similar biological crust on exposed land in the Neoproterozoic. Through increased weathering, photosynthesis, and burial of carbon, an early colonization of land by eukaryotes may have impacted Neoproterozoic climate and resulted in increased levels of oxygen permitting further development of animal life.