TUNNELING THROUGH TIME: HORIZONTAL GENE TRANSFER STRATIGRAPHY DATES THE RADIATION OF PROKARYOTE GROUPS, THE ORIGIN OF METHANOGENESIS, AND THE MICROBIOME

Horizontal gene transfers (HGTs), ubiquitous throughout the Tree of Life, may be harnessed to provide geological age constraints for clades lacking body fossils, trace fossils, or biomarkers. In one case, we observe that HGTs from methanogenic lineages (members of Archaea) into Cyanobacteria (themselves estimated to have diverged by the Great Oxidation Event) indicate methanogenesis evolved in the Palaeoarchaean, much older than previously suspected. Separately, HGTs from Gammaproteobacteria into arthropod lineages indicate major radiations of Proteobacterial crown groups in the Neoproterozoic, in tandem with the Second Oxidation Event. We propose that the coincident timeline represents a diversification of the animal microbiome, as new bacterial habitats within differentiated metazoan tissues rapidly became available for colonization. Given the lack of reliable fossils, our methods have sweeping applications to estimating the age of origin and radiation of microbial clades across the entire Tree of Life, which is critical given their pervasive role in biogeochemical regulation of Earth history.