Mapping geological events and nitrogen fixation evolution onto the timetree of the evolution of nitrogen-fixation genes.

Nitrogen is essential for all organisms, but biological nitrogen fixation (BNF) occurs only in a small fraction of prokaryotes. Previous studies divided nitrogenase-gene-carrying prokaryotes into Groups I to IV and provided evidence that BNF first evolved in bacteria. This study constructed a timetree of the evolution of nitrogen-fixation genes and estimated that archaea evolved BNF much later than bacteria and that nitrogen-fixing cyanobacteria evolved later than 1900 million years ago (MYA), considerably younger than the previous estimate of 2200 MYA. Moreover, Groups III and II/I diverged ∼2280 MYA, after the Kenorland supercontinent breakup (∼2500-2100 MYA) and the Great Oxidation Event (∼2400-2100 MYA); Groups III and Vnf/Anf diverged ∼2086 MYA, after the Yarrabubba impact (∼2229 MYA); and Groups II and I diverged ∼1920 MYA, after the Vredefort impact (∼2023 MYA). In summary, this study provided a timescale of BNF events and discussed the possible effects of geological events on BNF evolution.