A data-driven model of the evolution of N-fixation: the data supports one gain of a precursor, about 100 million years ago, as underlying subsequent evolution of the root symbioses. See the full paper in Werner et al. 2014, Nature Communications. Also if you’ve ever wondered whether a particular species can fix nitrogen, the complete dataset is freely available via data dryad.
A data-driven model for the evolution of N-fixing nodulation in plants