THE REQUIREMENT FOR BIOAVAILABLE FIXED NITROGEN IS A FUNDAMENTAL CONSTRAINT FOR ALL LIFE ON EARTH MAKING BIOLOGICAL NITROGEN FIXATION ONE OF THE MOST IMPORTANT EVOLUTIONARY BREAKTHROUGHS IN THE HISTORY OF OUR PLANET. ALL BIOLOGICAL NITROGEN FIXATION IS CATALYZED BY THE METALLOENZYME NITROGENASE IN A COMPLEX ENERGY-INTENSIVE REACTION THAT REDUCES ATMOSPHERIC N2 GAS INTO BIOAVAILABLE AMMONIUM. WITHOUT THIS PROCESS ONLY A FRACTION OF OUR BIOSPHERE COULD BE SUSTAINED AND IT IS LIKELY TO HAVE PLAYED A CRUCIAL ROLE IN THE TIMING AND TRAJECTORY OF THE EVOLUTION OF LIFE. HOWEVER THE ORIGINS OF N2 FIXATION REMAIN ELUSIVE PARTLY DUE TO COMPLEXITIES OF INTERPRETING ANCIENT SIGNATURES OF THIS LIFE-SUPPORTING REACTION. THE N STABLE ISOTOPE COMPOSITION (D15N) OF BIOMASS PROVIDES A POWERFUL TOOL FOR RECONSTRUCTING PRESENT AND PAST N CYCLING BUT ITS INTERPRETATION HINGES ON A COMPLETE UNDERSTANDING OF THE ISOTOPIC SIGNATURE OF N2 FIXATION. RECENT INVESTIGATIONS INTO THE METAL COFACTOR VARIANTS OF NITROGENASE REVEALED THAT THE MO- V- AND FE-ONLY ISOFORMS OF THE ENZYME IMPART DIFFERENT ISOTOPE FRACTIONATIONS DURING N2 FIXATION IN VIVO AND THAT MO-NITROGENASE CAN FRACTIONATE N IN EXCESS OF 10 UNDER IN VITRO CONDITIONS. THIS CHALLENGES THE TRADITIONAL VIEW THAT N2 FIXATION ONLY IMPARTS SMALL INVARIABLE ISOTOPE EFFECTS (~0-2) AND HAS LED TO NEW ESTIMATES THAT N2 FIXATION BY MO-NITROGENASE MUST HAVE EVOLVED BY 3.2 GA. YET THE MECHANISTIC BASIS FOR CHANGES IN THE FRACTIONATION OF N2 FIXATION REMAIN LARGELY UNKNOWN PREVENTING A FULL UNDERSTANDING OF THE VARIABILITY OF THIS KEY PARAMETER IN N CYCLE RECONSTRUCTIONS AND RESULTING IN LARGE UNCERTAINTIES IN THE INTERPRETATION OF 15N IN PAST AND PRESENT SYSTEMS.