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Nitrogenase, which catalyzes the ATP-dependent reduction of dinitrogen (N2) to ammonium (NH4+), has modulated the availability of fixed sources of nitrogen since early in Earth history. The most common form of nitrogenase today requires a complex metal cluster containing molybdenum (Mo), although alternative forms exist which contain vanadium (V) or only iron (Fe). It has been suggested that Mo-independent forms of nitrogenase (V and Fe) were responsible for N2 fixation on early Earth because oceans were Mo-depleted and Fe-rich. The fundamental requirement for fixed forms of nitrogen (N) for life on Earth, both today and in the past, has led to broad and significant interest in the origin and evolution of this fundamental biological process. One key question is whether the limited availability of fixed N was a factor in life’s origin or whether there were ample sources made available from abiotic processes or through delivery of bolide impact materials to support this early life. If the latter, the key questions become what were the characteristics of the environment that precipitated the evolution of this oxygen sensitive process, which form of nitrogenase arose first, when did this occur, and how was its subsequent evolutionary history impacted by the advent of oxygenic photosynthesis and the rise of oxygen in the Earth’s biosphere. Deep insights into such questions can be gained through phylogenetic analysis of key duplication and fusion events in genes that encode for proteins required to synthesize the molybdenum co-factor responsible for N2 reduction in the context of the evolutionary history of the genes that encode for nitrogenase structural proteins. In this presentation we will focus on new insights into these profound questions that together challenge traditional models for the evolution of biological N2 fixation and provide the basis for the development of new conceptual models that explain the stepwise evolution of this highly complex, life sustaining process.