text: The origin of nitrogenase has been of interest to paleobiologists and is an area of active research. Nitrogenase is thought to have evolved sometime between 1.5-2.2 billion years ago (Ga) although some isotopic support showing nitrogenase evolution as early as around 3.2 Ga. Nitrogenase appears to have evolved from maturase-like proteins, although the function of the preceding protein is currently unknown.
  Nitrogenase has three different forms (Nif, Anf, and Vnf) that correspond with the metal found in the active site of the protein (Molybdenum, Iron, and Vanadium respectively). Marine metal abundances over Earth’s geologic timeline are thought to have driven the relative abundance of which form of nitrogenase was most common. Currently, there is no conclusive agreement on which form of nitrogenase arose first.