LIFE × EVOLUTION
Nitrogen Fixation and Nitrogenase
at least 3.2 billion years ago
Nitrogenase is an enzyme that breaks the triple bond joining the two atoms of atmospheric nitrogen, which makes nitrogen usable by living organisms, and it was already at work at least 3.2 billion years ago.
Nitrogen fixation is the conversion of atmospheric nitrogen into a form that organisms can use. Four fifths of the air is nitrogen, in molecules of two atoms joined by a triple bond, which is among the most stable bonds in chemistry. Almost no living organism can break this bond. Nitrogenase breaks it: nitrogenase is an enzyme, a protein that speeds up a chemical reaction. Nitrogen fixation thus turns the atmosphere itself into fertiliser. Rocks more than three billion years old have the isotopic signature of nitrogenase, the proportions of heavy and light nitrogen that the enzyme leaves behind. Researchers have reconstructed ancestral forms of the enzyme from the genes of living organisms and expressed them in the laboratory, that is, had cells produce them. These ancestral forms leave the same signature today. One difficulty remains. The version of the enzyme recorded in the rocks depends on molybdenum, a metal that oxygen releases from rock. Molybdenum should have been scarce before oxygen existed, and the enzyme was nonetheless already at work.
Why it matters
Every protein and every strand of DNA requires nitrogen. By making the nitrogen of the air usable, nitrogenase raised the limit on the biomass, the total mass of living matter, that the planet could support.
Dating & uncertainty
The nitrogen isotope ratios measured in rocks 3.2 to 2.75 billion years old correspond to the ratios that nitrogenase produces. The rocks therefore give a minimum age for the enzyme and not the date of its origin. Rucker and colleagues (2026) reconstructed ancestral enzymes that reproduce these ratios, which supports this reading of the rocks.
Earth history as one day
07:05:0116 h 55 min before midnight
if Earth’s 4.54 billion years were compressed into a single day