Abiotic sources of fixed nitrogen sustained early ecosystems for several hundred million years after the origin of life

Abstract Nitrogen plays a crucial role in controlling biological productivity. However, it remains unknown how Earth’s earliest ecosystems accessed bioavailable forms of nitrogen. Here, we present genomic evidence that the last universal common ancestor (LUCA) possessed genes for importing ammonium into the cell, but the first organisms with all three catalytic nitrogen fixing genes emerged at least 1 billion years later. Similarly, enzymatic pathways for accessing nitrogen from urea and nitriles appear to predate biological nitrogen fixation. Our results indicate that Earth’s earliest biosphere was maintained by environmental sources of ammonium and other N-bearing compounds, possibly derived from a combination of processes such as hydrothermal activity, photochemistry, rock weathering, lightning or impact events. Biological nitrogen fixation may have emerged in response to an increase in biological nutrient demand or due to declining abiotic supplies of ammonium, urea and nitriles.