The ability to use atmospheric nitrogen to support more widespread life was thought to have appeared 2 billion years ago. Now, a study led by Dr Eva Stüeken from the University of Washington has found evidence that 3.2 billion years ago, life was already pulling nitrogen out of the air and converting it into a form that could support large and diverse biosphere.
Early Earth. Image credit: Christine Daniloff.
Dr Stüeken and her colleagues from the University of Johannesburg in South Africa and the University of Washington’s Department of Earth & Space Sciences analyzed 52 rock samples ranging in age from 2.75 to 3.2 billion years old, collected in South Africa and northwestern Australia.
The rocks were formed from sediment deposited on continental margins, so are free of chemical irregularities that would occur near an underwater volcano. They also formed before the atmosphere gained oxygen, 2.3 – 2.4 billion years ago, and so preserve chemical clues that have disappeared in modern rocks.
Even the oldest samples, 3.2 billion years old showed chemical evidence that life was pulling nitrogen out of the air.
The ratio of heavier to lighter nitrogen atoms fits the pattern of nitrogen-fixing enzymes contained in single-celled organisms, and does not match any chemical reactions that occur in the absence of life.
“Imagining that this really complicated process is so old, and has operated in the same way for 3.2 billion years, I think is fascinating. It suggests that these really complicated enzymes apparently formed really early, so maybe it’s not so difficult for these enzymes to evolve,” said Dr Stüeken, the first author of a paper published in the journal Nature.
Genetic analysis of nitrogen-fixing enzymes has placed their origin at between 1.5 and 2.2 billion years ago.
“This is hard evidence that pushes it back a further billion years,” said co-author Prof Roger Buick of the University of Washington.
Fixing nitrogen means breaking a tenacious triple bond that holds nitrogen atoms in pairs in the atmosphere and joining a single nitrogen to a molecule that is easier for living things to use.
The chemical signature of the rocks suggests that nitrogen was being broken by an enzyme based on molybdenum, the most common of the three types of nitrogen-fixing enzymes that exist now.
Molybdenum is now abundant because oxygen reacts with rocks to wash it into the ocean, but its source on the ancient Earth is more mysterious.
Dr Stüeken and her colleagues hypothesize that this may be further evidence that some early life may have existed in single-celled layers on land, exhaling small amounts of oxygen that reacted with the rock to release molybdenum to the water.
“We’ll never find any direct evidence of land scum one cell thick, but this might be giving us indirect evidence that the land was inhabited. Microbes could have crawled out of the ocean and lived in a slime layer on the rocks on land, even before 3.2 billion years ago,” Prof Buick said.
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Eva E. Stüeken et al. Isotopic evidence for biological nitrogen fixation by molybdenum-nitrogenase from 3.2 Gyr. Nature, published online February 16, 2015; doi: 10.1038/nature14180
