While it isn’t known exactly how life first appeared on Earth, scientists do know a great deal about the conditions that life needs to exist, and geological records provide clues about what the atmosphere was like. A new study published in Nature provides evidence that 3.2 billion years ago, forms of life were able to pull nitrogen out of the air. Nitrogen is crucial to the formation of genetic material and would have allowed a greater number of organisms to live. This paper pushes back the earliest evidence of thriving life by over a billion years.
"People always had the idea that the really ancient biosphere was just tenuously clinging on to this inhospitable planet, and it wasn't until the emergence of nitrogen fixation that suddenly the biosphere become large and robust and diverse," co-author Roger Buick said in a press release. "Our work shows that there was no nitrogen crisis on the early Earth, and therefore it could have supported a fairly large and diverse biosphere.”
Buick and his team analyzed rock samples that dated back 2.75-3.2 billion years. The rock's remarkable preservation is due to the fact that they came from the borders of continental shelves, not near volcanoes that would have adulterated them.
The samples were analyzed for their nitrogen isotopes in order to see if life forms were grabbing atmospheric nitrogen and converting it into a usable product. Much of the nitrogen in the atmosphere would have been triple bonded, which would have made it impossible for simple early organisms to use.
However, there are enzymes which are able to break one of those bonds, converting the nitrogen into a form that could bond with other molecules. DNA has indicated that these enzymes first appeared around 2 billion years ago, but even the samples that were 3.2 billion years old provided evidence of nitrogen isotopes that were altered in this manner.
"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," lead author Eva Stüeken added. "It suggests that these really complicated enzymes apparently formed really early, so maybe it's not so difficult for these enzymes to evolve.”
There are currently three primary enzymes responsible for nitrogen fixation, and the one most similar to what was seen in the rock samples is based on molybdenum. However, molybdenum was not thought to have been abundant at the time. It is possible that the molybdenum was released by simple microbial life living on rocks, though it will be hard to confirm with much certainty.
"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," Buick concluded. "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.”
The researchers will continue to analyze these ancient samples for other elements that might have affected early life on Earth.
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