An analysis of volcanic rocks revealed large quantities of nitrogen compounds that were almost certainly formed by volcanic lightning. This process could have provided the nitrogen necessary for the evolution and development of early life forms.
Nitrogen is a key component of amino acids that are strung together to make the proteins on which all life depends. Although nitrogen gas is abundant, plants cannot convert it into a usable form like they can carbon dioxide.
Instead, plants get much of their nitrogen from bacteria that can “fix” the gas by converting it into nitrogen compounds, such as nitrate. But nitrogen-fixing bacteria didn't exist when life evolved, according to Slimane Bekki at the Sorbonne University in Paris, so there must have been a non-biological source from the start.
Lightning caused by thunderstorms is a possible origin. This produces a relatively small amount of nitrates today, but it could have been significant early in Earth's history. The famous Miller-Urey experiment conducted in the 1950s demonstrated that lightning in the Earth's early atmosphere could have produced nitrogen compounds, including amino acids.
Bekki and his colleagues showed that another source could have been lightning that occurs in ash clouds during some volcanic eruptions.
When collecting volcanic deposits in Peru, Turkey and Italy, researchers were initially surprised to find large amounts of nitrates in some layers. An isotopic analysis of these nitrates showed that they were of atmospheric origin and had not been emitted by volcanoes. But Bekki says the quantities were too large to have been created by lightning during thunderstorms. “It was the amount that was really surprising,” he said. “It’s really huge.” This means the nitrates were likely generated by volcanic lightning.
“When you look at the different possibilities, the most likely was volcanic lightning,” says Bekki. “We know you get a lot of lightning during a massive volcanic eruption.”
Tamsin Mather at the University of Oxford says the team's conclusion makes sense. “We expect volcanic eruptions like the ones studied in the paper to generate significant lightning, so it's entirely possible that volcanic lightning gave rise to this signal,” she says.
It has been suggested that life first evolved around volcanoes, and the team's findings show that there could have been an abundance of nitrogen compounds in this environment, Bekki says.
The idea that volcanic lightning played a key role in the origin of life is not new. Jeffrey Bada at the Scripps Institution of Oceanography in California has already shown that volcanic lightning passing through volcanic gases can produce molecules such as amino acids. “This article only reinforces what I published,” he says.