The Great Dying mass extinction event 252 million years ago saw the planet drenched in acid rain like vinegar.
The phenomenon was the worst mass extinction event ever to take place, with 93-97% of all species wiped out.
What led to the event is unknown, but the most popular theory is that erupting volcanoes led to massive climate change. Researchers have now found that extremely caustic acid rain caused by climate change may well have played a role in the extinctions, Smithsonian.com reports.
In their analysis, published in the journal Geology, researchers examined traces of ancient soils from what is now Northern Italy. They found rocks containing the same substance that gives vanilla its flavour and scent.
Vanillin is produced by plants and found in wood, and it cannot survive on its own in the ground – its discovery in the rocks was therefore surprising and suggested a lack of bacterial degradation.
It is known that acidifying milk flavoured with vanilla helps protect and prolong the flavour because the low pH deactivates the enzyme that would normally target the taste.
Researchers tested the possibility that these ancient rocks could be similarly sensitive to acid.
"Our data fits the idea that acid rain caused the microbes to cease functioning," Henk Visscher, a paleoecologist at Utrecht University in the Netherlands, said.
Researchers then went on to establish just how acidic the rain was by estimating soil acidity.
"Intermittently low and high ratios of vanillic acid to vanillin detected in latest Permian and earliest Triassic deposits of the southern Alps, Italy, support concepts of pulses of severe acidification during the main phase of the biosphere crisis," the team wrote.
Mark Sephton, a geologist from the Imperial College London, said: "For the first time, we can say that soils from this time had an acidity similar to that of vinegar."