The exact causes behind the Permian-Triassic mass extinction have been much debated.
Two separate pulses of CO2 into the atmosphere - a "one-two punch" - may have helped fuel the die-off, new research suggests.
Changes to ocean acidity would have been one of the consequences, according to the study in Science journal.
Computer models suggested that this CO2 may have been released by massive bouts of volcanism from the Siberian Traps, now represented as a large region of volcanic rock in northern Eurasia.
The Permian-Triassic mass extinction, which took place 252 million years ago, wiped out more than 90% of marine species and more than two-thirds of the animals living on land.
The event is thought to have played out over a 60,000-year period and acidification of the oceans lasted for about 10,000 years.
The team led by Dr Matthew Clarkson from the University of Edinburgh analysed rocks unearthed in the United Arab Emirates - which were on the ocean floor at the time.
The rocks preserve a detailed record of changing oceanic conditions at the time. They then developed a climate model to work out what drove the extinction.
The researchers think the rapid rate of release of carbon was a crucial factor in driving the ocean acidification.
The carbon was released at a similar rate to modern emissions. Dr Clarkson commented: "Scientists have long suspected that an ocean acidification event occurred during the greatest mass extinction of all time, but direct evidence has been lacking until now.
"This is a worrying finding, considering that we can already see an increase in ocean acidity today that is the result of human carbon emissions."
Acidic oceans helped fuel the biggest mass extinction in the history of life on Earth, a study says.