Scientists on March 23 said they had ruled out a key hypothesis to explain Earth's greatest extinction, when 95 percent of marine species and 70 percent of land species were wiped out.
Dubbed "the Great Dying" or "the mother of all mass extinctions," the catastrophe occurred around 250 million years ago at the end of the Permian era.
The event may have unfolded over millions of years, and an increasing number of clues testify to its severity, include the discovery worldwide of eerie, fossilised, mutant plant spores. What is unclear, though, is what caused it. British researchers, reporting on Sunday in the journal Nature Geoscience, rule out a leading theory that the oceans became starved of oxygen and rich with sulphide, causing marine life to die out.
Clouds of hydrogen sulphide - the same chemical that comes from rotting eggs - rose from the seas and, abetted by methane released as a by-product of intense vulcanism, attacked the ozone layer, the stratospheric shield that filters ultraviolet-B light from the Sun.
On the ground, life was ravaged, goes this theory. Living things were poisoned by toxic levels of hydrogen sulphide and their DNA was shredded by solar radiation.
A team led by David Beerling of the University of Sheffield in northern England created a two-dimensional computer model of atmospheric chemistry to test this notion. According to their calculations, the lower levels of the atmosphere in the tropics would have acted as an oxidising buffer, preventing the hydrogen sulphide from seriously damaging the ozone layer.
"These gases seem unlikely to be the cause of coincident terrestrial biotic eextinctions," the paper says. Other theories still in the arena include an impact, or series of impacts, by an asteroid, similar to the event believed to have wiped out the dinosaurs some 62 million years ago.
A "deep impact" of this kind would have generated a dust storm that smothered the planet, obscuring the Sun and shrivelling vegetation.
Another idea is that the planet was convulsed by a brief but fierce period of vulcanism which caused a lethal mixture of acid rain and global warming.