Lindsay Hays remembers having conversations with paleontologists about whether trilobites would taste good. “They’re like horseshoe crabs, basically a shell with legs,” she says.

Trilobites, once ubiquitous, suddenly and mysteriously died out 250 million years ago. It turns out they weren’t the only ones. Something big changed on the planet at that time, wiping out 70 percent of land organisms, and 90 percent of marine life. Trilobites didn’t stand a chance.

“This Permian-Triassic extinction is unlike anything we’ve seen before,” says Lindsay. Today, no one knows what caused this extinction on such a scale. Lindsay, a molecular paleontologist and graduate student at MIT, is investigating one of the main suspects—poisoning.

“Toxic hydrogen sulfide welled up in the deepest layers of the ocean, and burped its way up into the upper layers,” she explains. This gas would have replaced the oxygen in the water, killing all organisms that needed it to survive. In an anoxic, or oxygen-less environment, dead marine life sank to the seabed, where they were petrified in oil shales. These oil shales are Lindsay’s dig.

Sifting through sediment from shale reserves dating back to the Permian-Triassic period, Lindsay has found evidence of a microbe called chlorobiaceae. This creature thrived on hydrogen sulfide, paddling cheerfully in the upper layers of poison-infused waters while its marine neighbors choked and sank into the ocean. Dying a natural death, chlorobiaceae made its way into the same oil shales.

Finding traces of this microbe is careful work. “We don’t get big fossils,” Lindsay explains. Her job is to look for “molecular fossils,” pieces of microbial residue that serve as evidence for the presence of chlorobiaceae. Her most recent find is “isorenieratane,” which chlorobiaceae used to photosynthesize. The presence of this molecule in shales from 250 million years ago puts chlorobiaceae at the ocean scene during the Permian-Triassic extinction. And chlorobiaceae could only have lived in the presence of hydrogen sulfide. With this evidence, Lindsay has the poison gas pinned.

Lindsay points out that identifying an anoxic ocean is only a first step to solving the mystery of the Permian-Triassic extinction. The preponderance of hydrogen sulfide was probably one of several factors that, together, precipitated the extinction event in the ocean and on land. The larger question of what triggered the incursion of our planet by this toxic gas remains unanswered.

Some things Lindsay is sure about. “This extinction event is clearly related to changes in the chemistry of the ocean and the atmosphere.” Chemical change? Isn’t that what is behind climate change today? Does Lindsay’s evidence predict dire threats to life on our planet by another mass extinction? “I’d say it was more a dire warning,” she answers. “Not knowing the causes for the Permian-Triassic extinction, do we really want to be making the changes to the environment we are making now?”