That could happen again today, no asteroids required.

We have to understand those changes if we want to survive.

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The next step will be to probe these rocks for telltale signs of that nasty bacteria and hydrogen sulfide.

Until there's more evidence, the detectives won't all agree that the case is closed.

The Nevada rocks offer some support for the idea, at least for the Permian.

This whole area was once at the bottom of an ocean, and tests on the rocks have revealed that, in the years leading up to the extinction, the deep ocean water here had lost its oxygen.

As a result, the deeper waters have lost their oxygen.

There are bacteria that can thrive under those conditions, and those bacteria produce hydrogen sulfide.

But then, about 250 million years ago, almost everything on sea and land died. That happened about 65 million years ago, when an asteroid the size of Mount Everest slammed into our planet, leaving a giant crater near what is now Mexico. To test his idea, he designed a computer model to simulate the Permian world.