When Life Nearly Ended

"The few species that survive could proliferate so that there is just as much abundance as before, but less diverse. We actually see evidence that the abundance is much lower after the extinction."

So, the planet had lost many of its species, and those that survived were barely making it.

How long did all that take? It may seem likely that the greatest extinction ever must have taken millions of years. But, the evidence suggests that's not necessarily so.

That fine line in the rock that marks the extinction "probably doesn't represent more than 100,000 years," Payne said. "That's still a very long time scale for humans, but when you think about the geological time scales of hundreds of millions of years, then 100,000 years is nearly instantaneous."

A process that could have taken much longer may have fed upon itself in a runaway series of global changes. Some scientists believe that is what is happening today.

There is yet another story told in the rocks. The sequence at which various species succumbed is very revealing. The first to go were the trilobites, those primitive arthropods that had survived since the Paleozoic.

"There's never been a trilobite found since that extinction," Payne said. "The other groups that were hit hardest were two groups of corals that are distant relatives of modern corals."

Next, came the brachiopods, similar to clams, that stayed on the bottom as filter feeders. Some survived the extinction, and are still around, but they are very rare. Relatives of modern starfish and sea urchins and sea cucumbers, called crinoids, also lost most of their diversity.

"The group that made it through in the best shape were the mollusks, largely clams and snails," Payne said. And they came through big time. "Nowadays, if you go to the beach, they are a very large component in what you are going to find among the shells."

During the few million years after the extinction, some brachiopods and crinoids began to recover, but they never caught up with the mollusks.

Payne theorizes that the mollusks survived because they were more mobile, and, thus, better equipped to make it in a changing world. Life on Earth had survived its greatest challenge, though, perhaps, barely. But, what could have caused such a disaster?

The mass extinction coincided with the largest known volcanic event in the past 600 million years. Volcanoes erupted across Siberia, throwing gases and ash into the atmosphere, and covering an area the size of the United States with basalt nearly four miles high.

Some scientists believe magma intruded into vast fields of coal, sending enormous quantities of carbon dioxide and methane — known as greenhouse gases today — into the atmosphere.

The Earth probably cooled briefly because ash shaded the sun, then warmed. The oceans turned acidic from fallout, making life very difficult, both on land and on sea. It took millions of year for the planet to fully recover.

It's a bit sobering.

Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.

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