New research published this month reinforces the view that this event, called the End of Permian - or "Mother of all Extinctions" - was set off not by an asteroid impact but by an internal disturbance that released a burst of carbon dioxide into the atmosphere and heated the planet.
It's not the heat itself but the resulting reordering of the living world that would cause a massive die-off, said Pennsylvania State University geoscience professor Lee Kump. Natural selection proceeds. Life extinguishes other life.
According to a theory that Kump proposed several years ago to explain the Mother of all Extinctions, excess heat hinders oxygen from dissolving into the oceans. Oxygen-starved oceans become hostile to fish, plankton, and other familiar life, but friendly to sulfur-using bacteria. They poison other species by exhaling hydrogen sulfide.
Peter Ward, a professor of biology and earth and space sciences at the University of Washington, popularized Kump's theory in his 2007 book Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future.
In his vivid visualization: "Most of the shoreline is encrusted with rotting organic matter. Silklike swaths of bacterial slick now putrefying under the blazing sun, while in the nearby shallows mounds of similar mats can be seen growing up toward the sea's surface. . . . From shore to the horizon, there is but an unending purple color - a vast, flat, oily purple, not looking at all like water . . . no fish break its surface, no birds of any kind . . . we are under a pale green sky and it has the smell of death and poison."
Is this our future? Kump says there's no way to know, but the rise in carbon dioxide that triggered the end-Permian is comparable to modern projections if we do nothing to curtail use of fossil fuels worldwide over the next century or so.
The full effect will not appear immediately. On the other hand, it is not out of the question that such a hostile takeover could have started by 3978, the fictional year those lost movie astronauts found their planet overtaken by other apes.
Ward says the view of the end-Permian as a global-warming event represents a shift in thinking from the 1990s, when he and scientists suspected it was triggered by an asteroid. The evidence was overwhelming by then that a space rock had killed off the dinosaurs 65 million years ago. This event was one of five major mass extinctions seen in the fossil record, he said, and it seemed reasonable that a similar cause triggered some of the other four. Now, Ward says, the most widely accepted explanation for the end-Permian is that a confluence of volcanic eruptions and the ignition of enormous beds of coal liberated tons of atmospheric carbon, setting off a chain of events.
The resulting devastation illustrates "the fragility of our biosphere," said Douglas Erwin, a paleobiologist at the Smithsonian Institution and one of the authors of a paper on the end-Permian recently published in Science. "You can wipe out roughly 90 percent of everything in the oceans in 100,000 years without an impact," he said.
Still, said Kump, while global warming can devastate life in the oceans by starving it of oxygen, that doesn't explain what killed off so much life on land. The fossil record shows mass extinctions of land plants and animals, including a number of mammal-like creatures whose demise left more room for dinosaurs.
That's where the sulfur-consuming bacteria come in - proliferating in newly favorable conditions and exhaling great clouds of hydrogen sulfide that waft over the continents.
When he first proposed the idea, Kump said, there was little evidence to back it up. Now, however, scientists have begun to find traces of chemicals in ancient rock that suggest a worldwide flourishing of the poison-spewing microorganisms, sometimes called green sulfur bacteria.
Not that our future is a done deal, he said. Another warming period, 55.8 million years ago, called the Paleocene-Eocene Thermal Event (PETM), did not result in this same poisonous bloom. Kump is now trying to figure out why the PETM was so different from the end-Permian, and which scenario is more likely to come.
Even if we're not gassed by toxic bacteria, the past does suggest that heat is the planetary norm. Sixty million years ago, temperatures were about 30 degrees Fahrenheit higher than today's and palm trees grew as far north as London, said Paul Pearson, a professor at the School of Earth and Ocean Sciences at Cardiff University in the United Kingdom. Then, too, carbon dioxide levels were comparable to projections for a couple of centuries from now.
"What we're seeing now is just the beginning of what could be a massive change," he said.
The scientists aren't sure what's going to happen as far ahead as 3978. Pearson is certain, though, that what humans do this century will have an enormous impact on whomever, or whatever, is alive then.
Contact staff writer Faye Flam at 215-854-4977 or firstname.lastname@example.org. Read her evolution blog at www.philly.com/evolution.