About 252 million years ago, Earth suffered the biggest mass extinction event in its history, known as the Great Dying. The atmosphere filled with carbon, the planet baked in a warmer climate, and the oceans acidified. When it was over, 90% of all species on the planet were extinct.
Scientists have long blamed volcanoes for triggering this catastrophe by pumping the atmosphere full of greenhouse gases. But a team of scientists have identified a new suspect: life itself, in the form of microbes.
In this scenario, microbes called methanogens evolved to gorge themselves on the organic remains of dying species, emitting methane as a byproduct. They pumped the atmosphere full of methane, raising the temperature and nearly wiping out life. The research, first presented by team leader Daniel Rothman at the annual conference of the American Geophysical Union in December 2012, was published online this week in the Proceedings of the National Academy of Sciences.
But although this scenario is plausible, it’s not airtight—the evidence is largely circumstantial.
Rothman and his team first speculated that microbes were behind the Great Dying based on the speed at which heat-trapping carbon moved from the atmosphere into Earth’s oceans about 252 million years ago. Based on measurements from southern China and Austria, the team found it happened at an exponential rate. This, they argue, is the fingerprint of the exponential explosion of life, not volcanic activity. The carbon from volcanoes would have seeped into the ocean more gradually, slowing down as the ocean became saturated with carbon. But an exploding population of methane-producing organisms in the oceans could have kept belching the gas even after the oceans were saturated.
There’s another piece of evidence: sometime within the last 500 million years, one class of methanogens, called Methanosarcina, evolved a new way to metabolize carbon in the form of acetate, a compound that would have been plentiful in ocean sediments littered with dead plants, animals, and microbes. The team analyzed the genomes of 50 methanogens, including direct descendants from the mutation as well as distant cousins, to pinpoint when the mutation occurred. By simulating gene mutation rates, they estimate the shift occurred 240 million years ago, give or take about 41 million years. That puts it at around the time of the Great Dying. It would have given the hungry microbes a new food source, allowing them to convert acetate into methane and furthering a vicious cycle where methane caused extinctions, deaths produced acetate, and microbes produced more methane. It’s a classic recipe for exponential growth.
But methanogenic life needs more than just decomposing corpses—they also need nickel so their enzymes can fold into the right shape to perform their biological functions. The metal is scarce on the Earth’s surface, but volcanoes churn it up from deep in the planet. Rothman’s team also found a spike in nickel concentration in the geological record at a site in southern China right around when they think the microbes went into exponential overdrive. That led them to argue that volcanoes weren’t the main culprit, but that they served as a catalyst, providing the raw biological material that allowed Methanosarcina to flourish.
This picture fits together, but not everyone thinks it’s a better explanation.
Some scientists are not so quick to relegate the volcanoes to a supporting role, however. What the researchers have found is part of the connection between the volcanoes and the die-out, says palaeobiologist Douglas Erwin of the Smithsonian National Museum of Natural History in Washington DC. But he warns that the evidence is “hardly conclusive”.
And not all of the details are nailed down. Rothman’s team thinks the nickel they see in south China came from Siberian volcanoes, the ones that most scientists think caused the mass extinction in the first place. But Marc Reichow at the University of Leicester is skeptical, reports Akshat Rathi for The Conversation:
He argues that there is no evidence that the increased nickel came from Siberian volcanoes. Rothman agrees that current data cannot identify the source of the nickel. “This is an interesting hypothesis, but I think that Great Dying was the doing of many ‘kill mechanisms’ rather than just a single mechanism suggested here,” Reichow said.
What is clear is that microscopic life can have enormous consequences across the entire planet. After all, if bacteria hadn’t evolved the ability to perform photosynthesis 2.5 billion years ago, pumping the Earth’s atmosphere full of oxygen, we—and the rest of the animal kingdom—wouldn’t be here today.
In the 1970s, scientist James Lovelock proposed the famous Gaia hypothesis—that life modifies its environment to suit itself, like a self-sustaining superorganism. Rothman’s theory could serve as an example of an anti-Gaian theory, or what paleontologist Peter Ward calls the Medea hypothesis—that life is ultimately suicidal and kills itself off. For now, at least as far as Rothman and colleagues’ findings are concerned, the jury is still out.