Here's my evolution column for this week. It will also appear in Monday's Philadelphia Inquirer:
Like the economy, evolution is not so much a progression as a series of crashes and recoveries. Periodically the Earth undergoes some assault or internal spasm that shakes off some 70 to 90 percent of all species.
Since the dawn of complex life some 600 million years ago, the Earth has suffered five such "mass extinctions," followed by a recovery in which some of the winners spread out into the niches left by the losers.
The most extreme of those disasters, called the end Permian event, happened about 250 million years ago — at a time of great diversity. The seas were full of fish, the land was covered in plants and thoroughly colonized by insects, reptiles and amphibians, said David Jablonski, a paleontologist at the University of Chicago. "Then it was almost lights out," he said. Ninety percent of all living species went extinct.
Jablonski has been studying mass extinctions and the rebound periods by studying one organism that's pulled through all five — bivalves, which include clams, scallops and oysters. He found, to his surprise, that mass extinctions seem to change the rate of evolution – that is, the rate at which new species emerged either slowed down or sped up. What was weird, he said, was that evolution generally seems to have a steady natural pace, but that pace changed after each extinction event. Life would go on, but not as before.
He and colleague Andrew Krug published their findings in this month's issue of the journal Geology.
Mass extinctions are interesting not just because they're dramatic, but because our present biosphere would be very different if these mass extinction events didn't periodically reboot the system. Mass extinctions temporarily change the rules determining what is most likely to survive, Jablonski said. Evolution is redirected.
Body size matters less during a mass extinctions — in the last event, big dinosaurs and little dinosaurs died off, but not big crocodiles or small mammals. And features we consider good adaptations, such as the giraffe's neck or elephant's trunk, might not help at all in a mass extinction, he said. Specialists are vulnerable, no matter how exquisitely adapted they are to their environments. If you depend on a narrow range of food or symbiotic helpers, trouble awaits.
What's helpful for getting through a mass extinction is diversifying — spreading out across broad geographic areas.
Scientists almost unanimously agree that an asteroid impact triggered the mass extinction that killed of the dinosaurs 65 million years ago. In that event, about three quarters of all living species perished. They've made a case that a different kind of event triggered the even more deadly mega-extinction 250 million years ago. That one is thought to have started with a rise in carbon dioxide from volcanoes and burning coal beds. The CO2 not only warmed the planet but changed the chemistry of the oceans. Oxygen became scarce and the oxygen-starved oceans become hostile to fish, plankton, and other familiar life, but friendly to sulfur-using bacteria, which poisoned other species by exhaling hydrogen sulfide.
Despite such poisonous conditions, a few complex species did survive, including land animals that were our ancestors.
After the most recent mass extinction — the dinosaur-killing event — some tiny mammals survived and then flourished as never before, spreading out into different habitats, with different diets and different sizes, some taking to the water and some to the air. With no more competition from dinosaurs, there was room for a boom in new mammals. Ten million years down the road, there were bats in the skies and proto-whales in the seas—an inconceivably long delay by human standards, but the blink of a geological eye.
Clams and other bivalves have been doing something similar through all the major mass extinctions. They are long-term survivors, and in between extinctions, they've fanned out into many diverse forms. There have been clams that were the size of and shape of fire hydrants and others shaped like ice cream cones building ancient reefs now preserved as the white limestone cliffs along the Mediterranean Sea. Some bivalves are predatory – which is pretty amazing for such sluggish animals. Some are filter feeders and some are parasites on sea cucumbers or crabs.
What Krug and Jablonski found looking back over the last 200 million years of bivalve history was that even after that initial evolutionary burst that inevitably seems to follow each mass extinction, the long-term evolutionary rate was permanently changed aftermath of the last three mass extinctions, the pace of bivalve evolution slowed down twice and sped up once. Each time, the new rate of evolution held for tens of millions of years. This may have some bearing on the future, since scientists are trying to figure out whether we're currently embarking on the sixth mass extinction. Organisms go extinct all the time, but some biologists see signs it's happening much faster than the normal background rate.
The problem with identifying a current mass extinction is that we don't know how many species are alive today or how many are going extinct without ever being discovered, said Jablonski.