Over the past 500 million years, there have been dozens of episodes of extinction on the planet, the most famous being the titanic event that swept away the dinosaurs.
Scientists have assumed that the time it takes for the living world to bounce back is proportional to the damage done: More time should be required to fill all of the Earth’s empty corners with newly evolved life after gargantuan extinctions and less time should be required after a more minor die-off.
But in an unexpected finding, researchers reported Thursday that it takes a long time to recover from these large-scale extinctions–around 10 million years–and more intriguing, the recovery time appears to be the same, whether the original destruction was one of the grander or one of the more minor events.
An ominous implication of the new research, some scientists say, is that humans may already or will soon have destroyed enough species that it will require a 10 million years for the planet to recover–20 times longer than humans have already existed and longer than many scientists predict humanity itself is likely to persist into the future.
Scientists say the new study also will prompt paleontologists to rethink how life evolves after major extinctions.
The study, by James W. Kirchner, an Earth scientist at the University of California at Berkeley, and Anne Weil, a paleobiologist at Duke University, appears Thursday in the journal Nature.
“I think it’s a surprise to everyone,” said Douglas Erwin, a paleobiologist at the National Museum of Natural History at the Smithsonian Institution. “We’re just at the beginning of understanding biotic recoveries following extinctions.”
Mike Foote, a paleobiologist at the University of Chicago, said the paper would change how people look at the fossil record, adding, “It’s going to be seminal.”
The researchers examined an enormous set of data gathered by the late Jack Sepkoski, a paleontologist at the University of Chicago. The data document when new kinds of marine animals appear in the fossil record and when they disappear or go extinct.
Scientists expected that with every additional species that becomes extinct, the longer the recovery should take. What researchers found instead was that after extinctions, big and small, it took about 10 million years for the proliferation of new kinds of organisms to peak and then begin to tail off. The decrease in the proliferation of new creatures is seen as a sign that ecosystems have recovered and are so full of diverse life that there is little opportunity left for new organisms.
Scientists said the new study suggests that the evolutionary rebuilding after an extinction might work differently than had been envisioned.
Many paleontologists had previously pictured extinctions as being the equivalent of wiping the pieces off a chessboard. In such a situation, though the pieces are gone, the spaces on the chessboard–or in nature, the niche for an algae-eating fish, say, or a long-lived evergreen tree–are still there, each simply needing to be refilled by a newly evolved species.
But the new study, Erwin said, suggests instead that “when you wipe the pieces off the chessboard, the size of the chessboard shrinks.” That is, with the loss of a species comes the loss of opportunities for the existence of other organisms as well — like those that would make their living by preying upon or parasitizing that organism.
“The bottom line,” said E.O. Wilson, an evolutionary biologist at Harvard University, “is that we had better take care to hold on to the biodiversity that still exists.”
