A cataclysmic mass extinction that devastated life on Earth millions of years ago is the unlikely reason such a rich variety of life is found in the oceans today, scientists have discovered.
Around 250m years ago at the end of the Permian era the Earth experienced its most dramatic loss of life, when an estimated 95% of marine species and 70% of land animals were wiped out. Scientists are uncertain what caused the extinction, but many suspect rapid environmental upheaval caused by vast volcanic eruptions were at least in part to blame.
Scientists at the Field Museum of Natural History, in Chicago, used a new database of fossil records to study how lifeforms in the oceans changed over 545m years. Instead of finding a gradual rise in different species, they spotted a sudden explosion in marine life shortly after what paleontologists call "the great dying".
Fossils of organisms known to be much older than 250m years reveal a picture of the oceans dominated by plant-like creatures that anchored themselves to sediments and filtered nutrients from the water washing past them.
For 10m years after the mass extinction life in the oceans showed little sign of recovery, but fossils dating from 240m years ago and younger showed that more complex ecosystems quickly emerged, along with the first creatures to become mobile and actively search for their food.
The scientists behind the study believe that by nearly wiping out life in the oceans the playing field was levelled between formerly dominant marine species and others eking out an existence on the margins. In the following millennia organisms jostled for space and carved out niches. The research, published in the journal Science today, is the first to explain how modern ocean life came to be.
"It tells us that there was not an inexorable trend towards modern ecosystems," said lead scientist Peter Wagner at the FMNH. "If not for this one enormous extinction event at the end of the Permian, then marine systems today might still be like they were 250m years ago."
Marine animals to emerge after the extinction included snails, clams and crabs. Other species, such as sea lilies and lamp shells, dominant before, became less common.
According to marine ecologists, over-fishing and other threats to marine life are damaging some ecosystems so much they are beginning to resemble the sparse oceans of 550m years ago. "The asteroid that wiped out the dinosaurs couldn't manage that," Dr Wagner added.