A mass extinction is hard enough for Earth's biosphere to handle, but when you chase it with prolonged oxygen deprivation, the biota ends up with a hangover that can last millions of years.
Such was the situation with the greatest mass extinction in Earth's history 250 million years ago, when 90 percent of all marine animal species were wiped out, along with a huge proportion of plant, animal and insect species on land.
A massive amount of volcanism in Siberia is widely credited with driving the disaster, but even after the immense outpourings of lava and toxic gases tapered off, oxygen levels in the oceans, which had been depleted, remained low for about 5 million years, slowing life's recovery there to an unusual degree.
The reason for the lingering low oxygen levels has puzzled scientists, but now Stanford researchers have figured out what probably happened. By analyzing the chemical composition of some then-underwater limestone beds deposited over the course of the recovery in what is now southern China, they have determined that while it took several million years for most ecosystems in the ocean to recover, tiny single-celled algae and bacteria bounced back much more quickly.
