From time to time, the planet Earth suffered mass extinctions. The last big one happened about 65 million years ago (Cretaceous ends, Paleogene begins) and wiped out the dinosaurs, and many other species. But the biggest one happened about 250 million years ago, and the event marks the transition from the Permian geological period to the Triassic. Roughly 90 percent of all marine life died, as well as nearly three-quarters of all land plants and animals! Now a new study finds that this great dying might have caused a lack of oxygen that left animals gasping for air.

Pangea Break-up (Courtesy: USGS)Back then, the Earth's continents were glued as a single large super-continent, called the Pangea. The reason behind the Great Dying is not known, and different hypotheses have been put forward (asteroid impact, global warming, supervolcanoes). The Lack-of-Oxygen scenario (put forward by Raymond Huey and Peter Ward of the University of Washington) might just be what really happened.
Oxygen currently makes up about 21% of our atmosphere. In the early Permian period it was 30%, from where it fell to 16% during the Great Dying, and to 12% after that. Such low levels of oxygen meant that animals at sea level breathed air similar to that at the top of a 17,400-foot mountain today! At higher elevations, the oxygen content would be still lower. Effectively, this would restrict the movements of the animals living during that time. Populations of animals might be fragmented, leading to their speedy demise.
Even though just the lack of oxygen might not be the sole reason, it could be a large factor in what happened. Perhaps the living kingdom suffered a double whammy, a lack of oxygen, coupled with some other major event that precipitated the mass extinction.