Scientists think they finally solved the mystery behind Earth's first mass extinction

The Earth has gone through five mass extinctions, and is now experiencing its sixth. The causes of our current event are pretty obvious, but until last month the underlying reason behind the planet’s first mass extinction some 444 million years ago was one of the biggest mysteries in science. Now, a pair of scientists with the Geological Survey of Canada and the University of Hull in England have given us a clearer picture of what they believe happened with a report published last month in the science journal Geology.

The first mass extinction event occurred during the Late Ordovician period. It’s a confusing period of time that ended with an estimated 85 percent of all marine life on the planet dead, and was punctuated with a sudden glaciation event that froze over the Earth. In the scientific community, it’s been widely believed that the first mass extinction event happened when a great freeze reduced the amount of water available for marine life to live in. Then, when the ice melted, ocean oxygen levels dropped, resulting in the extinction of the vast majority of water-based life.

But this theory has been questioned in the past. Other researchers have said the glaciation period that marked the end of the Late Ordovician extinction was something that came and went in regular cycles over millions of years. It didn’t make sense to them that this glaciation event would be special or unusual enough to kill off so many marine species within the same period of time.

So a pair of geologists set out to challenge the established narrative, and they believe they’ve found evidence that volcanoes were the true killers. It’s thought that, back then, there were frequent earthquakes and eruptions from ancient volcanoes as the Earth’s tectonic plates constantly shifted. The geologists think these volcanoes were spewing so much lava, carbon dioxide, and mercury that the planet went through a global warming event that caused the first mass extinction.

The scientists reached this conclusion after testing rocks from Scotland that date back to the Late Ordovician period. Using high heat, they were able to measure elements of mercury, molybdenum, and uranium emitted by the rocks — telltale signs that these stones came from an Earth with a volcanic environment and deoxygenated waters. Global warming can also cause the ocean to hold less oxygen, which can then kill off marine life.

This theory made more sense to the researchers, since the majority of extinction events occurred due to global warming. (Only the Cretaceous-Peleogene event was caused by a massive asteroid hitting the planet, resulting in global cooling that ended the reign of dinosaurs.) But there’s still more work to be done to support, or disprove, this newfound theory, and the geologists hope that these findings will spur additional tests on Late Ordovician-dated rocks from all around the world to gain a better picture of what the Earth looked like at the time. The findings could also help us learn what we might expect from our current, human-driven extinction event.