Mass Extinction Killed 85 Percent of Life on Earth 400 Million Years Ago, When Temperatures Dropped Extremely

Our world was defined by a unique history that went through a series of natural events that defined its character. Life on Earth was always fragile, and asteroid attacks, volcanic eruptions, or even the unpredictable Ice Age played a significant role in the balance. One such event was the Ordovician Mass Extinction, almost 400 million years ago, according to IFLScience. This period in time was extremely different from the world we know today, as life did not exist on land. There were no plants on the surface or any evidence of a plant community; hence, there were no animals without an existing food chain, yet this period witnessed great biodiversity.

“The climate was very different—it was a very warm period, and CO₂ levels were very high,” stated Richard Twitchett, Research Leader in the department of Earth Sciences at the UK’s Natural History Museum. The ocean was inhabited by a treasure trove of creatures with immense diversity and experimental evolution. “There were animals trying to exploit different lifestyles, but it's also diversification within body plans that are obviously becoming more successful for these warm Paleozoic seas,” Twitchett commented. However, these ecosystems took a slow hit, followed by a massive wipeout, when the planet’s temperature faced a drastic shift.

Despite significant research and efforts to understand the past, there were various missing pieces in the story. The climate faced a period of warming and a significant rise in sea levels, which instantly transitioned to the Ice Age. The landmass was engulfed by glaciers, which later gave way to a warming of the environment again, according to Discover Magazine. This shift in climate disrupted the way in which nutrients like oxygen, carbon, and nitrogen cycled through the ocean. The ocean circulation patterns would be significantly affected by the change from greenhouse conditions, likely enforced by new plants on land, to icehouse conditions.

The tipping levels of carbon dioxide in the atmosphere created an imbalance in the concentration of oxygen. This lack of oxygen was investigated through geochemical testing and computer modeling. Experts measured the levels of iodine in carbonate rocks to look for oxygen shifts at depths across time, according to Earth.com. “Upper-ocean oxygenation in response to cooling was anticipated, because atmospheric oxygen preferentially dissolves in cold waters,” stated Alexandre Pohl of UC Riverside. “We were surprised to see expanded anoxia in the lower ocean since anoxia in Earth’s history is generally associated with volcanism-induced global warming,” he added.

The sudden cooling influenced a response of anoxia, and the increasing ice reflected sunlight at an accelerated rate. The feedback loop of reflecting heat and light was a cycle that helped cope with the ice age, when things suddenly turned warm again. Life on the planet had to readjust to the rising temperatures and, at the same time, was suffocating from low oxygen. Regardless of the mystery surrounding the natural events, the result was the massive wipeout of 60 percent of genera and nearly 85 percent of species. The resilience of the planet to continue sustaining life was commendable, as it went through several such deadlines to stop functioning.