Earth has had some tough times. I mean really tough — five mass extinctions tough.
Mass extinctions are periods in time where large numbers of species on Earth go extinct. In fact, more than 90 percent of all organisms that have ever lived on Earth are extinct. At least five times in the last 500 million years, 50 to 90 percent of the species on Earth have been completely wiped out in a geological blink of the eye.
Although mass extinctions are fatal for most of the unfortunate species that happen to be alive at the time, they open the door for new species to emerge.
Photo credit: Joseph Smit
During the Ordovician-Silurian extinction, the majority of life was confined to the oceans, and it is believed to have been quite warm — especially in the tropics. The extinction, the second most devastating with 85 percent of all species going extinct, occurred 440 million years ago (Ma). The cause of the mass extinction is believed to be due to the expansion of the glaciers, resulting in dramatic sea-level drop as most of the world’s water became trapped as ice.
Late Devonian Extinction
Photo credit: Eduard Riou
The Late Devonian extinction occurred 360 Ma. Similarly to Ordovician-Silurian, the extinction was more devastating to warm-water marine life, but life on land also took a significant blow. The Late Devonian extinction resulted in a 70 percent extinction of all species, with the cause believed to be due to the expansion of glaciers.
Photo credit: Vassil/Wikipedia (CC BY-SA 3.0)
The Permian-Triassic extinction was Earth’s worst extinction — wiping out 95 percent of all species. It occurred 250 Ma. Scientists believe that an asteroid or comet caused the massive extinction, however a crater has never been discovered. Other theories suggest impact-triggered volcanism, or that flood volcanism from the Siberian Traps covered large stretches of land with lava and caused a loss of oxygen in the oceans.
Photo credit: Zina Deretsky/National Science Foundation
The Triassic-Jurassic extinction occurred 200 Ma, and resulted in the extinction of 50 percent of all species on Earth. Scientists believe it was likely caused by volcanic flooding that erupted from what is now the central Atlantic ocean, triggering the breakup of Pangea. Other scientists have suggested that an asteroid impact was the cause, however, a crater has never been found to validate the theory.
Photo credit: ScottAlbertAnselmo/Wikipedia (CC BY-SA 3.0)
The Cretaceous extinction, the one most familiar to us, occurred 65 Ma, and resulted in a 50 percent extinction of species on Earth, including dinosaurs. The extinction was caused by an asteroid impact, and evidence of the impact crater was found in the Yucatan Peninsula beneath the Gulf of Mexico. Some scientists believe that the cause of the extinction may have been the combination of global warming and the eruption of the Deccan Flats of India.
The Holocene Extinction?
Many scientists believe there is evidence we are currently in the sixth mass extinction, and the cause is solely because of humans. Animals are going extinct 100 times faster than the normal extinction rate. The five major human activities leading to this new mass extinction include:
Habitat destruction, including human-induced climate change
Over-harvesting by hunting and fishing
Introducing invasive species
It is estimated that by the year 2100, human activities will result in the extinction of more than 50 percent of species on Earth. The previous five mass extinctions took, on average, thousands and thousands of years, and humans are generating on par extinction levels in just hundreds. Not only do our actions threaten the biodiversity of our planet, they threaten the existence of humans as well.
But it is not all doom and gloom. Scientists have given a generation's worth of time to fix the problem. So there is hope, but the window of opportunity is closing quickly.