Of the many mysterious and unexplained events that have happened on Earth over millions of years, mass extinctions are perhaps the most perplexing. Mass extinctions involve the monumental loss of plant and animal species over short time. These events leave Earth ripe for evolutionary changes as new species develop to take the places of those lost. Scientists have discovered at least five different mass extinctions, referred to as the Big 5, over history when anywhere between 50% and 75% of life were lost. Countless questions remain unanswered about how and why these moments came to pass. Below is the information that experts have gathered so far.
The Mass Extinction Periods
Around 439 million years ago, 86% of life on Earth was wiped out. Scientists believe two major events resulted in this extinction: glaciation and falling sea levels. Some theories suggest that the Earth was covered in such a vast quantity of plants that they removed too much carbon dioxide from the air which drastically reduced the temperature. Falling sea levels were possibly a result of the Appalachian mountain range forming. The majority of the animal life lived in the ocean. Trilobites, brachiopods, and graptolites died off in large numbers but interestingly, this did not lead to any major species changes during the next era.
Late Devonian Extinction
Estimates propose that around 75% of species were lost around 364 million years ago. Information is unclear as to whether the late Devonian extinction was one single major event or spread over hundreds of thousands of years. Trilobites, which survived the Ordovician-Silurian extinction due to their hard exoskeletons, were nearly exterminated during this extinction. Giant land plants are thought to be responsible as their deep roots released nutrients into the oceans. The nutrient rich waters resulted in mass amounts of algal blooms which depleted the seas of oxygen and therefore, animal life. Volcanic ash is thought to be responsible for cooling earth’s temperatures which killed off the spiders and scorpion-type creatures that had made it on land by this time. A distant amphibian cousin, elpistostegalians, had also ventured onto land but became extinct. Vertebrae did not appear on land again until 10 million years later, the ichthyostegalians from which we all evolved. If the late Devonian extinction had not occurred, humans might not exist today.
This mass extinction, which occurred 251 million years ago, is considered the worst in all history because around 96% of species were lost. Ancient coral species were completely lost. “The Great Dying” was caused by an enormous volcanic eruption that filled the air with carbon dioxide which fed different kinds of bacteria that began emitting large amounts of methane. The Earth warmed, and the oceans became acidic. Life today descended from the 4% of surviving species. After this event, marine life developed a complexity not seen before and snails, urchins, and crabs emerged as new species.
The Triassic-Jurassic extinction happened between 199 million and 214 million years ago and as in other mass extinctions, it is believed there were several phases of species loss. The blame has been placed on an asteroid impact, climate change, and flood basalt eruptions. During the beginning of this era, mammals outnumbered dinosaurs. By the end, dinosaurs’ ancestors (archosaurs) reigned the earth’s surface. This extinction laid the path that allowed for the evolution of dinosaurs which later existed for around 135 million years.
Perhaps the most well-known of the Big 5, the end of the Cretaceous-Paleogene brought on the extinction of dinosaurs. A combination of volcanic activity, asteroid impact, and climate change effectively ended 76% of life on earth 65 million years ago. This extinction period allowed for the evolution of mammals on land and sharks in the sea.
Are We Next?
A heated debate in the scientific community is whether or not earth is heading into another mass extinction. Currently, the world is in the Holocene era, plants and animals are dying off at abnormally fast rates and life as we know it is in danger. This time, however, the cause is not volcanic activity nor asteroid impacts. Human activity is triggering a change in global climate which has increased species extinction to between 10 and 100 times faster than the norm. The evidence is pretty clear, we are headed toward the 6th mass extinction.