Understanding Extinction Events
An artist's rendering of the Chicxulub impact. (Source: Mini Museum)
Life on Earth is defined by cycles, an ebb and flow of bursts of life tempered by mass die-offs. Five extinction events have cut their way through Earth’s history, wiping out the majority of lifeforms on the planet. However, with each extinction, the stage was set for the next era of life to arise.
Mammal life was only able to flourish in the absence of the dinosaurs that had ruled the planet before the Cretaceous-Paleogene event, and dinosaurs came about after the Permian-Triassic extinction set the stage for them. An extinction is not merely the end of an era of life, but the beginning of the next too.
Although the extinctions themselves are easy enough to detect in the fossil record, their causes don’t always yield up as easily. The K-Pg extinction that eliminated the dinosaurs is endowed with the image of the Chicxulub impactor falling from the sky and eliminating the Earth’s species in one fell swoop. But in truth extinctions are slow processes that can stretch across millions of years, the sum total of a number of different intersecting catastrophes.
Luis and Walter Alvarez, the father-son team that originated the Alvarez hypothesis, standing next to the K-Pg extinction boundary. (Source: Lawrence Berkeley Laboratory)
The Late Devonian extinction is the best example of this phenomenon, taking place over at least 7 million years, across multiple distinct pulses. The extinction was the result of several interconnecting disasters that snowballed into each other: climate change spurring disruptions to sea levels causing oxygen depletion in the world’s oceans, killing off the majority of marine life. The Late Devonian illustrates just how difficult it is to pin down the exact cause of an extinction, but there are certain tools available.
As with any disaster, the first step in approaching an extinction is to look at the survivors. This is called the disaster taxa and is an invaluable tool in understanding extinctions. For example, if the disaster taxa is composed of creatures best suited to a cold environment, one can expect global cooling to have been the cause of the extinction. Disaster taxa are also primed to be the successors to their environment. The dinosaurs dominated the world after the Triassic-Jurassic extinction, while mammals took the throne after the Cretaceous-Paleogene eliminated their competition.
A chart tracking the major extinctions. (Source: Mini Museum)
Beyond the complexity of pinning down an extinction’s causes is defining an extinction itself. Traditionally, the extinctions are ordered into the Big Five, with a dozen other small die-offs punctuating the historical record, but this model is constantly being challenged. The Capitanian event, usually defined as a pulse of Permian may be considered a distinct extinction itself, while the End-Ediacaran extinction that preceded the Cambrian explosion is usually discounted because the primitive lifeforms of its day left behind few fossils, making the extinction difficult to study.
That being said, a sixth mass extinction is increasingly being accepted, the one taking place right now. The Anthropocene Extinction has seen the deaths of untold numbers of species, from those killed by the early hunter-gatherers, to those killed by pollution and deforestation today. With the previous extinctions, the process was a natural one and given to new bursts of life after the destruction. With the Anthropocene, the process is more artificial, with the potential to permanently damage the environment without easy recovery. However, like all other extinctions, the Anthropocene is a slow process, one with the potential to be reversed if drastic action is taken soon.
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