We’re naturally fascinated by the terminal–the end. Extinction seems like the ultimate end. Not only does an individual perish but an species disappears. We know that things ‘die out’ but extinction is not a concept many people spend much time thinking about. In fact, up until the dawn of the 19th century, no one thought a complete disappearance of a species was possible.
In 1796, Georges Cuvier–a brilliant French anatomist–was studying living and fossil elephants when he began to conclude that some fossil species no longer roamed the earth. Elephants are so large they’d be difficult to hide, Cuvier reasoned. Comparing the bones, he was sure that some types had gone extinct. And further discoveries of strange prehistoric animals confirmed his idea.
We now know that extinction is commonplace. Almost every species that ever lived is now gone. Our contemporaries on this planet are just the latest in a long line of lifeforms. Most of them will survive for only a million years, maybe two. Really successful species might make 10 million years. So the background extinction rate, as scientists call it, is slowly ticking away–established species vanishing and new species taking their place. But this rate is usually rather low and operating in millennia. The overall changes to the biota are gradual, yet on a more precise species or family level, we see punctuated equilibrium: rapid changes happening as species mix with each other, diseases are spread, or new territories are colonized.
This punctuated equilibrium can happen on a much larger scale. These are mass extinctions, where 75% of Earth’s species are exterminated very quickly. The extinction of the dinosaurs at the end of the Cretaceous period, 66 million years ago, is the most famous of these. But many people do not realize that the Cretaceous extinction event is one of five such events from the distant past.
The Ordovician ended 444 million years ago. When it did, 86% of species alive at that time were lost. An ice age altered sea levels, CO2 was pulled from the atmosphere, and temperatures plummeted.
The trilobites–one of life’s biggest success stories–were exterminated 375 million years ago at the end of the Devonian. 75% of species disappeared. The likely cause is the newly evolved terrestrial plant life releasing nutrients into rivers and oceans, causing algal blooms. The lack of oxygen choked the trilobites and other marine species out of existence.
Sound terrible? It’s not nearly as bad as The Great Dying at the end of the Permian, 251 million years ago. 96% of species were exterminated. 96%. How did it happen? Volcanic activity released extreme amounts of CO2 into the atmosphere. Bacteria fed off this, producing increasing amounts of methane–another greenhouse gas. Temperatures spiked, causing ocean acidification and the complete extermination of tabulate coral reefs (modern corals are altogether differently descended). Ocean acidification, greenhouse gas buildup…sound familiar?
Another mass extinction happened 200 million years ago, at the end of the Triassic period. We are not sure how this catastrophe occurred, but we know it took 80% of species with it. This dramatic event allowed the dinosaurs to rise to prominence, only for their own dynasty (of many, many species coming and going throughout the next 130 million years) to be wiped out by asteroid impact 66 million years ago. More than the dinosaurs were lost though. 76% of species vanished with them due to a mixture of extraterrestrial impact, its aftermath, volcanic activity, changing ecosystems, and a slew of other interconnected causes that we’re only beginning to unravel.
We humans are living through–and causing–the sixth mass extinction. Greenhouse gases are escalating temperatures. Oceans are acidifying. We don’t have the final dismal tally of how many species will be wiped out, but we know the causes. Our activity has pushed natural systems into overdrive. It is our responsibility to act accordingly when planning the future of our planet–for the moment, our only available home.