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Sixty-six million years ago, an extraterrestrial body—most likely an asteroid—collided with the Earth. The impact released energy roughly equivalent to 19,000 times the current global nuclear arsenal. The impact site at the Yucatan Peninsula of Mexico would have suffered earthquakes around magnitude 11. Evidence indicates that the impact caused regional tsunamis and a wave of thermal radiation that spread across the planet's surface. A blast of material was ejected from Earth’s surface, sending debris into the atmosphere with such force that it spread globally, and in some cases, escaped Earth’s gravity.
The ecological consequences of the collision form a now-familiar story. The rapid changes to the planet’s atmosphere—including darkness, cooling, and the release of massive amounts of sulfur that subsequently returned to the surface as acid rain—drove the most recent of the known mass extinctions. And all this may have been exacerbated by ongoing, climate-altering volcanic eruptions on the Indian sub-continent.
More than 70 percent of animals (at least the ones we know as fossils) became extinct, including the iconic non-avian dinosaurs. In the oceans, the ammonites—distant relatives of squid and octopuses, and one of the ocean's more dominant groups for over 250 million years—also became extinct. Tropical reefs that once stretched from the Western Interior Seaway in Arizona to the Caribbean Sea collapsed as their builders, the fantastic rudist clams, became extinct.
Planet Earth was an altered landscape. The planetary ecosystem, and its global economy, had collapsed in the ecological blink of an eye. Surviving lineages of organisms spread across the land into areas that had suffered heavy losses, and in some cases they began to diversify rapidly. Ecosystem recovery, however, would be a more protracted affair.
The asteroid impact scenario was a nightmare, and some people worry about the civilization-ending potential of a future impact. But the mass extinction, the losses of biodiversity, the collapse of entire ecosystems, and a lengthy period of recovery, were not unique events in Earth’s history. In fact, five global mass extinctions have occurred in the past 540 million years, though only the one described above has been associated with an asteroid impact. There were likely other global extinctions in the billions of years preceding, but the fossil record then is devoid of macroscopic organisms and is much more difficult to interpret. The possible causes of these extinctions include massive volcanism (“magmatic events”), climate warming, and climate cooling.
My favorite extinction, and the subject of a lot of my own work, is the Permian-Triassic event, which occurred about 252 million years ago. It was the greatest of all; more than 90 percent of known marine species and more than 70 percent of terrestrial species disappeared. This time the collapse was likely driven by extreme Siberian volcanism that altered the atmosphere, climate, and oceans. It took less than 60,000 years—a geological blink of the eye—for planet Earth to be transformed into what must have been a strange planet indeed. New species evolved within a few tens to hundred thousands of years, but it took considerably longer for ecosystems to stabilize and the extinctions to diminish.
The forces of nature that drive extinctions are tremendously powerful, whether they act on timescales of “seconds to impact” or play out over millennia. There have been no more global mass extinctions since the asteroid, in spite of the occasional extreme climatic event or volcanism. Yet today we stand on the threshold of a new one, driven by the newest force of nature: humans.
Humans are a part of nature. We evolved alongside millions of other species, and have been subjected throughout our history to the calamities of nature. But there is no doubt that we also stand apart from other species. Unique evolutionary attributes, and long histories of technological innovation, have enabled our species to manipulate planet Earth to an extent never before possible for a single species. We have spread over the planet, transforming species and landscapes in support of large and growing human populations, and extracting energy from the fossilized remains of past ecosystems to fuel our growth. Human expansion has brought many negative consequences, not the least of which are the extinctions of fellow species. We have become the next asteroid.
The earliest hints of human-caused extinctions date back to the end of the last ice age, some 12,000 years ago, when large mammals were hunted with increased social and technological efficiency. Since then, the hunting has continued, accompanied by landscape alteration, habitat destruction, pollution, and the disruption of key inter-species relationships.
Is the planet doomed to repeat the events of 251 or 66 million years ago: collapse, extinction, eventual recovery? What would the world look like to us if we lost even 50% of the species around us, and what would that mean for our own human well-being and survival?
These are heavy questions, and they demand answers, but here’s the optimistic bit. While the consequences of our actions may mimic those of asteroids and giant eruptions, our actions are conscious. We are an intelligent and cognizant force of nature. I think that an increasing number of modern humans are beginning to remember, or perhaps realize, that we remain a part of nature. Extinctions today are driven by our decisions—some of those necessary and some of them by choice. We can choose differently, and we can apply our remarkable intelligence to make our necessities less harmful to the planet.
As a scientist I am fascinated by the great extinctions of our past. As a person, I would like to be remembered as part of that pivotal moment in history when humans began to make good choices of global consequence.
Barnosky, A. 2014. Dodging Extinction. Power, Food, Money, and the Future of Life on Earth. University of California Press.
Kolbert, E. 2014. The Sixth Extinction: An Unnatural History. Henry Holt & Co.
Roopnarine, P. D. 2013. Omslagpunt voor de aarde (Tipping the Biosphere). In Meer!, M. Thieme (ed.). Uitgeverij Jan van Arkel, Netherlands. p. 87-98. (English translation here.)
Peter Roopnarine is Curator of Geology at the California Academy of Sciences.
Image: Visible Earth, NASA