Here we sit, with our lattes and AirPods, during the sixth mass extinction. As absurd as that sounds, a mass extinction doesn’t always mean explosive volcanoes and apocalyptic asteroids. Mass extinction is an event that at least 75% of species are killed due to environmental factors in a relatively short amount of time. And that’s in geologic time! Geologically speaking, a “short amount of time” is anything less than 2.8 million years. While that far exceeds our human lifespans, it’s a bit of a wake-up call. We could be losing three-quarters of Earth’s species, all while we obliviously take another sip of caramel flavored foam.
Species go extinct all the time. In fact, 99% of all species that have ever lived are extinct. Most often, new, more complex, species evolve and the more simple lifeform dies off. But in the case of mass extinctions, the normal narrative of “survival of the fittest” doesn’t quite fit. Extreme climatic changes and natural disasters cause massive die-offs. What causes these mass extinctions? Here, we’ll review the causes and consequences of the previous five mass extinctions, and analyze where we are today… on the precipice of the sixth mass extinction.
While some species of bacteria, like cyanobacteria, can leave imprints in the fossil record, many microscopic organisms don’t leave a trace. Documentation of Earth’s mass extinctions, therefore, focus mostly on animal species survival, with a lesser emphasis on plants. The impact of these extinction events on microbial communities is not well known. The percentages of species loss in this article reference plant and animal diversity, and don’t encompass the actual diversity of all lifeforms.
Based on geologic records, there are five recorded mass extinctions in Earth’s history. Each of these big five resulted in catastrophic destruction to life on Earth. Whether tiny trilobites or tyrannosaurus rexes, no species was immune. The five mass extinctions in chronological order were the Ordovician, the Devonian, the Permian-Triassic, the Triassic-Jurassic, and the Cretaceous-Paleogene extinction.
The first mass extinction in Earth’s history ended the Ordovician Period. Lasting 45 million years from 448 to 443 million years ago, life during the Ordovician looked nothing like Earth today. The only land was amassed in the supercontinent Gondwana, slowly shifting south throughout the period. Latitudes north of the equator were almost entirely ocean, and life on Earth reflected this watery existence. While plants had begun evolving terrestrial lineages, animals were still completely confined to marine life.
The Ordovician Extinction occurred around 440 million years ago.
Though completely aquatic, animal diversity exploded during the Ordovician period. Most notable are the marine invertebrates, including graptolites (tube-like creatures), trilobites (giant aquatic roly-polies), brachiopods (ancient clams), and conodonts (early eels). Some primitive fish, squid, slugs, and corals also found a home amidst the red and green algae.
A mild, temperate climate defined the general vibe of the Ordovician. However, Gondwana continued to move South throughout the period, eventually passing over the South Pole and causing massive glaciation. This started as a global cooling period. Freezing oceans caused sea levels to drop. Furthermore, water trapped greenhouse gases into glaciers and frozen seas. Removed from the atmosphere, global cooling accelerated and temperatures continued to decline.
Life around the globe felt the strain of colder waters, but ecosystems closest to the equator were hit the hardest. Unable to sustain life, massive die-offs began. While some members of each major animal group survived, 85% of species went extinct. Much of life could not adjust to the colder climate.
Next up in devastating events in Earth’s history: the Devonian Extinction. The Devonian period lasted from around 408 to 360 million years ago. Marine evolution defined the era with bony fish, early amphibians, and even sharks dominating the seas. In fact, the era is colloquially known as the “Age of Fishes.” The changing landscape and rapid evolution pushed some fish into freshwater ecosystems. Closer to land, this transition marked the beginnings of the first tetrapods (animals with four legs). By the late Devonian, these tetrapods would have taken their first steps on solid ground. While animals were still making their way to land, plants had made themselves right at home. Ferns, mosses, and trees covered the landscape, creating the first forests.
The Devonian Extinction happened around 360 million years ago.
Vertebrates and invertebrate animals ruled the waters. Early in the Devonian, spiders, scorpions, and wingless insects made their way to shore. By the end of the era, tetrapods evolved in the freshwater, and these proto amphibians also walked out of the water. Similar species of trilobites, gastropods, and brachiopods persisted in the Devonian, along with bony fish and sharks. More notably, perhaps was the explosion of vascular plant diversity on land.
In contrast to the other mass extinctions, the end of the Devonian Period happened more gradually. Over the course of 20 million years, a drawn-out crisis caused 75% of animal species to go extinct. Because it wasn’t a clear event like an asteroid impact or volcanic eruption, the exact cause of the extinction has been a bit puzzling. The best theory, however, targets those incredible plants as the culprit. The diversification of vascular plants released an influx of nutrients in the oceans. More nutrients caused giant algae blooms that depleted the oceans of oxygen (a phenomenon called anoxia). Additionally, periods of global cooling and global warming added extra stressors to the ecosystems. Between a lack of oxygen and a changing climate, marine animal communities experienced massive dieoffs, concentrated around the equator.
The third mass extinction at the end of the Permian Period takes the cake as the largest extinction event in Earth’s history. But before the event that took out most of Earth’s animal life, the Permian Period was pretty peaceful. Marine life still dominated, but terrestrial plants and animals were gaining their footing. The supercontinent Pangea accounted for most of the landmass, with the world’s waters combined in a single ocean known as Panthalassa.
The Permian-Triassic Extinction occurred around 250 million years ago.
On the plant front, trees evolved into gymnosperms (modern-day conifers) by developing a protective seed to house their offspring. Animals communities evolved more slowly, vertebrates and land animals, while advancing still were inconsequential compared to the vast numbers of marine invertebrates. The end of the Permian period changed everything, however, leading into the “Age of the Dinosaurs.”
The end of the Permian Period caused 96% of marine species to go extinct. That’s not just a mass extinction, but the most massive extinction! So what happened? A major volcanic eruption in Siberia is to blame. Releasing tons of greenhouse gases into the atmosphere, the Earth entered a period of rapid warming. Based on projections from the fossil record, ocean temperatures rose 10°C (about 20°F!). Before disaster struck, the Earth’s atmosphere was fairly similar to what it feels like today. After the eruption, nothing was the same. Because of the warming, oxygen levels fell by 80% in the oceans. Not only did this mark the end of the Permian Period, but also the mass extinction ended the Paleozoic Era.
The Triassic Period started the Mesozoic Era, but things didn’t happen in a flourish. The devastation at the end of the Permian Period returned life to a Precambrian state. Microbes and opportunistic species did the best. It wasn’t until the mid-Triassic period that coniferous trees regained control of the land and not until the end of the period that dinosaurs got their footing. Pangea continued to break apart, affecting the climate.
The Triassic Period ended around 210 million years ago.
Noted for a distinct lack of diversity, the Triassic period marked a rebuilding time. The destruction of the end-Permian extinction acted as a restart for many ecosystems. Notably, monocultures were common, with a single species dominating an area. Lycophytes, an early vascular plant, were the predominant flora, laying the groundwork for conifers and cycads.
Scientists aren’t totally clear on what caused the end of the Triassic period, though most have agreed upon a likely scenario. In the center of Pangaea lay a region called the Central Atlantic Magmatic Province (CAMP). Around the time of the end-Triassic extinction, extreme events of volcanism released tons of greenhouse gases into the atmosphere. The influx of carbon dioxide, sulfur, and methane caused a significant global warming event. Climate change combined with ocean acidification inevitably caused the extinction of 76% of marine and terrestrial species.
The Cretaceous Period lasted between 145 to 65 million years ago, ending in the Cretaceous-Tertiary Extinction. The supercontinent Pangaea continued to break up into the continents we know today. Separating into hemispheres, the continents adapted distinct climates and ecosystems that accelerated evolution by creating diverse niches. Plants, insects, reptiles, mammals, amphibians, and birds all diversified.
The Cretaceous-Tertiary Extinction occurred around 65 million years ago.
Welcome flowers! The Cretaceous Period welcomed the evolution of flowering species of plants. This crazy biodiversity of plants became a varied new food source for animals. Besides the fruit and nuts that attracted the appetites of all sorts of herbivores, the new wealth of flowers spurred the evolution of insects, including pollinators. In the Age of the Reptiles, these giant, scaly, coldblooded creatures dominated the land (dinosaurs), the sea (mosasaurs), and the air (pterosaurs). However, marine invertebrates, small mammals, and early birds also thrived.
Initially, there was some controversy as to whether the Cretaceous Period ended with another volcanic eruption or from a meteor impact. What geologists did know is the extinction event was disastrous. Upon discovery of a giant crater off the coast of Mexico’s Yucatan Penninsula, the asteroid impact theory prevailed. Marking the Cretaceous-Paleogene boundary is a band of iridium called the K-T boundary. Rarely found on Earth, iridium exists in high concentrations on asteroids. The impact of the meteorite set off a whirlwind of catastrophe, including acid rain, earthquakes, and global warming. Devastating plant life and large dinosaurs, only small mammals, reptiles, and avian dinosaurs survived.
Without a doubt, human activity impacts the world around us. So much so that scientists have given a new name to our current period: the Anthropocene. More than just a name, the Anthropocene has dire consequences for the species and ecosystems of planet Earth. Between urbanization, resource use, and climate change, we have started the sixth mass extinction.
Even with the most conservative estimates, the extinction rates are 100 times higher than normal. For example, over 400 vertebrate species have gone extinct in the last 100 years. At the “background rate” of extinction, scientists would expect that to take 10,000 years.
Four characteristics set the sixth mass extinction apart from the previous five. Each of these reflects the role that humans have in our world.
Bringing ruthless competition, deadly diseases, and destructive predators, invasive species threaten plants and animals with extinction. For example, the introduced brown snake predated the Guam broadbill bird to extinction in Micronesia. Whether intentionally or not, humans have introduced plants, animals, fungi, and microbes around the world, causing native species to go extinct.
Primary production references the food chain. At the base of the food chain are all the plants soaking up sunlight to make food for themselves, and, inevitably, the consumers higher up the chain. Normally, a giant diversity in herbivores, omnivores, and carnivores much up this food source. Today, however, a single species dominates the land and oceans around the globe. Humans use up to 40% of the world’s primary production for our uses!
Through competition and predation, species have put evolution pressures on each other throughout Earth’s history. But humans have taken this to another level. Through agriculture and the domestication of animals, we are directing and controlling gene flow. Even conservation efforts reflect this power play. Depending on which grants and research projects receive funding, humans are decided which species get “saved” and which disappear without a second glance. For better or worse, humans are “playing god.” Whether we admit it or not, we hold the marionette strings, controlling the fate of plants and animals around the globe.
The last piece that sets the sixth mass extinction apart from the others is the technosphere. A term coined by Duke University professor Peter Haff in the journal The Anthropocene Review, the technosphere refers to the extensive system that comprises technological systems, artifacts, and information. Haff argues that, similar to the atmosphere, the technosphere functions on its own. Obviously, humans have invented and created these tools, but an individual human cannot influence global technology independently. Technology is bigger than one person. As a result, tech functions at a societal level, and humans have lost control. Because of the amount of energy and resources the technosphere consumes, it has become a driver of the extinction crisis.
Understanding the disproportionate role humans have leads us to the forces behind the current extinction crisis. How does our influence threaten other species? We certainly aren’t trying to have such a negative impact…
To support the nearly 8 billion people on planet Earth, humans displace and destroy the habitats of other species. Habitat loss and destruction is the predominant cause of species extinction. Deforestation, modern agriculture, and urbanization certainly play a huge role in habitat loss.
Each of those smaller issues is just a symptom of the greater problem: climate change. Resource extraction already threatens species’ habitats. But while a simple policy can preserve a stretch of forest, we don’t have as clear control over the impact of our carbon emissions. Climate change further threatens ecosystems on land and sea. Similar to previous mass extinctions, a rapid increase or decrease of greenhouse gases caused a dramatic change to global climates. The difference today? Volcanoes, tectonic shifts, and meteorites can’t be blamed. Humans are causing global warming with our consumption of fossil fuels.
Is a sixth mass extinction inevitable? Estimates from MIT professor Daniel Rothman predict that a devastating extinction event will play out if we add 310 gigatons of carbon to the ocean. At our current rate of consumption, we will reach that number by 2100. Though it might take another 10,000 years for 75% of species to die off, the consequences for day-to-day life would be dire. Sea level rise, erratic weather patterns, and food insecurity would make human life dire, especially for underprivileged and marginalized communities.
We can hold on to some hope! Though human activity threatens the world’s ecosystems, we can also save them. Reducing carbon emissions and mitigating the effects of climate change could slow the rate of extinction before it is completely out of our control. It will take a concerted, meaningful effort, but changing the course of the sixth mass extinction could be possible. What can you do? Advocate for aggressive climate policies and support businesses that are mindful of their impact on the world’s ecosystems. We still have a chance to stop the sixth mass extinction!