Science shows that extinction risk is real. Over 4.5 billion years, Earth has experienced five global extinctions, and the clock can turn when the planet starts spinning again each day.
While the idea that a space rock could wipe out life in minutes is terrifying, there are many other factors that could drive the collapse of fragile life on Earth. Some stem from planetary dynamics, such as ice ages, climate shifts, or rapid sea-level changes, as well as the eruption of massive volcanoes. Others arise in ecosystems, including the spread of epidemics or competition among species for limited resources in the same area.
After five earlier mass extinctions, including the famous demise of the dinosaurs 65 million years ago, the planet today faces a sixth: a human-made extinction in several respects.
These are five extinctions the planet has endured across its history.
Ordovician-Silurian Extinction (439 million years ago)
This event is regarded as the second most intense mass extinction, and it stands as the earliest in time. The interval lasted roughly between 500,000 and 1 million years, with 85% of species disappearing, mostly in the seas. Brachiopods and bryozoans were among the hardest hit, along with trilobite, conodont, and graptolite groups. Although two explanations competed for why life collapsed, the causes remain under study as vast numbers of flora and fauna disappeared.
The prevailing theory suggests the great supercontinent Gondwana started drifting, first away from the equator toward the South Pole. Glaciation lowered sea levels, cooling the planet and altering ocean currents that once fed marine life. The extinction occurred in two phases: an initial shift as Gondwana moved toward the pole, followed by melting glaciers and shifting currents when the landmass returned toward the equator later on.
The second phase occurred at the end of the ice age, as the supercontinent moved back toward the equator, glaciers vanished, currents changed again, and sea levels rose. In recent years, a third theory has gained attention: an extraterrestrial trigger such as a nearby supernova explosion and gamma-ray pulse could have damaged the ozone and harmed aquatic life for a brief period. [citation]
Devonian-Carboniferous Extinction (367 million years ago)
The Devonian era marked a time of great oceanic diversity and surface prosperity, with jawed fish like the formidable 10-metre Dunkleosteus, sponges, mollusks, and the rise of vast coral reefs. It is often called the Age of Fishes for its long reign of about 70 million years. Yet this flourishing also ended with dramatic losses: roughly 83% of species disappeared over about 3 million years, primarily affecting marine life.
What makes this event distinct is the protracted mortality among benthic marine organisms, stretching over roughly 20 million years and not concentrated in a single moment. The extinction occurred in two phases, commonly referred to as the Kellwasser event and the Hangenberg event, underscoring a pattern of sustained stress rather than a single catastrophe. Climate shifts and ocean chemistry changes are the leading suspects, with volcanism and meteorite impacts also considered as contributing factors. [citation]
Permian-Triassic Extinction (251 million years ago)
Argument stands that this is the most devastating mass extinction in Earth history. It coincided with the emergence of large amphibians and early reptiles, the colonization of land by animals, and a roaring revival of marine life. Yet in about a million years, ecological collapse reshaped the biosphere. Estimates indicate only 3% of species survived, with marine life bearing massive losses and about 70% of terrestrial vertebrates vanishing. The recovery of life took a very long time due to the drastically reduced biodiversity.
The exact causes remain debated. Competing hypotheses include extreme volcanism, asteroid impact, and the release of greenhouse gases. Some researchers point to methane hydrates locked in the ocean floor, which could have unleashed runaway warming after destabilizing events. Modern evidence also links the era to Antarctica meteorite impacts, intense volcanism, and the release of toxic hydrogen sulfide into the seas, which together helped redefine ecological niches for the later rise of dinosaurs. [citation]
Triassic-Jurassic Extinction (210 million years ago)
During this interval, life on land and in the seas adapted to the breaking apart of Pangea and ongoing volcanic activity. Intense eruptions are linked to climate disruptions that may have killed a large share of species. The ejected gases, including sulfur dioxide and carbon dioxide, could have accelerated a climate shift, sometimes toward cooling if aerosols blocked solar input. The event is viewed as a series of biotic stresses rather than a single blow, leading to the gradual reshaping of ecosystems that allowed dinosaurs to rise to dominance.
Experts describe this period as a culmination of smaller extinctions occurring over about 18 million years. The disappearance of many species opened ecological space for dinosaurs to thrive in the subsequent eras. [citation]
Cretaceous-Paleogene Extinction (65 million years ago)
Famously responsible for ending the reign of the dinosaurs, this extinction is linked to an asteroid impact about 12 kilometres wide near the Gulf of Mexico. The immediate wave of devastation is clear, but the longer-term effects persisted for months and altered the atmosphere for years, suppressing photosynthesis as sunlight dimmed and temperatures fluctuated.
The event wiped out roughly 75% of Earth’s species and cleared the way for mammals to diversify. The aftermath included a dust-filled atmosphere that endured for about 18 months, followed by a cascade of changes in ocean currents and nutrient cycles that reshaped life at sea. The collapse of the food chain reshaped ecosystems and set the stage for a new era. [citation]
Toward a sixth extinction, many now see a human-driven pattern emerging. Humanity has reshaped the planet in ways that stress many species, with megafauna especially affected. After the last ice age, roughly 80% of the large-bodied species vanished. Today, estimates suggest one-fifth of birds, one-quarter of mammals, one-third of amphibians, and a large portion of plant life are threatened by human activities and climate change. The speed of today’s change stands out, as past extinctions unfolded over thousands to millions of years, while current trends unfold in decades. Projections show significant losses by mid-century if trends continue. [citation]
It is important to note that contemporary assessments emphasize the potential pace of anthropogenic change as a critical driver of the ongoing biodiversity crisis. [citation]