Exploring the Past: Understanding the 5 Major Extinctions on Earth and Seeking Answers for the Sixth Extinction.
What is Mass Extinction?
Mass Extinction refers to the widespread elimination of a significant percentage of a species population over a relatively short geological time frame.
Throughout Earth’s 4.6 billion-year geological history, there have been five major extinction events, all of which occurred during the Phanerozoic Eon, a period characterized by abundant animal life. This eon spans approximately 545 million years, beginning with the first appearance of diverse shelled animals and continues to the present day.
At times, it seemed as though Earth was facing its own apocalypse during the “uneasy transformations” of nature. Yet, the Blue Planet has demonstrated its resilience, enduring five significant mass extinctions…
1. Ordovician-Silurian Extinction
This was the first mass extinction event, occurring approximately 440-450 million years ago. During this period, several consecutive extinctions wiped out 17% of families and 50% of genera, making it the second-largest extinction in history based on the number of species lost.
By the end of the Ordovician period, around 49% of marine animal genera had completely disappeared, and other animal groups also faced significant declines. The leading hypothesis for this extinction is the gamma-ray burst from a nearby star, which drastically reduced carbon dioxide levels in the atmosphere. This phenomenon triggered a glacial period lasting between 0.5 to 1.5 million years.
The fluctuating sea levels during successive glaciations created numerous “ecological niches” on land. Biodiversity gradually diminished, particularly affecting species with restricted habitats along continental shelves and in tropical regions.
As the glaciers melted, sea levels rose, allowing the surviving families and genera to begin recovering, leading to an increase in biodiversity and the dawn of a new era.
2. Devonian Extinction
Archaeological evidence suggests that this was a protracted series of extinctions that may have lasted up to 20 million years. This extinction began around 360 million years ago, just before the transition from the Devonian to the Carboniferous period.
Prior to this extinction, the land was dominated by lower plants and the first insects, while oceans were teeming with massive coral reefs and a significant evolutionary surge in fish species.
Paleontologist McLaren posits that a large asteroid impact struck Earth, triggering tsunamis that devastated coastal ecosystems and disrupted deep-sea layers.
Another contributing factor was the robust growth of plants that reduced CO2 levels, resulting in a cooler climate that many species could not adapt to, leading to their extinction.
Marine organisms were the primary victims: coral reefs, home to countless marine species, faced mass die-offs, causing widespread extinction across numerous species. It is estimated that about 19% of families, 50% of genera, and 70% of species were wiped out during this event.
3. Permian-Triassic Extinction
This represents the most catastrophic extinction event in history, eradicating the vast majority of life on Earth and nearly resetting the biosphere. Paleontologists report that over 90% of marine species and 70% of terrestrial species were completely annihilated.
Archaeological evidence indicates that the primary cause of this phenomenon was intense tectonic activity affecting Earth’s crust, leading to fracturing and compression of landmasses.
The eruption of magma to the Earth’s surface engulfed everything in a sea of fire. Additionally, dust and carbon dioxide filled the atmosphere, creating a greenhouse effect that made Earth hotter than ever. Beneath the oceans, changing ocean currents wiped out many ecosystems, making life extremely fragile.
Another proposed contributor to this bloody mass extinction was the impact of a 500 km diameter asteroid on Earth. The remnants of this impact were discovered in Antarctica in 2006 by scientists from Ohio State University, USA.
It took a considerable amount of time for life to gradually recover from this event, with only a few lucky survivor organisms, including certain groups of reptiles.
4. Triassic-Jurassic Extinction
This extinction marked the boundary between the Triassic and Jurassic periods, occurring around 199.6 million years ago. This mass extinction had profound effects on terrestrial and marine life.
Many vertebrate species in the oceans and marine reptiles vanished, except for fish lizards and paddle-footed lizards. Invertebrates such as cephalopods, amphibians, and especially terrestrial reptiles, notably the archosaurs (excluding dinosaurs), were also heavily impacted. Approximately 23% of families and 48% of genera went extinct.
Scientists remain uncertain about what caused the end-Triassic extinction event. They speculate that a significant volcanic eruption may have occurred. However, recent studies have accurately indicated the timing of the extinction and the impact of a meteorite responsible for creating the Manicouagan Crater in Canada. This evidence suggests that the impact may have directly triggered this extinction.
This extinction event eliminated many large terrestrial species, paving the way for dinosaurs to dominate the planet throughout the Jurassic and Cretaceous periods. Dinosaurs ruled the land, while freshwater environments belonged to the ancestors of modern crocodiles (belonging to the archosaur clade), and the ancient lizard groups ruled the seas.
The Extinction that opened the Golden Age of Reptiles with Dinosaur Dominance
However, as the age of reptiles flourished, Earth faced the next catastrophe – the Cretaceous-Paleogene extinction.
5. Cretaceous-Paleogene Extinction
This extinction event occurred at the end of the Cretaceous period, approximately 66.5 million years ago, marking the end of the Mesozoic Era and the beginning of the Cenozoic Era with the Paleogene period. About 17% of families, 50% of genera, and 75% of species went extinct following this event.
Based on archaeological evidence gathered through research, scientists hypothesize that this extinction event was caused by one or more simultaneous disasters, such as the powerful impacts of asteroids (creating the Chicxulub crater in Mexico and the Boltysh crater in Ukraine) or due to falling sea levels and intense volcanic eruptions generating the “Deccan Traps,” which severely devastated Earth’s biosphere.
These geological events reduced light levels and rates of photosynthesis, leading to widespread destruction of Earth’s ecosystems. The climate became drier, and food chains were disrupted.
Dinosaurs were among the first vertebrate species affected by environmental changes, leading to a significant decline in species diversity. Alongside this, many species of plants and invertebrate animals also disappeared from Earth, paving the way for mammals to develop and gradually dominate.
The Cretaceous-Paleogene extinction event was uneven in its impact. Some organisms became completely extinct, others were severely affected, while the remaining species were hardly impacted at all.
Why Do Mass Extinctions Occur?
In general, when the biosphere experiences extreme pressures over extended periods, it can lead to mass extinctions. The higher the biodiversity, the greater the likelihood of mass extinctions occurring and the more significant the disruptions that can trigger a global mass extinction event.
In 2006, Allens and West proposed the “stress pulse” model, suggesting that mass extinctions typically involve two causes: prolonged stress—long-term pressure on ecosystems; and sudden stress events that arise and end abruptly—pulses. Their analysis of extinction rates across the oceans indicated that neither prolonged pressure alone nor sudden disasters alone would be sufficient to cause a significant increase in extinction rates!
Common Patterns of Extinction
Many scientists believe that mass extinctions are regular events, occurring in cycles. For instance, the 1984 paper by Raup and Sepkowski, titled “Mass Extinctions in the Geological History,” indicated that a mass extinction event occurs roughly every 26 to 30 million years.
In 2005, Rhodes and Mahler published the paper “The Cycle of Fossil Diversification,” proposing that the cycle of mass extinction is every 62 million years.
Explanations for the cyclical nature of mass extinctions include fluctuations in the plane of the Milky Way, spiral arms crossing the Milky Way, and fluctuations in geochemical variables. However, data on marine mass extinctions do not align with this cyclical pattern, leading to the conclusion that the existence of a common pattern of mass extinction remains merely a scientific hypothesis without conclusive evidence.
Is a Sixth Mass Extinction Looming?
All previous extinctions in history have been caused by natural events and occurred in cycles. Each simultaneous collapse has ushered in a new era, fostering the development of many resilient organisms. The question scientists now ponder is whether a sixth mass extinction is on the horizon.
In fact, signs of its beginnings are emerging… This time, however, it is not due to nature; rather, humans are orchestrating their own “self-destruction.” It will not be volcanic eruptions, asteroid impacts, or sudden sea level changes, but rather environmental pollution, habitat degradation, ecological imbalances, and climate change…
The current extinction rate is 4,000 times that of the dinosaur era, with every human impact leaving severe consequences for nature that are difficult to reverse. The over-exploitation of resources without conservation efforts is gradually erasing all forms of life.
Scientists predict that if the situation continues, within less than a century, a mass extinction will officially begin, and humanity may share the same fate as the dinosaurs. But after such an extinction, will humans be fortunate enough to survive and witness a revival of life?
