Mass Extinction

Extinction occurs when all members of a species, in the wild and in captivity, have died. Though extinction is a well-known and studied phenomenon in modern times, it was first described relatively recently in human history. In the late 18^th Century, a French naturalist named Georges Cuvier (1769-1832) was the first person to propose that species could become extinct and that life on Earth did not exist in a constant, unchanging state. While the exact mechanisms and varying extinction speeds were poorly understood then, Cuvier used examples of fossilized species that could no longer be found anywhere on Earth as evidence for the phenomenon. Over the next century, the idea was further bolstered by many scientists, including Charles Darwin. Today we know that over 99 percent of all species that have lived are now extinct.

Mass extinction definition

Mass extinctions, also known as extinction events, occur when there is a massive and sharp decline in global levels of biodiversity. When this occurs, the rate of extinction exceeds that of speciation (the rate at which new species arise). At least six mass extinction events are known to have occurred: the Ordovician-Silurian, Late Devonian, Permian-Triassic, Triassic-Jurassic, Cretaceous-Paleogene, and Holocene. The Holocene extinction event is currently ongoing.

Mass extinction events: a brief overview

In the rest of the article, we will walk through mass extinction events throughout history, from the longest ago to the most recent. The table below lists the extinction events covered in this article and their timeline.

Ordovician-Silurian mass extinction (440-450 million years ago)

Known as the oldest mass extinction event, it is estimated that 85% of species present on Earth at the time perished during the Ordovician-Silurian mass extinction. At the time, life had not yet moved onto land and was confined to the Earth’s vast oceans. It is believed that a dramatic shift in the Earth’s climate resulted in mass glaciation, thus lowering sea levels and temperature. This was then followed by a period of global warming, which caused sea levels and temperatures to increase. It is believed that the changes in sea temperature resulted in the mass die-off of most species.

Many brachiopod, conodont, coral, and trilobite species are believed to have become extinct during this event. Other theories regarding this extinction event suggest that the ocean may have become polluted by toxic metals or that large-scale volcanic eruptions may have been the cause.

Late Devonian mass extinction (360-375 million years ago)

During the late Devonian mass extinction event, it is estimated that 75% of living species became extinct. Though most species continued to live within the ocean, life had begun to adapt and evolve to living on land during this period. The Devonian Period is commonly known as the “age of fish” due to the vast diversity of oceanic species at the time.

This extinction event appears to have occurred gradually over a very long period, with estimates ranging from as little as 500,000 years to as long as 25 million years being proposed. Given the extended nature of this extinction event, no one cause has been identified, and it is possible that many smaller extinction events occurred during the period. One theory is that the burgeoning diversity of plant life, particularly on land, may have resulted in environmental changes, causing many marine species to disappear.

The largest mass extinction: Permian-Triassic (252 million years ago)

The Permian-Triassic extinction event is the largest known mass extinction in Earth’s history, with approximately 96% of marine and 70% of terrestrial species going extinct. Due to the massive reduction in global levels of biodiversity, this period is commonly referred to as the “Great Dying.” After surviving the previous two mass extinction events, trilobites became extinct during this period. Most of the period’s large synapsids, including the Dimetrodon, are also believed to have perished at this time. It is suspected that biodiversity levels may have taken up to 10 million years to recover fully.

Figure 3: A fossilized trilobite of the Cheirurus genus. Trilobites went extinct during the Permian-Triassic mass extinction. Source: Wikipedia

Triassic-Jurassic mass extinction (201 million years ago)

Following the devastating Permian-Triassic extinction event, the first dinosaurs and mammals appeared during the Triassic Period. During the Triassic-Jurassic mass extinction, it is estimated that around 80% of species became extinct, including most groups competitive with the dinosaurs, thus resulting in dinosaurs becoming the dominant terrestrial vertebrates of the Jurassic Period. Factors involved in this extinction event are suspected to have been similar to the Permian-Triassic event, including widespread volcanic eruptions, followed by global warming, sea level rise, and acidification. Other theories propose that melted permafrost may have released large amounts of CO₂ into the atmosphere, resulting in mass extinction.

Cretaceous-Paleogene mass extinction (66 million years ago)

Perhaps the best known mass extinction event, the Cretaceous-Paleogene extinction (also known as the K-Pg extinction) occurred around 66 million years ago. It is estimated that around 75% of the Earth’s species were extinguished at this time, including all non-avian dinosaurs, as well as many species of mammal, bird, reptile, etc. The large niches left by the extinction of non-avian dinosaurs allowed for massive diversification of mammal and bird species in their wake. The most widely accepted cause of this extinction event is a catastrophic meteor impact that occurred at what is modern-day Chicxulub on Mexico’s Yucatan Peninsula. This is known as the Alvarez hypothesis. Other proposed causes include massive volcanic eruptions at the Deccan Traps of what is now the Western Ghats in India.

Holocene mass extinction (11,700 years ago-present day)

The Holocene mass extinction, sometimes referred to as the Anthropocene mass extinction, is currently ongoing and is primarily driven by human activities. While this extinction event likely began around the time of the Agricultural (Neolithic) Revolution at the Fertile Crescent, the most dramatic changes in biodiversity have only been observed within the past two centuries. In fact, since the beginning of the 20th Century, species have been going extinct at a rate of over 1,000 times the normal, expected rate of extinction.

Early causes of extinction likely involved overhunting, which may have resulted in the extinction of some megafauna, such as the woolly mammoth (Mammuthus primigenius) and Megalania (Varanus priscus), as well as other animals, such as the fully terrestrial crocodiles of the Mekosuchus genus. In modern times, extinction is primarily driven by human overpopulation, habitat destruction, resource depletion, climate change, pollution, and overconsumption.

Of the over 40,000 species that the IUCN (International Union for the Conservation of Nature) Red List has assessed, 28% are currently threatened with extinction. This includes Critically Endangered species such as the Chinese alligator (Alligator sinensis), Siamese crocodile (Crocodylus siamensis), Javan rhinoceros (Rhinoceros sondaicus), Yangtze giant softshell turtle (Rafetus swinhoei), and many, many more.

Mass extinction timeline

The following (Fig. 4) is a timeline of the six mass extinction events detailed above.

Figure 4: A timeline of all six known mass extinction events. Source: National Geographic

Causes of mass extinction

Mass extinction can have many different causes. The five known mass extinctions that occurred prior to the arrival of humans are all suspected to have been caused by cataclysmic natural events, such as meteor impacts and widespread volcanic eruptions. The role that disease may play in mass extinction is unclear, as genetic diversity typically means that at least a small number of individuals of many species would be asymptomatic or resistant to the disease. This would be effective at preventing mass extinction, though it could facilitate localized extinction or the extinction of species with small populations and low levels of genetic diversity.

In modern times, the dramatic rise in extinction rates and loss of biodiversity is due to human activities. As the human population continues to expand, more habitat is lost, more resources are depleted, and the habitat that remains becomes polluted by synthetic materials (e.g., plastic) and other waste, resulting in increased pressure on biodiversity and species extinction.